* Department of Biochemistry, New York University Medical Center, New York 10016; Department of Biological Sciences,
Stanford University, Stanford, California 94305; and § Flanders Interuniversity Institute for Biotechnology and University of
Ghent, B-9000 Ghent, Belgium
The production of native /
tubulin heterodimer in vitro depends on the action of cytosolic
chaperonin and several protein cofactors. We previously showed that four such cofactors (termed A, C, D,
and E) together with native tubulin act on
-tubulin folding intermediates generated by the chaperonin to
produce polymerizable tubulin heterodimers. However, this set of cofactors generates native heterodimers
only very inefficiently from
-tubulin folding intermediates produced by the same chaperonin. Here we describe the isolation, characterization, and genetic analysis of a novel tubulin folding cofactor (cofactor B) that
greatly enhances the efficiency of
-tubulin folding in
vitro. This enabled an integrated study of
- and
-tubulin folding: we find that the pathways leading to
the formation of native
- and
-tubulin converge in
that the folding of the
subunit requires the participation of cofactor complexes containing the
subunit and
vice versa. We also show that sequestration of native
-or
-tubulins by complex formation with cofactors
results in the destabilization and decay of the remaining
free subunit. These data demonstrate that tubulin folding cofactors function by placing and/or maintaining
-and
-tubulin polypeptides in an activated conformational state required for the formation of native
/
heterodimers, and imply that each subunit provides information necessary for the proper folding of the other.
THE cytoskeleton of eukaryotic cells consists of three
distinctive networks: microfilaments, intermediate
filaments, and microtubules. In common with other
proteins, the proper functioning of the subunits from which
these networks are assembled depends on their three- dimensional structure. Classical experiments using small
enzymes as model systems have established that the formation of correct tertiary structure can occur spontaneously (Anfinsen, 1973 Studies on the bacterial homologue of c-cpn, GroEL/
GroES, have led to a wealth of genetic, biochemical, and
structural information on the mechanism of facilitated
folding (for review see Horwich et al., 1995 The in vitro c-cpn-mediated folding of actin and In the case of While cofactors A, C, D, and E participate in the pathway leading to native In Vitro Folding Assays
The purification of c-cpn from rabbit reticulocyte lysate and the generation of unfolded 35S-labeled In some experiments, the reaction products were purified by gel filtration, mixed with native brain tubulin, and assayed for their ability to cocycle through multiple rounds of polymerization and depolymerization as
described (Gao et al., 1993 Purification, Peptide Sequence Analysis, Cloning, and
Expression of Cofactor B
Cofactor B was purified from a crude extract of bovine testis tissue (Gao
et al., 1994 Table I.
Purification Scheme for Cofactor B
) and have led to the idea that all the
information required for the proper folding of a protein
resides in the amino acid sequence. However, the subunit
proteins from which microfilament and microtubule networks are assembled
i.e., actins and tubulins, respectively
do not fold spontaneously; they require the action
of cytosolic chaperonin (c-cpn)1 (Chen et al., 1994
; Ursic et
al., 1994
; Vinh and Drubin, 1994
), a multisubunit toroidal
complex that generates potentially productive folding intermediates via multiple rounds of ATP hydrolysis (Frydman et al., 1992
; Gao et al., 1992
; Tian et al., 1995a
; Lewis et
al., 1996
).
; Hartl, 1996
). In
contrast with GroEL/GroES, there is biochemical and genetic evidence that the target range of c-cpn is quite limited (Lewis et al., 1996
). Furthermore, neither GroEL/ GroES nor its mitochondrial homologue, Hsp60/Hsp10,
can generate productive actin or tubulin folding intermediates (Tian et al., 1995a
). It therefore seems possible that
c-cpn may have evolved to overcome specific kinetic traps
on the folding pathways of its target proteins.
-tubulin requires only chaperonin, target protein, and ATP
(Gao et al., 1992
; Melki et al., 1993
). In contrast, the productive folding of
- and
-tubulin requires the additional
presence of GTP (both
- and
-tubulin are GTP-binding
proteins), a set of protein cofactors, and native tubulin
(Gao et al., 1993
). Quasinative tubulin folding intermediates (termed IQ) that already contain the GTP-binding pocket are produced via one or more ATP-dependent cycles of interaction with c-cpn (Tian et al., 1995b
); however,
in the absence of cofactors, these intermediates cannot
proceed to the native state.
-tubulin, interaction of c-cpn-generated
intermediates with four cofactors (designated A, C, D, and
E) and native tubulin leads ultimately to the production of
de novo folded
-tubulin subunits that form functional
heterodimers (Tian et al., 1996
). Yeast homologues of
three of these cofactors (A, D, and E) have been identified
and shown to give rise to cytoskeletal phenotypes when mutated or deleted. The Saccharomyces cerevisiae homologue
of cofactor A, RBL2, can compensate for the lethal overexpression of
-tubulin when it too is overexpressed (Archer et al., 1995
). Deletion of CIN1 (Chromosome Instability 1), the gene encoding the yeast homologue of
cofactor D, results in supersensitivity to the antimicrotubule drug benomyl, cold sensitivity, and chromosome loss
in mitosis (Stearns et al., 1990
; Hoyt et al., 1990
). The CIN1 protein appears to act in the same pathway as the
products of two other yeast genes, CIN2 and CIN4, although neither of these is related to any of the known
mammalian tubulin folding cofactors. The PAC2 protein,
which was identified as a synthetic lethal in conjunction
with the chromosome instability mutant CIN8, is the yeast
homologue of cofactor E (Hoyt et al., 1997
). No homologue of cofactor C can be discerned via database searches
of the yeast genome; however, homologues are identifiable in evolutionarily distant eukaryotes (e.g., Arabidopsis, Caenorhabditis elegans). None of the genes encoding
cofactor homologues are essential for viability in yeast, although cofactors are required for the productive folding of
an essential protein (tubulin) in vitro.
-tubulin, these cofactors do not support the efficient in vitro folding of
-tubulin (Tian et al.,
1996
). Here we describe the isolation, characterization, and
functional analysis of cofactor B, a protein cofactor that
participates in the efficient generation of native
-tubulin.
