(Received for publication, July 15, 1996, and in revised form, October 8, 1996)
From the Department of Pharmacology, University of
California at San Diego, La Jolla, California 92093-0636 and
§ CNRS, Unité de Recherche Associée 1455, Institut Fédératif de Recherche Jean Roche,
Université de la Méditerranée, Faculté de
Médecine Secteur Nord, 13916 Marseille Cedex 20, France
Fasciculin, a selective peptidic inhibitor of
acetylcholinesterase, is a member of the three-fingered peptide toxin
superfamily isolated from snake venoms. The availability of a crystal
structure of a fasciculin 2 (Fas2)-acetylcholinesterase complex affords an opportunity to examine in detail the interaction of this toxin with
its target site. To this end, we constructed a synthetic fasciculin
gene with an appropriate leader peptide for expression and secretion
from mammalian cells. Recombinant wild-type Fas2, expressed and
amplified in Chinese hamster ovary cells, was purified to homogeneity
and found to be identical in composition and biological activities to
the venom-derived toxin. Sixteen mutations at positions where the
crystal structure of the complex indicates a significant interfacial
contact point or determinant of conformation were generated. Two
mutants of loop I, T8A/T9A and R11Q, ten mutants of the longest loop
II, R24T, K25L, R27W, R28D, H29D, Pro30, P31R, K32G,
M33A, and V34A/L35A, and two mutants of loop III, D45K and K51S, were
expressed transiently in human embryonic kidney cells. Inhibitory
potencies of the Fas2 mutants toward mouse AChE were established, based
on titration of the mutants with a polyclonal anti-Fas2 serum. The
Arg27, Pro30, and Pro31 mutants
each lost two or more orders of magnitude in Fas2 activity, suggesting
that this subset of three residues, at the tip of loop II, dominates
the loop conformation and interaction of Fas2 with the enzyme. The
Arg24, Lys32, and Met33 mutants
lost about one order of magnitude, suggesting that these residues make
moderate contributions to the strength of the complex, whereas the
Lys25, Arg28,
Val34-Leu35, Asp45, and
Lys51 mutants appeared as active as Fas2. The
Thr8-Thr9, Arg11, and
His29 mutants showed greater ratios of inhibitory activity
to immunochemical titer than Fas2. This may reflect immunodominant
determinants in these regions or intramolecular rearrangements in
conformation that enhance the interaction. Of the many Fas2 residues
that lie at the interface with acetylcholinesterase, only a few appear to provide substantial energetic contributions to the high affinity of
the complex.
Fasciculins, selective inhibitors of acetylcholinesterase
(AChE),1 but not butyrylcholinesterase, are
7-kDa peptides with four disulfide bridges isolated from mamba venoms.
They belong to the family of three-fingered snake toxins that includes
the selective nicotinic receptor blockers, -neurotoxins (1-3) and
-neurotoxins (4, 5), the subtype-specific muscarinic receptor
agonists (6, 7), the L-type calcium channel blockers,
calciseptine and FS2 (8, 9), the GPIIb-IIIa antagonist and platelet
aggregation inhibitor, RGD-containing dendroaspin (or mambin) (10, 11), and the cell membrane lytic cardiotoxins (or cytotoxins) (12, 13).
Despite a highly conserved structural motif, the toxins in this family
are directed to diverse targets, yet their individual modes of action
are highly selective. The three-finger scaffold, that is also found in
proteins isolated from sources other than snake venoms and devoid of
toxic activity (14-17), constitutes a structural motif which
accomplishes a plethora of precise, but eclectic, functions.
Fasciculins selectively inhibit mammalian and electric fish AChEs with Ki values in the pico- to nanomolar range (18-22). Several kinetic studies have shown that the fasciculin-AChE complex, despite its high affinity and extremely slow rate of dissociation, possesses residual catalytic activity (20, 23-26): in the complex, both substrate and catalytic site inhibitors are able to access the active center of the enzyme, located at the bottom of a deep and narrow gorge (27). On the other hand, fasciculin is competitive with propidium (18, 20, 24), a peripheral site inhibitor of AChE (28) which binds at an allosteric site presumed to be at the rim of the gorge (cf. Ref. 29). Three aromatic residues, that lie at the gorge rim, were found to be critical for fasciculin binding to AChE by site-directed mutagenesis of the enzyme (21).
