(Received for publication, September 27, 1996, and in revised form, January 8, 1997)
From the Department of Biochemistry and Molecular Biology, Indiana University School of Medicine, Indianapolis, Indiana 46202-5122 and the § Department of Biological Sciences, University at Albany/State University of New York, Albany, New York 12222
Protein phosphatase 2A (PP2A) is a major cellular
serine/threonine protein phosphatase, present in the cell in a variety
of heterotrimeric forms that differ in their associated regulatory B-subunit. Cloning of the mammalian B subunit has allowed the identification of a highly homologous Saccharomyces
cerevisiae gene, RTS1. Disruption of the gene results
in a temperature-sensitive growth defect that can be suppressed by
expression of rabbit B
or B
isoforms. The B
subunit is much
more effective in restoring normal growth at 37 °C than B
.
Immunoprecipitated Rts1p was found associated with type 2A-specific
protein phosphatase activity that is sensitive to 2 nM
okadaic acid, but not to 100 nM phosphatase inhibitor-2,
and to be phosphorylated in vivo. However, overexpression of RTS1 was unable to suppress the cold sensitivity,
defective cytokinesis, and abnormal cell morphology resulting from
defects in the CDC55 gene, which encodes the yeast homolog
of a different B subunit of another form of 2A phosphatase,
PP2A1. These results indicate that Rts1p is a yeast homolog
of the mammalian B
subunit and that the various regulatory B-subunits
of PP2A are not functionally redundant but direct the enzyme to
distinct cellular functions.
Serine/threonine protein phosphatases are ubiquitous enzymes that participate in many diverse biological processes. The enzymes are broadly classified as type 1 (PP1),1 type 2A (PP2A), 2B (PP2B), and 2C (PP2C), based on substrate specificity, catalytic properties, metal ion requirements, and sensitivity to various inhibitors (1-3).
The 2A protein phosphatases are heterotrimeric complexes consisting of a core dimer, composed of a catalytic (C2) and a regulatory (A) subunit, associated with one of the variable regulatory B-subunits (4-6). Two mammalian C2 (7-9) subunit isoforms have been identified which are highly conserved in the living kingdom. Three genes, PPH21, PPH22, and PPH3, encoding homologs of mammalian C2, have also been found in the budding yeast, Saccharomyces cerevisiae (10-12). Disruption of any one of these genes has no significant effect, whereas disruption of PPH21 and PPH22 causes a severe growth defect (12). However, the double mutants can survive, provided that the PPH3 gene is intact (10, 11). The PP2A catalytic subunits have been shown to be essential for normal cell cycle progression in S. cerevisiae and for the organization of the actin cytoskeleton (13), as well as for the maintenance of normal cellular morphology (11). Other reports implicate the phosphatase in regulation of glycogen metabolism in yeast (14). The regulatory A subunit has been identified in cells of diverse origin. Two isoforms of the A subunits have been characterized in mammalian cells (15) and one in Drosophila (16). In S. cerevisiae, the TPD3 gene encodes a homolog of the mammalian subunit (17). Mutations in this gene greatly diminish cell growth at 14 and 37 °C. At the low temperature, cytokinesis appears to be affected, whereas at 37 °C, transcription by RNA polymerase III is impaired.
In contrast to the relative paucity of the A and C2 subunit isoforms,
the B-subunits2 of PP2A show great
diversity. Three families, B, B, and B", have been found associated
with the PP2A1, PP2A0, and PCSM,
respectively (18-21). Three isoforms of B subunit have been identified
in mammals (21-23), and cDNAs encoding some 13 isoforms of the B
subunit have been isolated recently (24-26). Cloning of the cDNA
encoding the B" polypeptide suggests the existence of two alternatively spliced variants (27). Even though all of the B-subunits associate with
the same dimeric C2·A core, no obvious homology has been found among
the B, B
, and B" forms. The abundance of the diverse B-subunit
isoforms and their association with the relatively invariant set of C2
and A subunits has led to the suggestion that the B-subunits are
responsible for targeting specific phosphatase 2A holoenzymes to
distinct cellular locales and/or for conferring specificity toward
appropriate substrates (4, 5, 28).