We demonstrate that the proper folding of
- and
-tubulin is interdependent, and present evidence that this occurs
via convergent pathways that result in the formation of a
multimolecular
- and
-tubulin and cofactor-containing
complex. The reaction cycles that lead to the formation of
this complex define a role for cofactor function in generating and maintaining
- and
-tubulins in an activated energy state that is required for the formation of the native
/
tubulin heterodimer.
Materials and Methods
and
target proteins via their expression in
Escherichia coli were done as described previously (Gao et al., 1992
,
1993
). C-cpn-mediated folding reactions were done at 30°C in folding buffer (Tian et al., 1995a
) and contained 0.2 µM c-cpn, 1 mM ATP, 0.1 mM GTP, and one or more of cofactors A, B, C, D, and E present in varying amounts with respect to c-cpn (see text). Cofactors A-E were either
purified from crude extracts of bovine testis as described (cofactor A, Gao
et al., 1994
; cofactor B, see below; cofactors C-E, Tian et al., 1996
), or (in
the case of cofactors A-C) from extracts of host Escherichia coli
BL21DE3 cells engineered for their expression. Some folding reactions
(see text) were supplemented with 2.5 µM native bovine brain tubulin as
described previously (Tian et al., 1996
).
). In others, the reaction products were supplemented with 2.5 µM native tubulin and treated by digestion at 30°C for 30 min with 10 µg/ml subtilisin so as to generate carboxy-terminally truncated tubulin (Sackett et al., 1985
). The proteolytic reaction was quenched
by the addition of PMSF to 5 mM. All in vitro folding reaction products
were analyzed on 4.5% nondenaturing polyacrylamide gels containing 0.1 mM GTP as described (Gao et al., 1992
).
; Tian et al., 1996
) by fast performance liquid chromatography
using the steps shown in Table I. To improve the efficiency of folding assays containing column fractions, unfolded 35S-labeled target protein was
first cycled with c-cpn, ATP, and GTP so as to form quasinative (IQ) intermediates (Tian et al., 1995b
). Peptide sequence analysis of the purified
protein was done as described previously (Gao et al., 1994
; Tian et al.,
1996
). A virtual homologue encoding human cofactor B was assembled
from the WashU-Merck EST database using the Wisconsin Package, version 8.1 (Genetics Computer Group, Inc., Madison, WI). A full-length cDNA encoding human cofactor B was generated by PCR using a cDNA template prepared from human testis mRNA (Clontech, Palo Alto, CA)
following procedures recommended by the supplier. The amplified cDNA
product was inserted into the pET23 expression vector, and the recombinant protein was purified after induction of host E. coli BL21DE3 cells.
The procedure used for the purification of recombinant human cofactor B
was identical to that used for purification of the bovine protein (Table I),
except that step 4 was omitted.
Isolation of FB and Glutaraldehyde
Cross-linking Experiments
The products of a c-cpn-mediated -tubulin folding reaction done with
unlabeled target protein (Tian et al., 1995b
) and purified bovine cofactor
B (equimolar with respect to c-cpn) in the presence of
-[32P]GTP were
applied to a 0.75 × 7.5-cm SW gel filtration guard column (TosoHaas,
Montgomeryville, PA) equilibrated and run in folding buffer (Tian et al.,
1995a
) containing 50 µM GTP and purified rabbit hemoglobin (0.1 mg/
ml) included as a stabilizing agent. The FB
complex emerged from this
column at 2.9 ml and was effectively resolved from
-tubulin/c-cpn binary
complex. In experiments done to identify the presence of unstable
-tubulin-cofactor complexes, aliquots (15 µl) of this material were incubated
for 5 min at 30°C with a fivefold molar excess (with respect to cofactor B) of either cofactor D, E, or both. The products of these reactions were stabilized by the addition of glutaraldehyde to 0.03% and incubation at 30°C
for an additional 5 min. The cross-linking reaction was quenched by the
addition of ethanolamine to 0.1 M and the reaction products were analyzed on nondenaturing polyacrylamide gels as described (Gao et al.,
1993
).
In Vitro Translation, Purification of 35S-labeled Tubulin, and Backreactions with Tubulin Folding Cofactors
Full-length mouse - or
-tubulin cDNAs (Gao et al., 1993
) were expressed by coupled transcription/translation in 50 µl of TNT lysate
(Promega, Madison, WI) containing [35S]methionine. Native bovine brain
tubulin was added to 1 µM, the mixture was incubated for an additional 30 min at 30°C, and the reactions were cleared of particulate material by centrifugation at 200,000 g for 20 min at 4°C in a rotor (TL100; Beckman Instruments, Inc., Fullerton, CA). The concentration of NaCl was adjusted
to 0.25 M, and the labeled native tubulin was purified free of cofactors using a microcolumn of DEAE-Sephacel (Pharmacia Fine Chemicals, Piscataway, NJ) (Tian et al., 1996
) contained in a glass-wool plugged polypropylene pipette tip. After elution with 0.5 M NaCl, the tubulin was dialyzed
to reduce the salt concentration to ~0.1 M and incubated at 30°C for various times in the presence of nucleotide and one or more purified cofactors. Reaction products were analyzed on nondenaturing polyacrylamide
gels as described (Gao et al., 1993
).
UV Cross-linking Experiments
A backwards reaction of native tubulin with cofactors (see above) was assembled in folding buffer (Tian et al., 1995a) containing equimolar
amounts of native tubulin heterodimer, cofactors C, D, and E, and 15 µM
-32P-labeled GTP (sp act 200 Ci/mmol). This mixture was incubated for
30 min at 30°C and exposed to UV irradiation at 254 nm for 20 min as described (Eriksson et al., 1982
), and the reaction products were analyzed on
an 8% SDS polyacrylamide gel.