The recently solved x-ray structures of the Fas2-mouse AChE and Fas2-Torpedo AChE complexes revealed that three domains of Fas2 anchor it to the enzyme, and delineated a large contact area consistent with the low dissociation constant of the complex; the Fas2 and AChE residues participating in the binding interface were unambiguously established, and major hydrophobic interactions were identified (30, 31). The structural analyses, however, did not reveal to what extent each contact between Fas2 and AChE contributes to the overall binding energy. In addition, the central loop of Fas2 was found to fit snugly at the gorge entrance so that the entry of other molecules, even as small as water, should be precluded. The mode of inhibition of fasciculin therefore appeared to be total occlusion of substrate entry at the mouth of the active-site gorge. To reconcile the disparity in the kinetic and structural data, either a second portal for entry of substrate and catalytic site inhibitors in the complex or a conformational change in the enzyme, not obvious in the crystal structure, opening a gap between the gorge wall and the bound peptide, have been proposed (30).
Complete understanding of the chemistry of the Fas2-AChE association requires a functional map of the binding surfaces. By site-directed mutagenesis of a synthetic Fas2 gene, we have generated new probes aimed at analyzing the individual contributions of the fasciculin residues to complex formation and conformation. In a mammalian system, we expressed a fully processed recombinant fasciculin, rFas2, that is undistinguishable from the natural, venom-derived Fas2. Fourteen mutants, encompassing 16 amino acid residues distributed among the three loops (fingers) of Fas2, were designed based on both the kinetic and structural data. We show that common determinants are identified by the structural and the mutagenesis approaches, but only a few of the many Fas2 residues residing at the binding interface provide the critical contacts required for enzyme inhibition.
HEK-293 and CHO-K1 cells were obtained from
American Type Culture Collection. Ultraculture and serum-free Ultra-CHO
cell culture media were from BioWhittaker, and serum-free Dulbecco's
modified Eagle's cell culture medium from Life Technologies, Inc.
L-Methionine sulfoximine, polyethylene glycol 8000, protease-free BSA, 5,5-dithiobis-(2-nitrobenzoic acid),
acetylthiocholine iodide, and the gel-filtration molecular weight
markers (Mw-GF-70 Kit) were products of Sigma. The
prepacked FPLC columns, Mono S HR 5/5 and Superose-12 HR 10/30, were
from Pharmacia Biotech Inc. Dialysis tubing (Spectra/Pore6) was from Spectrum Medical Industries. The BCA kit for protein assays was from
Pierce. Prestained protein molecular weight standards for SDS-PAGE were
from Life Technologies, Inc. 125I-Na (2100 Ci
mmol
1) was from Amersham. Complete and incomplete
Freund's adjuvants were from Difco Laboratories. Normal rabbit serum
and goat anti-rabbit whole serum were from Jackson ImmunoResearch
Laboratories. All buffers were made with deionized water from a
Millipore MilliRO/MilliQ system.
The pGS expression vector was a gift
from Scios Nova Inc. (Mountain View, CA). Purification of Fas2 from
Dendroaspis angusticeps venom has been described previously
(32). Concentrations of stock solutions were determined from their UV
spectra (276 nm = 4900 M
1
cm
1). Wild-type AChE from mouse recombinant DNA was
expressed, concentrated, and titrated as described previously (33, 34).
The polyclonal anti-Fas2 serum was obtained by immunization of a Blanc
du Bouscat Evic rabbit with subcutaneous multi-site injections of
purified Fas2 (100-150 µg). Primary and booster injections were made
with complete and incomplete Freund's adjuvants, respectively
(35).