Recent cloning of the mammalian B subunits (24) allowed identification
in GenBank of a highly homologous, 53-56% identical, yeast gene,
RTS1 (accession number U06630[GenBank]), whose function is not
clearly understood. This gene is unrelated to CDC55, the yeast homolog of the B subunit, whose disruption results in a cold-sensitive phenotype and morphologically aberrant cells (22). RTS1 was isolated independently by two laboratories, using
different screening approaches. Evangelista et al. (29)
isolated RTS1 as a multicopy suppressor of a ROX3
gene mutation. The ROX3 gene encodes an essential nuclear
protein that functions in the global stress response pathway,
controlling the level of CYC7 transcription. Shu and
Hallberg (30) have isolated the same gene as a high copy suppressor of
hsp60-ts mutant alleles. Disruption of RTS1 results in a temperature-sensitive phenotype and reduction of the
mRNA levels of the mitochondrial chaperones Hsp60p, Cpn10p, and
Mge1p at the restrictive temperature (30), whereas CYC7 mRNA levels were increased (29).
In this paper, we present evidence that RTS1 encodes the
S. cerevisiae homolog of the mammalian B subunit of
PP2A0. The mammalian B
and to a lesser extent the
isoform were able to suppress the temperature-sensitive phenotype
associated with the RTS1 disruption. Moreover, Rts1p was
found associated with type 2A-specific protein phosphatase activity and
was phosphorylated in vivo. However, the RTS1
gene could not restore normal growth or morphology to a
CDC55-disrupted strain. These observations indicate that
S. cerevisiae, like mammalian cells, possesses multiple
forms of B-subunit. Most importantly, our results demonstrate that the various B-subunits of PP2A are not functionally interchangeable but
direct the enzyme to distinct cellular functions.
The following yeast strains were used: RZ53-6
(MAT ura3-52 trp1-289 ade1 leu2-3, 112);
RZ53-6
rts1, identical to RZ53-6, except for the
replacement of the wild-type RTS1 allele with the rts1::URA3 disruption (29); and
AHY86 (MATa leu2-3, 112 ura3-52 cdc55::URA3; 22). The cells were grown either
in YPD (1% yeast extract, 2% Bacto-peptone, 2% glucose) or in
synthetic complete (SC) medium (31) lacking the selective nutrient.
Solid media contained 2% agar.
Oligonucleotide primers were synthesized by the Biochemistry Biotechnology Facility at Indiana University School of Medicine on an Applied Biosystems synthesizer model 394. Restriction endonucleases were obtained from New England BioLabs or from Life Technologies, Inc. All recombinant DNA manipulations followed standard procedures (32).
Plasmid pDB20, containing the URA3-selective marker, was obtained from Kelly Tatchell (Louisiana State University, Shreveport). It was derived from the Escherichia coli-yeast shuttle vector YEp352 (33) by insertion of a 2-kb alcohol dehydrogenase (ADH1) promoter/terminator fragment (34) into the multiple cloning site of the vector. To construct the pDB21 vector with the LEU2-selective marker, the ADH1 promoter/terminator region was excised from pDB20 with EcoRI and PstI and ligated into the Yeplac181 vector (35). In both pDB20 and pDB21, a HindIII site was eliminated from the multiple cloning region such that the HindIII site between the ADH1 promoter and terminator was unique.
For expression in yeast, the complete B subunit coding sequence,
tagged at the NH2 terminus with a hexahistidine sequence (His-tag) was excised with NcoI and BamHI as a
1.74-kb fragment from plasmid B
·pET-15b (24), blunted, and
subcloned into the blunted unique HindIII site of pDB21 to
yield pDB21(His-B
). The complete B
subunit coding sequence was
assembled by ligating the 1,481-bp AflIII-EcoRI
fragment from clone BR6-2 with the 533-bp EcoRI-PvuII fragment from clone BR6-1 (24) at the
EcoRI site. The resulting 2,014-bp fragment was subcloned
into the blunt ended BamHI site of pTZ18U (U. S.
Biochemical Corp.) to yield pTZ18U(B
). The hemagglutinin epitope
tag YPYDVPDYA (HA-tag) was introduced between the first and second
codon of the open reading frame by polymerase chain reaction
amplification of a 300-bp NdeI-HindIII fragment.
The resulting fragment was ligated to the
HindIII-SmaI 1,570-bp fragment of B
cDNA
excised from pTZ18U(B
). The 1,870-bp tagged B
DNA was blunted
and inserted into the blunt ended HindIII site of pDB21.