GTP Hydrolysis Experiments
In experiments to determine the effect of GTPS in folding reactions,
c-cpn-mediated
-tubulin reactions were first done in the presence of 1 mM ATP and 50 µM GTP so as to generate IQ intermediates (Tian et al.,
1995b
); this procedure avoided interference of GTP
S in the ATP-dependent cycling of c-cpn. Equimolar amounts of cofactors B, C, D, and E
(with respect to c-cpn) and native tubulin (2.5 µM) were then added with
or without 1 mM GTP
S and the incubation continued at 30°C for 1 h. In
the case of
-tubulin, c-cpn-mediated
-tubulin reactions were done in
the presence of 1 mM ATP, 50 µM GTP, and a molar equivalent (with respect to c-cpn) of cofactor D so as to generate FD
intermediates (Tian et
al., 1996
). Cofactor C, cofactor E, and native tubulin were then added with
or without 1 mM GTP
S and the incubation continued at 30°C for 1 h.
The extent of hydrolysis of -32P-labeled GTP was measured in folding
buffer (Tian et al., 1995a
) by incubation at 30°C for 1 h in reactions containing 15 µM GTP (sp act 200 Ci/mmol), purified native brain tubulin (0.5 µM), and/or one or more cofactors each present at ~1.5-fold molar excess
with respect to tubulin. Reaction products were analyzed by TLC on phospho-ethyleneimine plates as described (Spiegelman et al., 1977
), and the
yield of labeled GTP and GDP was quantitated using a phosphorimager.
Manipulations in S. cerevisiae
Media for yeast growth and sporulation were as described (Sherman et al.,
1983). SGal medium was made as for synthetic dextrose medium except that
glucose was replaced with 2% galactose. Benomyl (98.6% pure; maintained
in 10 mg/ml in DMSO at
20°C) was a gift from E.I. duPont de Nemours,
Inc. (Wilmington, DE) Growth of strains on solid media was assayed by
spotting suspensions of cells in water onto plates using a 32-point multipronged inoculating manifold (Dan-Kar Corp., Wilmington, MA). Yeast
cells were transformed by the lithium acetate method (Ito et al., 1983
);
transformants carrying plasmids were selected on synthetic complete medium lacking the appropriate nutrient.
ALF1 was disrupted by the PCR method described by Amberg et al.
(1996). Four oligonucleotide primers were synthesized as follows: ALF1.1:
CGCAGCTCCACCCATTAATTTGACGC; ALF1.2: GCCTCGAGGGGTCCAACCCTTGGTTT; ALF1.8: ATGGTTAGAGTTGTCATAGAGCAGATTGTACTGAGAG; ALF1.9: TCATCATCGCTCTCCACGTCCTGTGCGGTATTTCACAC. Amplification of ALF1 from S. cerevisiae genomic
DNA with ALF1.1 and ALF1.2 was followed by creation of a fusion between sequences flanking ALF1 and either the URA3 or HIS3 gene. The
fusion was created by PCR using the plasmids pRS313 and pRS316
(Sikorski and Hieter, 1989
) as the source of marker DNAs. These constructs were transformed into a wild-type diploid strain (TPS507; Marschall et al., 1996
). The genotypes of two of each of the resulting alf1:: URA3/ALF1 and alf1::HIS3/ALF1 transformants were confirmed by PCR, sporulated, and dissected.
Cofactor B: A Participant in the -Tubulin
Folding Pathway
We originally identified two crude fractions from rabbit
reticulocyte lysate that together include the activities required (in addition to c-cpn) to yield correctly folded - and
-tubulin in in vitro folding reactions (Gao et al., 1993
).
These crude fractions contain cofactors A, C, D, and E, all
of which have been purified and shown to participate in
the pathway leading to correctly folded
-tubulin (Tian et
al., 1996
). However, c-cpn-mediated
-tubulin folding reactions containing these cofactors do not generate appreciable quantities of native
-tubulin, as judged by the very
low yield of radiolabel comigrating with authentic tubulin
heterodimers on a nondenaturing polyacrylamide gel (Fig. 1 A). We therefore reasoned that one or more additional
cofactors must exist that contribute to the efficient production of properly folded
-tubulin.
To isolate such cofactor(s), we fractionated a crude extract of bovine testis tissue by anion exchange chromatography and assayed the emerging proteins in in vitro c-cpn-
mediated -tubulin folding reactions (see Materials and
Methods). Native bovine brain tubulin was added (Gao et
al., 1993
), and the reaction products were analyzed by nondenaturing polyacrylamide gel electrophoresis. A fast-
migrating species was generated in these reactions without supplementation with cofactors A, C, D, and E (Fig. 1 B).
We used this assay as a method to purify to homogeneity
the protein responsible for the generation of this product.
The purified protein (which we termed cofactor B) migrated with an apparent mass of 130 kD upon gel filtration
(Fig. 1 C), and consisted of a single polypeptide of 38 kD
upon analysis by SDS-PAGE (Fig. 1 D) and 27,561 D by mass spectrometry.
The product of in vitro c-cpn-mediated -tubulin folding reactions supplemented with purified bovine cofactor
B alone comigrated with native tubulin in our gel assay. To
see if this material was indeed native, we tested its ability
to copurify with unlabeled bovine brain tubulin through
several cycles of polymerization and depolymerization and
found that it failed to cycle efficiently (Fig. 1 E). However,
when a cofactor B-containing
-tubulin folding reaction
was supplemented with a mixture of purified
-tubulin- folding cofactors (i.e., cofactors A, C, D, and E), the reaction product (the yield of which was greatly enhanced
compared with a parallel reaction lacking cofactor B; see
below) cocycled with native tubulin without loss of specific
activity, demonstrating that correctly folded
-tubulin was
efficiently produced in this experiment (Fig. 1 F).