A cDNA encoding rFas2 and the leader peptide from erabutoxin a (36) was synthesized as two sets of complementary oligonucleotides of ~130 base pairs in length, that were annealed, ligated together, and cloned into the expression vector pGS. rFas2 was expressed from the cytomegalovirus promoter while glutamine synthetase under the control of the Rous sarcomal virus-long terminal repeat was used as a dominant selectable marker conferring resistance to a low level of methionine sulfoximine. The pGS-rFas2 plasmid was transfected into CHO-K1 cells by calcium phosphate co-precipitation with glycerol shock. Individual cell colonies which successfully integrated the rFas2 and adjacent GS genes were selected in glutamine-free Ultraculture medium in the presence of 25 µM methionine sulfoximine and grown to confluence. Clones which produced the highest Fas2 activity were then further amplified using either 250 µM or 500 µM methionine sulfoximine.
Mutagenic oligonucleotides (usually 18 mers) were used in generating Fas2 mutants according to Kunkel et al. (37). Mutations were done on either M13 or single-stranded pBluescript templates. The entire nucleotide sequences of the mutated plasmids were verified using dideoxy sequencing. Transient transfections into HEK-293 cells employed the pGS plasmid and coprecipitation with calcium phosphate and glycerol shock. To ascertain transfection efficiencies, 5 µg of a pcDNA-3 expression vector containing the Escherichia coli lacZ gene were cotransfected (38). The cells were rinsed and placed in serum-free medium 24 h after transfection. Secreted fasciculin activity was examined 48 to 72 h later.
ChromatographyUltraculture medium containing the expressed
rFas2 (10 ml/10-cm plate) was harvested every 3-4 days, centrifuged
(4 °C, 10 min) to remove cell debris, extensively dialyzed against
50 mM NH4Ac, pH 7.5, 0.01% (w/v)
NaN3, and filtered through 0.22-µm cellulose acetate
filters. rFas2 was purified by cation-exchange and size-exclusion FPLC
(Pharmacia) performed at 4 °C in NH4Ac, pH 7.5, with
flow rates of 0.5 ml min1. The dialyzed and filtered cell
culture medium (up to 500 ml) was loaded on a Mono S column previously
equilibrated with 50 mM NH4Ac, then the column
was washed extensively with the same buffer. Elution of the rFas2
fraction was performed with a 50-200 mM NH4Ac
gradient over 60 min, followed by an isocratic step at 200 mM NH4Ac. The eluted rFas2 fraction was
lyophilized, redissolved in 100 mM NH4Ac, and
loaded as a 200-µl sample on a Superose-12 column equilibrated in 100 mM NH4Ac. For the final cation-exchange step,
the pooled rFas2-containing fractions emerging from successive gel
filtrations were diluted twice and loaded on the Mono S column equilibrated in 50 mM NH4Ac. Elution of rFas2
was performed under isocratic conditions with 100 mM
NH4Ac.
All Fas2 mutants were concentrated and partially purified from the culture medium through the first ion-exchange step, then further concentrated by ultrafiltration. To prevent cross-contamination, separate Mono S columns were used for rFas2 and the Fas2 mutants, and the columns were fully regenerated after each use.
ElectrophoresisSDS-PAGE under reducing conditions used the
discontinuous system of Laemmli (39) with a 20% resolving/5% stacking
gel. Samples were denatured in 500 mM Tris-HCl, pH 6.8, in
the presence of 5% (v/v) -mercaptoethanol and 4% SDS, at 90 °C
for 2 min, then loaded onto the gel in the presence of 10% (v/v)
glycerol and 0.01% bromphenol blue. Isoelectric focusing was performed
with pH 3-10 precast gels (Novex) as specified by the manufacturer. Staining was by silver nitrate.
Amino acid analysis of rFas2 (5 nmol), previously hydrolyzed with 6 M HCl and 1% (w/v) phenol for 20 h in vacuo, was carried out on a ABS Auto Analyzer. Automated Edman analysis of rFas2 (250 pmol) was conducted on a ABS sequenator. Equivalent samples of venom-derived Fas2 were analyzed in parallel.