This procedure yielded the plasmid pDB21(HA-B
).
Plasmids pDB20(HA-RTS1) and pDB21(HA-RTS1) were constructed by inserting the HA-epitope tag between the first and second codon of the open reading frame of RTS1 using YEp195RTS1 (29) as template for polymerase chain reaction amplification of a 672-bp NdeI-SpeI fragment. This fragment was then ligated at the SpeI site of the remaining 2.07-kb SpeI-BamHI portion of RTS1 excised from YEp195RTS1. The resulting 2.75-kb fragment was subcloned into the NdeI-BamHI sites of the pET-15b vector (Novagen). The 2.45-kb NdeI-Bst BI fragment obtained by partial digestion of pET15-b·RTS1 was then blunt ended and subcloned into the blunted HindIII site of pDB20 or pDB21.
Plasmids pDB20(HA-CDC55) and pDB21(HA-CDC55) were constructed as
follows. The plasmid pTSV31(CDC55) containing the CDC55 gene on a 6-kb SacI-HpaI fragment (22) was used as
template for polymerase chain reaction to amplify a 849-bp fragment
containing an HA-tag at the 5 end of the coding sequence, flanked by
an NdeI site. The 849-bp NdeI-BamHI
fragment was joined at the BamHI site of the remaining
2.28-kb portion of the CDC55 sequence in which an XhoI linker had been added at the 3
end. The resulting
NdeI-XhoI fragment was then ligated into the
corresponding sites of pET15-b vector. The 2.43-kb
NdeI-Bpu1102 I fragment, containing the HA-tag followed by the complete coding sequence and ~800 bp of
3
-untranslated sequence, was blunt ended and ligated into the
filled-in HindIII site of pDB20 or pDB21. All polymerase
chain reaction products were verified by dideoxynucleotide sequencing
(36).
Yeast cells, transformed by the
procedure of Ito et al. (37) or of Eble (38), were grown
with vigorous shaking at 30 °C in liquid synthetic medium. The
cultures were harvested at late logarithmic or early stationary phase,
washed with water, and pelleted in tared microcentrifuge tubes. The
weight of packed cells was determined. The cells were either used
immediately or stored frozen at 80 °C. All subsequent steps were
carried out at 0 °C.
The cells were resuspended in 3 volumes of homogenization buffer
consisting of 50 mM Tris-HCl, pH 7.5, 1 mM
EDTA, 1 mM dithiothreitol, and 0.1 M NaCl, and
containing a mixture of protease inhibitors (1 mM
phenylmethylsulfonyl fluoride; 1 mM
N-p-tosyl-L-lysine
chloromethyl ketone; 0.1 mM benzamidine; 10 µg of
leupeptin/ml; and aprotinin, antipain, and pepstatin A, at 5 µg/ml
each). Where indicated, instead of NaCl, the buffer contained 50 mM NaF and 10 mM potassium phosphate.
Chilled acid-washed glass beads (0.45-0.50-mm diameter) were added up
to the meniscus level, and the suspension was vortexed at a maximum
speed in twelve 30-s bursts. The crude extract was collected by
centrifugation for 2 min at 1,500 × g followed by centrifugation at 18,000 × g for 10 min at 4 °C.
The supernatant (fraction S T) was removed, and the pellet was
extracted with the same buffer containing 1% (v/v) Triton X-100. The
suspension was left on ice for 15 min and then centrifuged at
18,000 × g for 10 min at 4 °C. The supernatant was
collected (fraction S + T), and the pellet was suspended in
homogenization buffer without Triton X-100 (fraction P). Aliquots of
all fractions were mixed with 5 × SDS sample buffer at a 4:1
ratio (1 × sample buffer is 12.5 mM Tris-HCl, pH 6.8, 0.5% SDS, 1%
-mercaptoethanol, 0.002% bromphenol blue, 10%
glycerol) and analyzed by SDS-PAGE.
Approximately 50 µg of protein of the S T fraction and the
corresponding volumes of the S + T and pellet fractions were resolved
by SDS-PAGE and transferred to nitrocellulose as described previously
(39). After blocking, the filters were incubated for 1-2 h at room
temperature with the appropriate antibodies. The anti-HA epitope 12CA5
monoclonal antibody was from Babco Laboratories. Antibodies against
recombinant B
subunit were raised in rabbits and affinity purified.