Cofactors B and E Contain the CLIP-170 Microtubule Binding Motif
We obtained partial amino acid sequence data from purified
cofactor B and used this information to search for homologues in the WashU-Merck EST database. This search resulted in the identification of multiple overlapping cDNAs
encoding a human protein of 27,394 D, with 80% amino
acid identity within the region covered by the bovine peptides sequenced. No other related sequences were identified. To confirm that this homologue indeed encodes cofactor B, we cloned a cDNA encoding the human protein,
expressed it in host E. coli cells, purified the recombinant
protein, and demonstrated its activity in in vitro c-cpn-mediated -tubulin folding assays (see below). The complete
amino acid sequence encoded by our human cofactor B
cDNA (Fig. 2) revealed a region of homology with cofactor E (Tian et al., 1996
). This region is also homologous to
a motif (the CLIP-repeat) that is tandemly repeated
within the microtubule-binding domain of CLIP-170, a microtubule-associated protein that links endocytic vesicles
to microtubules (Pierre et al., 1992
). The amino acid sequence of cofactor B identifies an open reading frame (YNL148c) encoding an unknown protein in S. cerevisiae
with 32% and 52% amino acid sequence identity and similarity, respectively, to human cofactor B. We term this
yeast gene ALF1 (Alpha tubulin Folding 1).
Genetic Analysis of the Yeast Homologue of Cofactor B
To determine the effect of eliminating its expression in S. cerevisiae, we disrupted the ALF1 gene in a diploid strain
and subjected the resulting heterozygote to tetrad analysis.
In 16 out of 20 tetrads, all four spores were viable, and the
disruption marker segregated 2:2, indicating that ALF1 is
not essential for viability. Haploid alf1 null strains were
tested for temperature sensitivity and sensitivity to the antimicrotubule drug benomyl; increased sensitivity to
benomyl is a common phenotype among mutants affecting
the microtubule cytoskeleton (Stearns et al., 1990). In
comparison with wild-type, alf1 null strains were supersensitive to benomyl, growing poorly on 5 µg/ml. For comparison, the CIN1, CIN2, and CIN4 genes (the first of these
being the yeast homologue of cofactor D) are among the
most benomyl-supersensitive mutants known (Hoyt et al.,
1990
; Stearns et al., 1990
), failing to grow on 5 µg/ml
benomyl (Fig. 3). alf1 null strains were not cold sensitive.
Genetic interactions between ALF1 and other genes involved in microtubule function were tested by making
double mutants. alf1::URA3 cin1::HIS3, alf1::URA3 cin2::
LEU2, and alf1::URA3 cin4::LEU2 double mutants were
all viable, as was an alf1::URA3 cin1::HIS3 cin2::LEU2 triple mutant. The phenotypes of all these mutants were indistinguishable from the single cin mutants. In contrast, when we attempted to make double mutants between alf1::
URA3 and tub1-1, a mutation in the essential yeast -tubulin gene, we were unable to recover the double mutant by
tetrad analysis. This result indicates that alf1::URA3 and
tub1-1 are synthetically lethal.
Cofactor Requirements for Productive
-Tubulin Folding
We compared c-cpn-mediated 35S-labeled -tubulin folding reactions supplemented with either purified bovine cofactor B or recombinant human cofactor B alone, and analyzed the products on a nondenaturing gel. We found that
-tubulin folding reactions done with either bovine or human cofactor B yielded products that have different electrophoretic mobilities under native conditions; the product of reactions done with bovine cofactor B comigrates with
native tubulin, whereas the corresponding product generated in reactions done with human cofactor B runs more
slowly (Fig. 4 A). It follows that these species cannot be
free
-tubulin molecules; rather, they must be cofactor B/
-tubulin complexes, which we term FB
.
We took advantage of our observation that human FB
and native tubulin migrate differently on nondenaturing
gels to determine those cofactors required for c-cpn-mediated
-tubulin folding in vitro. C-cpn-mediated
-tubulin
folding reactions containing ATP, GTP, and native brain
tubulin were supplemented with subsets of the set of cofactors (A, B, C, D, and E) that we identified as sufficient
for proper
-tubulin folding (Fig. 1 F). In reactions done
with each of the cofactors alone, we found that c-cpn-
mediated
-tubulin folding in the presence of human cofactor B resulted in the generation of the characteristic
FB
intermediate, whereas none of the other cofactors on
their own yielded a recognizable product (Fig. 4 B). In
folding reactions from which each cofactor was omitted in
turn, we found that the absence of cofactor A did not influence the production of material that migrated as native tubulin, whereas omission of cofactor B resulted in a
greatly reduced yield of this product. In contrast, omission
of either cofactors C, D, or E completely eliminated the
production of material that migrated as native tubulin,
yielding the intermediate characteristic of folding reactions containing (human) cofactor B alone (FB
) and, in
reactions containing cofactors D and E but lacking cofactor C, an additional slower moving band (highlighted with
an asterisk) (Fig. 4 C). The nature and significance of this
species is addressed below.
To obtain further evidence that the product migrating as
authentic tubulin in these experiments was indeed native,
we took advantage of the observation that native tubulin
treated with subtilisin results in truncated molecules that
have reduced mobilities on nondenaturing gels, but that
nonetheless retain their capacity to polymerize into microtubules (Sackett et al., 1985). A control reaction done with
native bovine brain tubulin yielded the expected pair of
bands, one formed as a result of subtilisin cleavage of only
the
subunit, and one formed as a result of cleavage of both
and
subunits (Bhattacharyya et al., 1985
) (Fig. 4
D, lanes 1 and 2). Subtilisin treatment under identical conditions of the products of an
-tubulin folding reaction
done in the presence of human cofactor B alone showed
that the FB
fast-moving band was completely destroyed
by the proteolysis step (Fig. 4 D, lanes 3 and 4). In contrast, the subtilisin-treated products from a c-cpn-mediated
-tubulin folding reaction containing cofactors B, C,
D, and E were indistinguishable from those derived from authentic tubulin (Fig. 4 D, lanes 5 and 6). In addition, as
in the case of the products of a folding reaction done with
cofactors A, B, C, D, and E (Fig. 1 F), the products of a
folding reaction containing cofactors B, C, D, and E cocycled efficiently with native brain tubulin through three
successive cycles of polymerization and depolymerization
(data not shown). We conclude that cofactors B, C, D, and
E participate in the pathway leading to correctly folded
-tubulin.