Liquid-phase Radioimmunoassay (RIA)Lactoperoxidase-catalyzed radioiodination of Fas2 to a
specific radioactivity of ~1200 Ci mmol1 has been
detailed elsewhere (20). The titer of the anti-Fas2 serum, defined as
the serum dilution which binds 50% of the 125I-Fas2 added
to the sample, was determined as a 316,000-fold dilution. For standard
RIAs, 125I-Fas2 (10,000 cpm) was incubated at 37 °C for
90 min with the anti-Fas2 serum at its titer in 200 µl of 50 mM
NaH2PO4/Na2HPO4, pH
7.5, 0.1 mg ml
1 BSA, and then the samples were diluted
with buffer to 500 µl and incubated overnight at 4 °C. Double
immunoprecipitation of the antigen-antibody complexes was performed at
4 °C with successive additions of normal rabbit serum (50 µl of a
1/50 dilution), goat anti-rabbit serum (50 µl of a 1/20 dilution),
and polyethylene glycol (400 µl of a 20% (w/v) solution). Samples
were centrifuged (10,000 × g, 25 min, 4 °C), and
the radioactivity of the pellets was determined. Titration of the
unlabeled rFas2 and Fas2 mutants by RIA was based on competition with
125I-Fas2 for complexation with anti-Fas2 serum and
comparison to a Fas2 standard curve.
Relative AChE activities (40)
were recorded at room temperature by microtitration on a
Vmax kinetic microplate reader (Molecular Devices Corp.) at 405 nm, in the presence of 0.5 mM
acetylthiocholine iodide and 0.3 mM
5,5-dithiobis-(2-nitrobenzoic acid), in 50 mM
NaPO4 buffer, pH 8.0, 0.1 mg ml
1 BSA (20).
The activities of purified rFas2 and of the Fas2 mutants were monitored
by inhibition of 5-10 pM AChE after a 1-h incubation at
37 °C and 14-18 h at room temperature.
Culture media, 72 h after transfection, were screened for immunoreactivity and inhibitory activity directly or after 10-20-fold concentration by ultrafiltration. The Fas2 mutants were titrated by RIA, and their inhibitory activities were quantitated by AChE inhibition assay after partial purification and concentration by cation exchange of the media (100-200 ml) and ultrafiltration of the chromatographic fractions. The Fas2 mutants were submitted to at least two independent transfection-chromatography-assay sequences.
Recombinant Wild-type Fasciculin
Expression of rFas2 in CHO Cells and PurificationSynthesis
of the oligonucleotides encoding rFas2 enabled us to use codons of high
usage frequency for mammalian cells and place restriction sites at
convenient locations within the open reading frame and flanking
regions. To ensure secretion and processing, a sequence encoding the
leader peptide of a structurally related three-fingered toxin,
erabutoxin a (36), was joined at the 5 end (Fig. 1). A
ScaI site was generated at the linkage between the two
sequences by replacing Thr at position
1 by Ser.
Selected clonal cells at confluence secreted up to 1.7 µg
ml1 (0.25 µM) of rFas2 in the 3-day
intervals between medium changes; this rate continued for up to 10 weeks. Purification employed three steps on 500-ml batches of media
(Fig. 2). Initial cation exchange led to removal of most
medium proteins with a 440-fold enrichment (in pmol of Fas2 per mg of
total protein) in rFas2, with more than 90% of recovery (Fig.
2A). A gel-filtration step purified rFas2 1500-fold (Fig.
2B). Final removal of trace contaminants was achieved by a
second cation-exchange chromatography performed in isocratic conditions
(Fig. 2C). rFas2 migrated in the same positions as the
venom-derived Fas2 in the second and third chromatographic steps. An
overall purification of 2200-fold was achieved in ~50% yield.
Physical and Biological Characterization of rFas2
SDS-PAGE in
reducing conditions (Fig. 3, inset) and
isoelectric focusing analysis (not shown) each revealed a single, sharp band that migrated at a position identical to the venom-purified Fas2.
Parallel runs on the amino acid analyzer yielded identical compositions
for rFas2 and Fas2. Amino-terminal sequencing of native rFas2 yielded a
single sequence, TMXYSHTTTS, identical to that of Fas2, with
a blank third step (noted as X) for non-reduced Cys3. UV absorbance of natural Fas2 was used to estimate
concentration of rFas2 stocks. Radioimmuno- and AChE-inhibition assays
showed equivalent potencies for rFas2 and venom-derived Fas2 for
inhibiting mouse AChE and competing with 125I-Fas2 for
binding to a polyclonal anti-Fas2 serum (Fig. 3 and Table
I). Hence fully processed and correctly folded rFas2 is directly produced in mammalian cells.