The nitrocellulose membranes were incubated with
125I-protein A (ICN), and the filters were subjected to
autoradiography using Kodak X-Omat film with two DuPont Quanta III
screens at
80 °C. Protein concentration was measured on the S
T supernatant fraction by the method of Bradford (40), using bovine
serum albumin as a standard.
Yeast cell extracts were
prepared as above except that Triton X-100 was added (1% final
concentration) after disruption of the cells with glass beads. The
Triton-soluble fraction (200 µl) was incubated with 2-3 µl of
12CA5 antibody on ice for 60 min. The antigen-antibody complex was
harvested by the addition of 60-100 µl of a 50% protein G-agarose
slurry (Life Technologies, Inc.). After gentle shaking for 60 min at
4 °C, the samples were centrifuged for 10 s at 9,000 × g, the supernatant (unbound fraction) was removed, and the
protein G-agarose was washed five times with 500 µl of 50 mM Tris-HCl, pH 7.5, 0.1 mM EDTA, 200 mM NaCl, and protease inhibitors. The beads were
resuspended in 40 µl of 50 mM Tris-HCl, pH 7.5, 0.1 mM EDTA, 0.2% -mercaptoethanol, and used for
phosphatase assays and for Western analyses.
RZ53-6rts1 cells transformed with the
plasmid pDB21(HA-RTS1) were grown in liquid synthetic medium without
leucine to a density of 7.5 × 107 cells/ml. The cells
were then collected by centrifugation and suspended to a density of
6 × 107cells/ml in synthetic low phosphate medium
(41). Two 25-ml cultures were incubated with shaking for 3 h at
30 °C to deplete intracellular phosphate pools, after which time one
flask received 4 mCi of 32Pi (DuPont NEN), and
the incubation continued for 2 h. The labeled and unlabeled cells
were processed as above for preparation of cell extracts and
immunoprecipitation of Rts1p in the presence of 50 mM NaF
and 10 mM potassium phosphate. The immunoprecipitated material was resolved by 6% SDS-PAGE and transferred to polyvinylidene difluoride membrane. The membrane was washed with phosphate-buffered saline and blocked for 1 h with Boehringer Mannheim blocking
solution. After blocking, the membrane was air dried followed by
incubation for 1 h with anti-[HA]-antibody-peroxidase conjugate
(kindly provided by Sophie J. Boguslawski, Boehringer Mannheim) and
detection of HA-tagged Rts1p by the chromogenic substrate BM Teton
(Boehringer Mannheim), according to the manufacturer's
specifications.
Phosphatase activity was measured
using 32P-labeled phosphorylase a (1 mg/ml;
specific radioactivity 3,000-4,000 cpm/pmol) as a substrate (42). The
assays were carried out in a 50-µl reaction containing 50 mM Tris-HCl, pH 7.5, 0.1 mM EDTA, 0.2%
-mercaptoethanol, and 5 mM caffeine. Ten µl of
immunoprecipitated sample or 10 µl of a 100-fold diluted extract was
preincubated for 5 min at 30 °C with 100 nM inhibitor-2
or 2 nM okadaic acid, before the addition of the substrate.
Control reactions contained the vehicle buffers. The reactions were
carried out at 30 °C for 10 min and stopped by the addition of
trichloroacetic acid to a final concentration of 16%. The released
32Pi was quantitated as described previously
(42). To ensure linearity of the reaction, phosphate release was
limited to less than 20%. One unit of phosphatase activity is defined
as the amount of enzyme that releases 1 nmol of phosphate/min from the
phosphorylase a substrate.
Yeast cells were grown in selective media, and aliquots of stationary phase cultures were applied to glass slides. The cells were examined using Nomarski optics with 400-fold magnification on a Nikon Microphot-FXA microscope, and the images were captured using a video camera connected to an Apple Macintosh computer.
The mammalian B and the yeast Rts1p proteins have
~55% identity and 66% similarity if conservative replacements are
taken into account. Disruption of the RTS1 gene results in
temperature sensitivity for growth above 37 °C. The homology between
the mammalian B
subunit and the yeast gene product prompted us to
investigate whether the B
proteins could functionally substitute for
Rts1p and rescue the temperature-sensitive growth defect of an
rts1 deletion mutant. When rabbit B
and B
subunit
cDNAs were introduced into yeast cells, the parent strain RZ53-6
grew well at 30 °C, regardless of the isoform used (Fig.