Intermediates in the -Tubulin Folding Pathway
To see whether FB contained the nonexchangeable GTP
that is associated with native
-tubulin (Spiegelman et al.,
1977
), we did an
-tubulin c-cpn-mediated folding reaction containing cofactor B using unlabeled target protein
in the presence of
-[32P]GTP (Tian et al., 1995b
). Nondenaturing gel analysis of the reaction products showed that
the cofactor B-dependent fast-migrating band does indeed contain nonexchangeably bound GTP (Fig. 4 E, lane
1). We conclude that the
-tubulin in this complex is quasi-native in that it contains nonexchangeably bound GTP; its
sensitivity to cleavage by subtilisin, on the other hand,
demonstrates that it is nonnative.
Does FB behave as an intermediate in
-tubulin folding? To address this question, we isolated human FB
by
gel filtration, and found that it could be efficiently partitioned to the native state by the addition of cofactors C, D,
E, and native tubulin, as judged by the mobility of the reaction product on a nondenaturing gel (Fig. 4 E, lanes 2 and 3) and by the ability of this product to cocycle with native brain tubulin through multiple rounds of polymerization and depolymerization without loss of specific radioactivity (data not shown). The greater yield of native product
in this experiment compared with the starting material is a
reflection of their relative stability during native gel electrophoresis. We conclude that FB
is a bona fide intermediate in the pathway leading to native
-tubulin.
Target protein-cofactor complexes formed between
c-cpn-generated folding intermediates and cofactors D or
D plus E have been described in the pathway leading to
correctly folded -tubulin (Tian et al., 1996
). Since cofactors D and E also participate in the
-tubulin folding pathway (Fig. 4 C), we reasoned that similar complexes might
be generated (at least transiently) in
-tubulin folding reactions containing these cofactors. Indeed,
-tubulin folding reactions done in the absence of cofactor C yielded an
additional band that might be an intermediate in the
-tubulin folding pathway (Fig. 4 C, asterisk). To further characterize such complexes, we incubated the isolated, labeled
FB
intermediate in the presence of cofactors D and E either alone or together, with or without added native tubulin. In initial experiments, we observed very low yields of
labeled bands with intermediate mobilities on nondenaturing gels. However, when the products of reactions done
with FB
and cofactors D or E (or both) were reacted briefly with glutaraldehyde so as to cross-link and thereby
stabilize any complexes that might have formed, analysis
of the reaction products on a nondenaturing gel revealed
the appearance of additional bands (Fig. 4 F). These data
suggest the formation of unstable
-tubulin complexes
containing cofactor E (FE
) and cofactors D and E. Moreover, the generation of the complex formed by reaction with
cofactors D and E (Fig. 4 F, asterisk) is completely dependent upon the inclusion of native tubulin, suggesting that
this complex contains both
- and
-tubulin (see below).
The intermediates identified in these experiments might
play a part in the post-FB
pathway leading to the generation of native
-tubulin. Indeed, low levels of labeled
bands with the same migration properties are evident in
kinetic analyses of
-tubulin in vitro translation reactions
(Zabala and Cowan, 1992
).
Native - and
-Tubulin Exist in Activated
Conformational States
In principle, the pathways going from unfolded to correctly folded tubulins should be reversible, providing an
opportunity to examine intermediates generated by the
action of cofactors on the native tubulin heterodimer. To
study the - and
-tubulin folding pathways in reverse, we
labeled each subunit (separately) with [35S]methionine by
translation in vitro. The resulting native tubulin heterodimers were resolved from cofactors on a column of
DEAE-Sephacel (Murphy et al., 1977
), taking advantage
of the fact that native tubulin binds much more tightly than
any of the cofactors to the anion exchange resin (Tian et al.,
1996
). This labeled tubulin was then incubated with various
combinations of purified cofactors, and the reaction mixtures were analyzed by native gel electrophoresis (Fig. 5 A).
Incubation of native tubulin heterodimer containing 35S-labeled subunit with a fivefold molar excess of cofactors
B or E alone did not result in any shift in the mobility of
the radiolabeled species. However, reaction with cofactor
D resulted in a complete loss of radiolabeled
-tubulin. Incubation with cofactors D and E caused a quantitative
shift of the labeled protein to a species that comigrates
with a band that appears in in vitro c-cpn-mediated (i.e.,
forward)
-tubulin folding reactions (Fig. 4 C, asterisk), as
well as with the species seen in a
-tubulin backreaction
containing the same cofactors (Fig. 5 A, asterisk). Finally,
reaction of tubulin with recombinant human cofactor B
and either cofactor D or E resulted in the formation of the
FB
intermediate (compare Figs. 4, A-C and 5 A). In the
case of reactions done with native tubulin heterodimer
containing 35S-labeled
subunit, incubation with cofactor
D or with cofactor D plus E resulted in a quantitative shift
of label to slower migrating species; these species correspond in mobility to cofactor/
-tubulin complexes formed
in in vitro c-cpn-mediated (i.e., forward)
-tubulin folding
reactions containing the same cofactors (Tian et al., 1996
).
The data shown in Fig. 5 A suggest that cofactor D can
interact with native tubulin, disrupting the heterodimer.
To rule out the possibility that the tubulin heterodimer
might be denaturing during the course of these experiments because of the relatively low tubulin concentration,
we repeated the incubation of native heterodimer with a
1.5-fold molar excess of cofactor D using a tubulin concentration of 2.5 µM, i.e., above the reported dissociation constant of 1 µM (Detrich and Williams, 1978). In this case
the labeled
-tubulin band was also quantitatively shifted
(Fig. 5 B). These data confirm that cofactor D can bind to
the native
-tubulin subunit, disrupting the heterodimer
and forming a stable (FD
) complex.