|
Mutant Fasciculins
Selection of Positions for Mutation of Fas2The overall net
positive charge (+4 at neutral pH) of Fas2, coupled with its
association at a peripheral anionic site on AChE, suggested an
electrostatic attractive component with the cationic amino acids on
Fas2 (Fig. 4). Cationic positions unique to the fasciculins were considered first. Hence, Arg11,
Arg24, Lys25, Arg27,
Arg28, Lys32, and Lys51 were
mutated, but Arg37 and Lys58, being invariant
among the three-fingered toxins and with their side-chains only partly
solvent-accessible (41, 42), were not studied. Second,
His29, supposedly unprotonated at neutral pH, is located at
the tip of loop II, a position corresponding to the critical
Asp31 in erabutoxin a (43). Third, Pro31, whose
cis configuration constrains the conformation of the tip of
loop II (30, 41), was modified to Arg, a residue and position essential
for the -neurotoxin activity (43, 44). Fourth, a doublet of anionic
residues, Asp45-Asp46, at the tip of loop III,
is found only in the fasciculins and "synergistic-type" toxins, all
from mamba venoms (45). All
-neurotoxins have a positively charged
residue at position 45. Finally, the recently solved structures of two
Fas2-AChE complexes (30, 31) revealed that several neutral residues
participate in the interaction of Fas2 to AChE. Hence,
Thr8-Thr9, Pro30,
Met33, and Val34-Leu35, were also
mutated.
By substituting residues found in homologous toxins, perturbations in
secondary structure might be minimized. Based on sequence homology and
superimposition of crystal structures in the members of the
three-fingered peptide family, Fas2, erabutoxin b, -cobratoxin,
-bungarotoxin, and cardiotoxin VII4 (Fig. 4A), we mutated
Arg11
Gln (R11Q), Arg24
Thr (R24T),
Lys25
Leu (K25L), Arg27
Trp (R27W),
Arg28
Asp (R28D), His29
Asp (H29D),
Pro31
Arg (P31R), Lys32
Gly (K32G),
Asp45
Lys (D45K), and Lys51
Ser (K51S)
(Fig. 4B). The close fit and potential for hydrogen bonding
of certain residues at the binding interface of the Fas2-AChE complex
suggested that side chains on Thr8, Thr9,
Met33, Val34, and Leu35 be
minimally perturbed. Accordingly, we changed them into Ala (T8A/T9A,
M33A, and V34A/L35A mutants). Finally, to shorten loop II and possibly
create a gap between the bound Fas2 and the entrance of the gorge of
the enzyme, we deleted Pro30 (
P30 mutant).
Expression
from transiently transfected cells enabled direct analysis of activity
and/or peptide production in the cell culture media. Analysis of AChE
inhibition and RIA titer allowed an initial classification of the Fas2
mutants. Class I comprised the T8A/T9A, K25L, R28D, H29D, D45K,
V34A/L35A, and K51S mutants, which displayed significant AChE
inhibition and immunoreactivity. Class II comprised the R24T, R27W,
P30, P31R, K32G, and M33A mutants, which displayed significant
immunoreactivity but less AChE inhibition. Class III comprised only the
R11Q mutant, which displayed AChE inhibition but in complete curves
immunoreactivity curves. Production yields, determined by RIA titration
of rFas2 and the Fas2 mutants secreted into the media, ranged between
0.5 and 5 pmol ml
1 in a 3-day period.
Elution positions of the
Fas2 mutants on the cation-exchange resin were monitored by RIA and
AChE inhibition assay for the class I and class III mutants and by RIA
for the class II mutants. Representative profiles are shown in Fig.
5. Compared to rFas2 (top panels), 10 of the
14 mutants showed elution positions consistent with their net charge.
Mutants T8A/T9A, P30, M33A, and V34A/L35A, all possessing the same
net charge as rFas2, eluted at the same ionic strength as rFas2.