1), whereas overexpression of RTS1 led to
somewhat slower growth. The growth at 30 °C of a mutant
rts1 strain transformed with RTS1, B
, or
B
subunit DNA was similar to that of the parent RZ53-6 strain, but
the mutant strain transformed with vector alone grew less well even at
the permissive temperature. Approximate doubling times in liquid SC medium without leucine were 5.1 h for the mutant cells transformed with vector only and 3.4, 3.7, and 3.3 h for cells transformed with B
, B
, and RTS1, respectively. The generation
time of an isogenic wild-type strain was 3.2 h.
The difference in the ability of the individual subunits to rescue the
mutant strain became readily apparent when transformed cells were
incubated at 37 °C (Fig. 1). Under these conditions, pDB21(His-B) and pDB21(HA-RTS1) restored normal growth to the mutant, whereas pDB21(HA-B
) was less effective. The approximate generation times at 37 °C for the cells transformed with vector only, B
, B
, and RTS1 were 32, 9.0, 23, and 8.0 h, respectively, compared with 6.7 h for the parent strain. These
results indicate that the cDNAs encoding the mammalian B
subunits
can functionally replace the yeast RTS1 gene even though the
and
isoforms are not equally efficient. Analysis of cell
morphology revealed that, unlike B
, B
subunit expression
resulted in abnormal multiple and elongated cells (Fig.
2), a phenotype somewhat reminiscent of that observed in
cdc55 mutants (22).
The ability of the mammalian B cDNAs to rescue the temperature
sensitivity of the yeast mutant was paralleled by the level of
expression of the proteins. As shown in Fig. 3, B
,
B
, and Rts1p polypeptides could be detected by Western blotting of
yeast extracts. Although the majority of each species was found in the insoluble fraction, a significant amount was present in the Triton X-100-soluble fraction. The apparent molecular weight of the proteins on SDS-PAGE, 55,000 for His-B
and 90-95,000 for HA-Rts1p, deviates from the predicted 60,000 and 86,000, respectively, suggestive either
of post-translational modifications or abnormal mobility caused by
intrinsic properties of the polypeptides. The observation that B
expressed in E. coli (24) migrates with an apparent molecular weight similar to that seen in yeast implies that the abnormal mobility of this isoform is probably not due to
phosphorylation. Western blots of extracts prepared from
rts1 mutants transformed with pDB21(HA-RTS1) revealed the
presence of two or three discrete bands between 92 and 95 kDa (Fig.
3C and Fig. 4). When cell extracts were
prepared in the presence of the serine/threonine phosphatase inhibitors
NaF and potassium phosphate, an upward shift in the mobility of the
lower band of the Rts1p was observed (Fig. 4A). Consistent
with this initial observation, 32P labeling of yeast cells
and immunoprecipitation of Rts1p showed that Rts1p is phosphorylated
in vivo (Fig. 4C). Three bands were detected by
immunoblotting, and all three were labeled with 32P. The
slowest migrating species contained the highest ratio of 32P to protein, suggesting that it was the most heavily
phosphorylated.
Rts1p Is Associated with PP2A Activity
Since the mammalian B
subunit isoforms were able to rescue the rts1 phenotype, an
important question was whether Rts1p was associated with PP2A activity.
Measurements of phosphorylase phosphatase activity in
immunoprecipitates from extracts of yeast expressing HA-Rts1p indicated
the presence of phosphatase activity that was insensitive to 100 nM inhibitor-2, a protein phosphatase 1-specific inhibitor,
but almost completely inhibited by 2 nM okadaic acid (Table I), a concentration that preferentially inhibits
PP2A. Analysis by immunoblotting indicated that the monoclonal antibody 12CA5 almost quantitatively immunoprecipitated the HA-Rts1p (Fig. 5). Determination of phosphatase activity in extracts
from rts1 mutant cells indicated that ~30% of the
activity could be inhibited by 2 nM okadaic acid (data not
shown). However, we do not know what proportion of this activity can
associate with the yeast B
subunit homolog.
|
Rts1p Expressed in cdc55 Strains Does Not Suppress the Mutant Phenotype
Cdc55p is highly homologous to the mammalian B subunit of PP2A1 (22). Disruption of CDC55 resulted in a cold-sensitive growth phenotype and abnormal multiple elongated cells at the restrictive temperature. Since the various B-subunits of PP2A associate with the same C2·A core, we examined whether Cdc55p and Rts1p could play interchangeable roles.