Are free -tubulin subunits bereft of their
-tubulin
partners unstable in solution or merely destabilized during
native gel electrophoresis? To address this question, we incubated native tubulin dimer 35S-labeled in its
subunit
with a 1.5-fold molar excess of cofactor D; immediately
thereafter (which we define as t = 0 min), or after an additional 15 min of incubation at 30°C (defined as t = 15 min)
so as to fully form the FD
complex, the reaction was supplemented with one of several components and the incubation continued for an additional 30 min. When native tubulin was added at t = 0 min, there was no significant loss
of labeled tubulin relative to the input counts. However,
addition of native tubulin after a 15-min delay resulted in a
total loss of counts (Fig. 5 C). We next tested whether free
-tubulin generated in cofactor D-containing reactions could be captured either by other cofactors or by mitochondrial chaperonin (mt-cpn), which we have previously
shown to recognize and bind nonnative forms of
-tubulin
(Tian et al., 1995a
). Addition of cofactors B or E or mt-cpn
at t = 0 min resulted in the binding of
subunits to these
proteins. However, when added at t = 15 min, cofactors B
or E failed to capture detectable levels of labeled
-tubulin. In contrast, mt-cpn added at t = 15 min was still able to
capture about half of the input
-tubulin radioactivity (the remainder may have irreversibly aggregated). Finally, in a
control experiment in which native tubulin dimer containing 35S-labeled
-tubulin was incubated with mt-cpn alone,
only a small proportion of the
subunit was captured, reflecting a modest amount of denaturation of the input heterodimer. These data demonstrate that, in the absence of the
subunit, the conformation of
-tubulin rapidly decays to
a form that is incapable of heterodimerization or interaction with cofactors, and that is recognized as nonnative by
mt-cpn. Since cofactor B alone does not interact with native tubulin (Fig. 5 A), the data in Fig. 5 C suggest that it binds to
-tubulin in a conformation intermediate between the
native state and the nonnative forms recognized by mt-cpn.
The action of cofactors B and E on native tubulin results
in the sequestration of -tubulin as FB
(Fig. 5 A), suggesting that cofactor E, like cofactor D, interacts with native tubulin. Because cofactor B on its own does not interact with native tubulin, it must acquire its bound
subunit
via FE
, which is unstable, but can be stabilized by chemical cross-linking (Fig. 4 F). FB
can also be formed when
cofactor D sequesters the
subunit and the remaining
subunit decays to a state recognizable by cofactor B. Since
a combination of cofactors B and E results in the formation of a stable FB
complex, this allowed us to investigate
the fate of the
subunit in the absence of its partner
-tubulin subunit. We found that incubation of native tubulin
dimers with cofactors B and E resulted in the destabilization of
subunits as assayed by their ability to be captured
by mt-cpn (Fig. 5 D). We conclude that both
and
subunits in the native tubulin heterodimer exist in metastable
conformations; if one subunit is removed by interaction with cofactor(s), the remaining free subunit rapidly decays
to a conformational state of lower energy that is incapable
of heterodimerization.
GTP Hydrolysis and Tubulin Folding
In the tubulin heterodimer, both and
subunits bind
one molecule of GTP, the latter exchangeably; GTP hydrolysis by
-tubulin is coupled to polymerization (Mitchison and Kirschner, 1986
). It has been reported that the
slowly hydrolyzable analogue GTP
S inhibits the production of native
-tubulin in in vitro translation cocktails
(Fontabla et al., 1993
; Paciucci, 1994
). We therefore tested
the ability of GTP
S to inhibit the folding of
- and
-tubulin using purified components. Both
- and
-tubulin post-c-cpn-mediated reactions were inhibited in the presence of
GTP
S, showing that GTP hydrolysis is indeed necessary
for cofactor-mediated
- and
-tubulin folding (Fig. 6 A).
Under these conditions, both
- and
-tubulin folding intermediates remained complexed with cofactors, appearing as identical doublets upon native gel electrophoresis. We also investigated the conversion to native heterodimer of
intermediates generated in the backreaction of native tubulin with cofactors D and E. We first formed tubulin/cofactor complexes in reactions containing cofactors D and
E, 10 µM GTP, and native tubulin heterodimer in which
either the
or
subunits were 35S-labeled. The addition
of cofactor C and unlabeled native tubulin to these intermediates results in the regeneration of native tubulin. This
reaction was blocked by the addition of a 100-fold molar excess (relative to GTP) of GTP
S, but not ATP
S (Fig. 6
B). Note that the same labeled doublet appears in the
products of both forward (i.e., c-cpn-mediated) and backward
- and
-tubulin folding reactions when these are
blocked by GTP
S (compare Fig. 6 A and 6 B). To see
which component(s) in this reaction hydrolyzes GTP, we
incubated cofactors and tubulin with
-32P-labeled GTP
either alone or in various combinations, and assayed for
conversion to
-[32P]GDP by TLC (Fig. 6 C). Neither native tubulin heterodimer nor cofactors D, E, or C alone
significantly hydrolyzed GTP. Upon coincubation, however, these components hydrolyzed GTP at a rate of at
least 0.4 min
1, comparable to the rate of ATP hydrolysis
by c-cpn (Melki et al., 1996
).
The minimal set of proteins necessary for GTP hydrolysis in these reactions consisted of tubulin and cofactors C
and D (Fig. 6 C). When native tubulin was preincubated
with a molar excess of cofactor D in this reaction so that
no free -tubulin subunits were present in cofactor-recognizable form (Fig. 5 C), GTP hydrolysis was unimpaired
upon addition of cofactor C (data not shown). Thus, the
GTP hydrolysis step in tubulin folding requires only
-tubulin and cofactors C and D. To see which of the proteins in
the fully constituted active complex binds and hydrolyzes
GTP, we incubated native tubulin with a molar excess of
cofactors C, D, and E in the presence of
-32P-labeled
GTP. Under these conditions, all tubulin subunits are in
cofactor complexes, and
-tubulin contains bound, unlabeled GTP, which it does not exchange or hydrolyze
throughout the post-c-cpn folding pathway (Fig. 4 E and
data not shown). Following UV irradiation, the only species containing labeled, cross-linked GTP was
-tubulin (Fig. 6 D), suggesting that the cofactors stimulate GTP hydrolysis by
-tubulin. It follows that the productive folding
of
- and
-tubulin requires the hydrolytic action of
-tubulin in the active complex.
Cofactor B Enhances the Folding of -Tubulin
We recently showed that the c-cpn-mediated folding of
-tubulin in vitro involves the action of four protein cofactors that we named A, C, D, and E (Tian et al., 1996
).