Mutants K25L, R27W, K32G, and K51S, which lost one net positive charge,
eluted at lower ionic strength than rFas2. Mutants P31R and D45K, which
gained one and two net positive charge(s), eluted at higher ionic
strengths than rFas2. In contrast, mutants R11Q, R24T, R28D, and H29D,
which lost either one or two positive charge(s), eluted at a similar
ionic strength as rFas2, suggesting that the initial and newly
introduced side chains at positions 11, 24, 28, and 29 are not
solvent-accessible and/or do not interact with the anionic matrix of
the column. Two mutants, R24T and R27W, eluted as several peaks, but
calculation of concentrations based on a simple reversible equilibrium
for ligand binding revealed that the additional peaks are minor
components. Whether these peaks reflect incomplete processing or
folding of the peptides, or slow conformational equilibria, has not
been ascertained.
Analysis of the Apparent Dissociation Constants (K
The Fas2 mutants, enriched in purity and
concentrated by cationic exchange and ultrafiltration, were titrated by
RIA and analyzed for inhibitory activity (Fig. 6). The
RIA profiles were generally consistent with the amounts of peptide
expected based on transfection efficiency and purification yields. In
contrast, the concentration dependences of the AChE inhibition curves,
relative to those of immunoreactivity, varied widely, illustrating
significant differences in the affinities of the Fas2 mutants for AChE.
The transiently expressed rFas2 sample, enriched and concentrated,
yielded the same AChE inhibition potency as the stably expressed and
fully purified rFas2 (cf. Fig. 3). Compared to rFas2, no
significant differences in inhibition potency were found for mutants
K25L, R28D, K32G, V34A/L35A, D45K, and K51S. In contrast, significant differences were found for mutants T8A/T9A, R11Q, R24T, R27W, H29D,
P30, P31R, and M33A. The inhibitory activities of the Fas2 mutants
toward mouse AChE, calculated from the dilutions for 50% inhibition in
the inhibition assays and 50% competition in the RIA titrations, and
normalized to Fas2, are reported in Table I. The apparent
K
i values that are also reported are expressed
relative to a Ki value of 2.3 pM for
Fas2 (21). Because of the limitations that may arise from slow
equilibration, from the disparity between added and free ligand, and
from using immunoreactivity to reflect peptide concentration, the
mutants with low dissociation constants are specified only by an upper limit.
Residue Contributions to Fas2 Inhibition Based on Mutagenesis Studies
Based on the relative inhibitory activities of the rFas2
mutants (Table I), the mutations can be classified in four categories: (i) those which cause little or no apparent effect on Fas2 activity, as
seen for mutants K25L, R28D, V34A/L35A, D45K, K51S (no change), or K32G
(3-fold decrease), (ii) those which cause a decrease by about one order
of magnitude in the Fas2 activity, as seen for mutants R24T
(13-fold) and M33A (8-fold), (iii) those which cause a decrease by two
or more orders of magnitude, as seen for mutants R27W (49-fold), P30
(192-fold), and P31R (625-fold), and (iv) those which cause an apparent
increase in the Fas2 inhibitory activity, as seen for mutants T8A/T9A,
R11Q, and H29D (18-, 6-, and 73-fold, respectively).
The contribution of the mutated residues to activity, relative to their
location in the three-dimensional structure of Fas2 bound to mouse AChE
(30), is shown in Fig. 7. The three residues that
dominate the AChE-inhibitory activity of Fas2, Arg27,
Pro30, and Pro31, form a subset located at the
tip of loop II. The surface area of this subset, 261 Å2,
represents only 6% of the total accessible surface area of Fas2 but
25% of the Fas2 area buried in the Fas2-mAChE complex. Fas2 residues
Thr8, Thr9, and Arg11, mutation of
which resulted in an apparent increase in Fas2 activity, form a second
subset located at the tip of loop I. The surface area of this subset,
300 Å2, represents 7% of the total accessible surface
area of Fas2 but 29% of the Fas2 area buried in the complex. Hence, a
small fraction of the many Fas2 residues which make up the overall
functional site provide the critical contacts required for the high
affinity interaction.