Attempts to transform RZ53-6 and rts1 mutant strains with the pDB21(HA-CDC55) plasmid did not yield transformants. These strains would not tolerate the presence of a high copy CDC55 plasmid, suggesting that high level of expression of Cdc55p might cause lethality in these strains. However, we do not believe this to be the case. More likely, the inability to transform these strains with pDB21(HA-CDC55) is due to the presence of the ade1 allele. Other yeast strains, such as F808 and DBY745, which harbor the same ade1 allele, could not be transformed by the multicopy CDC55 plasmid. In contrast, two unrelated strains that did not carry the ade1 allele, YNN27 and JC482, generated large numbers of transformed colonies with pDB21(HA-CDC55). Thus, it appears as though the ade1 mutation and the high copy CDC55 plasmid are unable to coexist for reasons that at this time are not understood. Perhaps high levels of Cdc55p and accumulation of the adenine precursor, phosphoribosylaminoimidazole carboxylate, combine to have a toxic effect on the cell.
The expression of Cdc55p and Rts1p in the cdc55 disruption
mutant transformed with pDB20(HA-CDC55) or pDB20(HA-RTS1) was then investigated. Western blot analysis revealed that both HA-Cdc55p and
HA-Rts1p were expressed in comparable amounts (Fig. 6).
Cells overexpressing Cdc55p or Rts1p were grown at the permissive
30 °C temperature and then transferred to 14 °C. After 3 days,
the cells were examined microscopically. The results demonstrated that
although pDB20(HA-CDC55) restored normal morphology to the mutant cells (Fig. 7), pDB20(HA-RTS1) had no effect on the
cdc55 phenotype. Therefore, even though both Rts1p and
Cdc55p associate with the same C2·A complex, they must be involved in
different functions in the cell and cannot substitute for one
another.
Several lines of evidence identify the RTS1 gene
product as the yeast homolog of the B subunit of PP2A0.
First, the mammalian B
subunit proteins and Rts1p show a high level of
amino acid homology (24). Second, two mammalian B
subunit isoforms are able to rescue the temperature-sensitive growth defect of the rts1 deletion strain (Fig. 1). Third, Rts1p
coimmunoprecipitates with protein phosphatase activity that is
insensitive to the PP1-specific inhibitor I-2 but is almost completely
inhibited by okadaic acid (Fig. 5 and Table I) at a concentration (2 nM) ineffective against PP1.
The changes in the proportion of the slower mobility form of Rts1p,
observed when cell extracts are prepared in the presence of protein
phosphatase inhibitors, and the metabolic 32P labeling
(Fig. 4) clearly indicate that Rts1p is phosphorylated in
vivo. The presence of at least three phosphorylated species of
different electrophoretic mobility suggests that the protein is
multiply phosphorylated at a minimum of three sites. Whereas the
catalytic subunit of PP2A is known to be modified directly by
phosphorylation (43) and by carboxyl methylation (44, 45), the present
paper provides evidence that the yeast B subunit undergoes
post-translational modification. Similar conclusions were recently
reached for the mammalian isoforms (46). We do not know at this time
the functional significance of the phosphorylation. However, control of
activity by phosphorylation of regulatory subunits is well established
for PP1 (4, 47, 48). Possibly a similar mechanism operates to control
PP2A0 activity and/or subcellular localization.
Expression of mammalian B and B
subunits rescues the growth
defect of the rts1 mutant at both 30 and 37 °C. B
restores growth completely, up to the wild-type level, whereas B
only provides partial complementation. It is unlikely that this is the
result of a different degree of expression of B
protein, since
Western analysis indicated that the polypeptide is present at a level
comparable to that of Rts1p (Fig. 3). These results suggest that the
two mammalian B
subunit isoforms may perform somewhat different
cellular roles. The two isoforms are ~70% homologous; and
interestingly the B
, but not the B
, polypeptide contains in its
COOH-terminal region a putative bipartite nuclear localization signal
that could direct the protein to the nucleus (49, 50). Indeed, recently
it has been reported (46) that the mammalian B
but not
can
localize to the nucleus. Rts1p has been shown to reside in the
cytoplasm (30). The potential nuclear localization of the B
, which
would increase PP2A activity in the nucleus, may also be responsible
for the abnormal morphology of the cells overexpressing the protein
(Fig. 2), either by affecting nuclear functions or by depleting other
pools of the enzyme.