Here we describe the purification, cloning, and functional
characterization of a protein (cofactor B) that acts in the
-tubulin folding pathway (Fig. 1). This cofactor greatly
increases the yield of native
-tubulin in c-cpn-mediated
folding reactions containing cofactors C, D, and E (Fig. 4
C). Cofactor B acts by capturing quasinative (IQ) intermediates generated by c-cpn in an ATP-dependent reaction (Fig. 4 E and Tian et al., 1995b
); these intermediates (and
all subsequent intermediates in the
-tubulin folding pathway) contain nonexchangeably bound GTP, as does native
-tubulin. Reaction with either human recombinant or bovine cofactor B results in species with differing electrophoretic mobilities in our native gel assay (Fig. 4 A); this
shows that cofactor B forms a complex with
-tubulin
folding intermediates. However, the
-tubulin in these
species is not in its native conformation, since it will not
exchange into added native tubulin heterodimers and, unlike native tubulin, it is extremely sensitive to proteolysis
by subtilisin (Fig. 4 D). The interaction of cofactor B with
-tubulin folding intermediates is consistent with our observation that a null mutation of the yeast cofactor B homologue, ALF1, is lethal in combination with the
-tubulin mutation tub1-1.
Convergence and Symmetry of the - and
-Tubulin
Folding Pathways
A simple model for tubulin folding that incorporates our
data on the interaction of purified cofactors with native tubulin and chaperonin-generated intermediates is presented in Fig. 7. Quasinative - or
-tubulin folding intermediates generated via ATP-dependent interaction with
c-cpn are captured by cofactors B and E (in the case of
-tubulin) or A and D (in the case of
-tubulin; Tian et al.,
1996
), forming tubulin intermediate/cofactor complexes,
i.e., FB
or FE
(in the
-tubulin pathway) or FA
or FD
(in the
-tubulin pathway). The FB
and FA
complexes
act as reservoirs, capable of accepting or delivering their
target protein to cofactors E and D, respectively. FE
and
FD
interact with each other to form the species (FE
/
FD
) marked with an asterisk in Figs. 3, 4, and 5. Addition
of cofactor C (FC) generates the active entity (the
/
-supercomplex; boxed in Fig. 7), which hydrolyzes GTP and produces native tubulin. The FD
state is also populated from
the backreaction between native
-tubulin subunits and
cofactor D, as is FE
from a backreaction between native
-tubulin subunits and cofactor E.
Among the tubulin intermediate/cofactor complexes depicted in our model, FD, FA
, and FB
are sufficiently
stable to allow their biochemical isolation. The tubulin intermediates in these complexes are convertible to the native state only in the presence of GTP, tubulin itself, and
other essential cofactors, i.e., C and E (in the case of FD
)
or C, D, and E (in the case of FA
and FB
) (Fig. 4 E and
Tian et al., 1996
). The FE
and FE
/FD
complexes are unstable during column chromatography and immunoprecipitation; hence the evidence for the existence of these species is necessarily circumstantial, but nonetheless compelling.
We infer the existence of FE
from the following considerations: (a) since cofactor B can be omitted from productive
-tubulin folding reactions (albeit with a >90% loss of
efficiency) (Fig. 4 C), cofactor E must be capable of capturing some c-cpn-generated intermediates, as it is the
only
-tubulin binding cofactor present. (b) FE
can be stabilized and visualized in in vitro
-tubulin folding reactions after chemical cross-linking (Fig. 4 F). FE
interacts
with FD
to form the FE
/FD
complex (marked with an
asterisk in Figs. 4, 5 and 6) whose existence is in turn supported by the following data: (a) because of the high affinity of cofactor D for
-tubulin, in the presence of excess
tubulin heterodimer and the absence of cofactor C, cofactor D must be bound to
-tubulin. Thus, the species
marked with an asterisk generated in
-tubulin forward and backward reactions containing cofactors D, E, and tubulin must contain
-tubulin in association with cofactor
D. Moreover, this species is not formed in
-tubulin folding reactions in which native tubulin is omitted (Fig. 4 F),
demonstrating that it is FD
and not cofactor D alone that
interacts with FE
. (b) A species of the same mobility as
that seen in
-tubulin folding reactions appears in
-tubulin forward and reverse reactions (Figs. 5 A and 6 B, asterisk) and has been shown to contain cofactors D, E, and
-tubulin (Tian et al., 1996
). Since
-tubulin cannot be productively folded without added native tubulin and cofactor E, we conclude that this too is the FE
/FD
complex.
When cofactor C is added to the FE/FD
complex, GTP
is hydrolyzed and native tubulin is released. Only this
combination of proteins results in the maximal rate of
GTP hydrolysis and the production of native tubulin.
Thus, all five proteins appear to constitute the GTPase
(the
/
-supercomplex) that is active in tubulin folding.
Although our model depicts these five proteins as interacting physically in a single complex, we cannot rule out a
more elaborate scheme In which subsets of them act in
rapid succession.
As presented in our model, the - and
-tubulin folding
pathways are symmetrical. Cofactors A and B act as reservoirs for the sequestration of tubulin folding intermediates; in this capacity, they could serve as buffers that
would protect the cell from an unbalanced production of
- or
-tubulin subunits. This notion is consistent with the
observation that the otherwise toxic induced overexpression of
-tubulin in yeast can be rescued by a corresponding induced overexpression of the yeast homologue of cofactor A (Archer et al., 1995
). In addition, the in vitro
folding of
-tubulin is very inefficient in folding reactions
that lack cofactor B but that contain native tubulin heterodimer (which is necessary to provide the partner subunit in the
/
-supercomplex) (Fig. 4 C). Similarly, in
c-cpn-mediated
-tubulin folding reactions, if native tubulin is added together with cofactors (rather than at the end
[Tian et al., 1996
]), the yield of native product is extremely
low in the absence of cofactor A (Tian, G., and N.J. Cowan, unpublished observations). Thus, cofactors B and
A shift the reaction equilibria: cofactors E and D seem to
preferentially accept
- or
-tubulin intermediates from
FB
and FA
, respectively, rather than from native heterodimer, favoring de novo folding over the backreaction
involving refolding of native subunits.