A clearer functional distinction exists between Cdc55p, the B subunit
of PP2A1, and Rts1p. Overexpression of Rts1p cannot suppress the cold-sensitive phenotype of a cdc55 mutant. The
cells grown at 14 °C remain defective in septation and cytokinesis
(Fig. 7). Therefore, in keeping with the idea of spatial or functional targeting, the B and B subunits of PP2A must channel the same phosphatase toward different cellular substrates or locales.
Furthermore, the somewhat slower growth observed in wild-type RZ53-6
cells overexpressing RTS1 may be suggestive of enrichment of
the PP2A holoenzyme containing Rts1p at the expense of other essential pools.
Both the mammalian and yeast PP2A may be involved in cell cycle
control. Carboxyl methylation of the catalytic subunit of PP2A has been
reported to correlate with cell cycle progression (51), and the
activity of a microtubule-associated PP2A fluctuates as the cells
traverse the cell cycle (28). The B subunit of Drosophila
PP2A was shown to be required for correct chromatid migration in the
anaphase (52). Mutations in both the yeast catalytic subunit gene (13)
and B subunit gene (22) result in cell cycle defects, although the two
display dissimilar morphological aberrations: multiple elongated buds
in the cdc55 mutant at the restrictive temperature and
small, deformed buds in the pph21 mutant. The
rts1 strain cells at either 30 or 37 °C appear to be
significantly enlarged, again indicating a distinct function of the B
subunit.
Mutations in the CDC55 and RTS1 genes result in phenotypes similar to those displayed by A subunit defects. Mutations in the yeast TPD3 gene, encoding the regulatory A subunit of PP2A, result in both cold- and temperature-sensitive phenotypes (17). At 13 °C, the cells resemble cdc55 mutants in that they show elongated buds, consistent with defective cytokinesis. The same mutation does not allow growth at 37 °C, a phenotype analogous to that of the rts1 mutant. The ability of one mutation to confer a dual phenotype is consistent with the regulatory A subunit being a component of different phosphatase holoenzymes containing either the Cdc55p or the Rts1p.
Several lines of evidence point toward an involvement of RTS1 in modulating cellular responses to stress conditions. First, overexpression of RTS1 suppresses a rox3 temperature mutation (29) and several hsp60-ts alleles (30), both genes known to respond to stress. Hsp60p resides in mitochondria, and Rox3p is localized in the nucleus but affects the level of expression of a mitochondrially located protein, iso-2-cytochrome c. The fact that RTS1 is required for expression of the mitochondrial chaperonins Cpn10p and Mge1p (30), which are defective in the hsp60-ts strains, suggests that this protein phosphatase subunit plays a role in regulating the expression of genes whose products are destined for mitochondria and/or involved in stress responses. Supporting this notion is the observation that, although RTS1 is not an essential gene, its disruption not only impairs growth at elevated temperature but also decreases utilization of nonfermentable carbon sources, a feature consistent with impaired mitochondrial function.
Second, disruption of the RTS1 gene itself confers temperature sensitivity and results in elevated expression of the CYC7 gene in response to osmotic stress and heat shock (29). These responses may occur through a common stress response element (53). The osmotic stress response is mediated, at least in part, by the activation of the Hog1p protein kinase pathway (54-56). Thus, we infer that the protein kinases and PP2A0 may share common substrates or that they may be targets for each other. Other protein phosphatases, PTP2 and PP2C, have also been implicated in the control of the osmosensing mitogen-activated protein kinase pathways in S. cerevisiae (56) and in Schizosaccharomyces pombe (57)
In conclusion, the present study demonstrates that the RTS1
gene encodes the S. cerevisiae homolog of the mammalian B
subunits of PP2A0 and that the Rts1p is phosphorylated
in vivo. Mammalian B
and, to a lesser extent B
, are
able to complement the temperature-sensitive defect of rts1,
but RTS1 cannot replace CDC55, the yeast homolog of the B subunit of PP2A1. Thus, the B-subunits are not
redundant but confer functional specificity to the various forms of
PP2A.