GTP Hydrolysis and Cofactor Function
The final step in the generation of native tubulin heterodimer involves the hydrolysis of GTP (Figs. 6 and 7). In
our previous description of the pathway leading to correctly folded -tubulin, we found that native
-tubulin
folding could be supported by GDP (Tian et al., 1996
).
However, our c-cpn preparations contained some nucleoside diphosphate kinase activity: hence, in reactions containing ATP and GDP, there was a resulting generation of sufficient levels of GTP to drive the folding reaction. The
data shown in Fig. 6 demonstrate conclusively that GTP
hydrolysis is necessary for the release of native tubulin
from cofactor complexes, whether these are formed in the
forward reaction in which urea-denatured tubulin target
protein is presented to c-cpn, or in the backreaction, in
which native tubulin heterodimer is incubated with cofactors. The combination of tubulin heterodimer and cofactors C, D, and E hydrolyzes GTP at a rate of at least 0.4 min
1 (Fig. 6 C). The continuous cycling of tubulin
through cofactor complexes and the concomitant hydrolysis of GTP could serve to stabilize the tubulin subunits;
this is consistent with the role of the yeast homologues of
cofactors D and E in maintaining microtubule stability in
vivo (Stearns et al., 1990
; Hoyt et al., 1997
). The GTPase
formed by these five proteins might serve other cellular functions: e.g., it is possible that cofactors C, D, and E
could bind to the GTP cap on polymerizing microtubules,
stimulating the hydrolysis of GTP by
-tubulin, and thus
influencing microtubule dynamics. It is therefore important to confirm that, in the
/
-supercomplex, one or more
of the cofactors is stimulating GTP hydrolysis by
-tubulin, as opposed to the tubulin-dependent stimulation of GTP hydrolysis by a cofactor. Current evidence suggests
that this is the case (Fig. 6 D).
Tubulin Cofactor Function In Vitro and In Vivo
When native tubulin dimer is incubated with excess cofactor D, the -tubulin subunit is efficiently sequestered as
FD
, disrupting the
/
tubulin heterodimer (Fig. 5). Under such conditions, the
-tubulin subunit rapidly loses its
ability to reassociate with
-tubulin. In the absence of
other cofactors, free
-tubulin simply "disappears," presumably because of aggregation via exposed hydrophobic surfaces. We confirmed this conclusion by showing that
free
-tubulin quickly becomes capturable by mitochondrial chaperonin, which binds nonnative
-tubulin states
(Fig. 5 and Tian et al., 1995a
). We conclude that, in the native heterodimer,
-tubulin exists in an activated state.
The converse experiment involving sequestration of
subunits by the action of cofactors B and E shows that
-tubulin is also in a metastable conformational state in the heterodimer (Fig. 5 D). The interaction between
and
subunits is therefore critical for the maintenance of functional tubulin heterodimers. This explains why it has never
proved possible to biochemically isolate
- or
-tubulins in
native form free from their partner subunits.
There are known homologues of cofactors A, B, D, and
E (but not C) in S. cerevisiae (Fig. 2 and Stearns et al., 1990;
Archer et al., 1995
; Hoyt et al., 1990
, 1997
). Deletion of
genes encoding these homologues results in microtubule
phenotypes such as supersensitivity to antimicrotubule
drugs, temperature sensitivity, and chromosomal instability. These yeast genes are not essential for viability, even
though they are single copy. Therefore, since microtubules are indispensable structures, yeast tubulins must be able to
reach the native state without the aid of the full complement of cofactors, although there is persuasive evidence
that c-cpn is required for tubulin folding in yeast (Chen et
al., 1994
; Ursic et al., 1994
). In that case, the cofactors
might be needed only to stabilize native yeast tubulin subunits and to prevent the accumulation of free
subunits
that would otherwise be lethal (Archer et al., 1995
); this
would be consistent with their mutant phenotypes. On the
other hand, the participation of cofactors C, D, and E is
absolutely required for the proper folding of mammalian
- and
-tubulin in vitro. The differences in cofactor requirements in vitro and in vivo could reflect either the existence of alternative in vivo folding pathways, a critical
difference in the folding requirements of yeast and mammalian tubulins, or both.
The complicated and energy-consuming post-c-cpn reactions involving cofactors may have evolved because native tubulin exists as a tight heterodimer. In the case of an
unrelated heterodimeric protein, bacterial - and
-luciferase,
if both subunits are allowed to refold from urea separately,
they attain stable structures that are unable to dimerize;
however, if they are allowed to refold together, they
dimerize into an inactive heterodimer, which then isomerizes to the active form of the enzyme (Baldwin et al., 1993
; Clark et al., 1993
). The luciferase heterodimer therefore
appears to act as a kinetic trap on the folding pathways of
the individual subunits; the subunits of the dimer are not
in their ground state conformation. This contradicts the
widely held tenet that all the information required to
reach the native state is contained in a protein's amino
acid sequence: like the propeptide in some proteases,
which can act in cis or in trans to promote correct folding,
luciferase is an example of one protein that affects the final conformational state of another. We have shown here that tubulin is another such example: the instability of isolated tubulin subunits implies that the attainment and
maintenance of the native conformation of
-tubulin depends on information derived from the
-tubulin subunit
and vice versa. We provide strong evidence that the function of the cofactors that act in the post-c-cpn tubulin folding pathways is to bring the
and
subunits together in a
supercomplex so that they can achieve this native (i.e., activated) conformation.
Received for publication 16 May 1997 and in revised form 1 July 1997.
Please address all correspondence to Nicholas J. Cowan, Department of Biochemistry, New York University Medical Center, 550 First Avenue, New York, NY 10016. Tel.: (212) 263-5809. Fax: (212) 263-8166.We thank R. Beavis for mass spectrophotometric analyses, and E. Hammel and N. Kallenbach for stimulating discussions.
This work was supported by grants (to N.J. Cowan and T. Stearns) and a National Research Service Award (to G. Tian) from the National Institutes of Health, and by grants from the Geconcerteerde Onderzoeksactie and the Belgian National Fund for Scientific Research (to C. Ampe). C. Ampe is a Research Associate of the National Fund for Scientific Research.
c-cpn, cytosolic chaperonin; mt-cpn, mitochondrial chaperonin.
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