Several groups have produced recombinant three-fingered snake
toxins, - and
-neurotoxins, as fusion proteins in heterologous bacterial expression systems, and some have reported site mutagenesis studies on these toxins (43, 44, 46-52). The lower activity of the
fusion proteins, compared to the natural toxins, however, precludes
direct analysis of the mutants before in vitro cleavage of
the hybrid by chemical or enzymatic means. More recently, a recombinant
-neurotoxin was produced in yeast with full activity, although
possessing a short amino-terminal extension due to altered signal
peptide cleavage (53). In our study, we used a mammalian system to
express secreted and fully processed rFas2 and 14 Fas2 mutants. Direct
expression of rFas2 in its functional conformation demonstrates that
the use of the signal peptide sequence of erabutoxin a and the change
at position
1 were not critical for post-translational events such as
folding, cleavage of signal peptide, and disulfide bond formation of
the rFas2 molecule in mammalian cells.
Owing to the use of complementary assays, RIA titration and AChE
inhibition, 14 transiently expressed Fas2 mutants could be quantitated
and functionally characterized without extensive purification. The
polyclonal anti-Fas2 serum is characterized by a high titer in
antibodies, likely to be mostly IgGs (35). The shapes of the RIA
curves, that generally extend over a concentration range wider than
expected for interaction of 125I-Fas2 with a single site
(Figs. 3 and 6), suggest that the serum contains at least two IgG
populations presumably directed toward distinct epitopes on Fas2. The
existence of several antigenic determinants on the fasciculin molecule
is in accord with previous reports of three to four distinct epitopes
simultaneously accessible to specific antibodies (Fab fragments)
directed against -neurotoxins and of two monoclonal antibodies
directed toward distinct areas of a cardiotoxin (cf. Ref.
54). Because of the heterogeneity of polyclonal antisera, as well as
the multiplicity of residues involved in the contact area between each
IgG and its specific epitope, single residue modifications of the
three-fingered toxins usually have little influence on polyclonal
antibody titers. Antigenic diversity proved to be generally useful for
quantifying the loss in the inhibitory activity of the Fas2 mutants
based on immunotitration of enriched samples.
Mutants T8A/T9A, R11Q, and H29D, however, displayed greater ratios of
inhibitory activity to immunochemical titer than did wild-type Fas2.
While this may reflect enhanced inhibitory affinity for AChE, several
caveats preclude a precise determination of a dissociation constant.
First, with a fasciculin dissociation rate constant of 4.0 × 103 min
1 (21), extended equilibration times
are required for fractional AChE occupation. Extensive dilution of AChE
to satisfy the condition of a Ki to total enzyme
concentration ratio that assures a reasonable fraction of free ligand
(55, 56) would have required unrealistic equilibration times for the
putative high affinity mutants. Second, as detailed below, the ratios
will be artificially high if a region of Fas2 is a dominant epitope, as
mutants of this region would show diminished immunoreactivity.
Loop II of Fas2 contains 16 residues, located between Cys22 and Cys39 (Fig. 4). Eleven residues, Arg24, Lys25, and all residues from Arg27 through Leu35, were mutated. The Arg27-Pro30-Pro31 subset dominates the inhibitory activity of Fas2 (Figs. 6 and 7 and Table I) and interacts with the peripheral anionic site of the enzyme (30). However, the influence of these mutations is particularly complex since they may cause at least three distinct changes. Substitution of Trp for Arg27 removes a positive charge, increases the volume of the side chain, and introduces an aromatic indole ring with hydrophobic properties. Thus, the specific conformation of Fas2 loop II is likely to be altered because of the loss of the stabilizing intraloop interaction that Arg27 establishes with Pro30 and its indirect influence on the Pro30-cis-Pro31 turn (30, 41). The bulky Trp side chain introduced at position 27 should experience steric hindrance with mAChE-Trp286 and would eliminate the potential hydrogen bonds of Arg27 with the carbonyl oxygens of Trp286 and Leu289 in mAChE. A similar steric occlusion might be responsible for the increased Ki seen for Fas2 chemically modified at Arg27 (57). Deletion of the amino acid residue Pro30 not only removes the cyclic side chain, but also alters both the length of loop II and the cis conformation of its tip. As a result, the cavity formed by mAChE residues Glu292, Ser293, Gly342, and Ile365 is likely to contain bound water molecules in the complex. The shrinking of loop II, as well as its enhanced flexibility, would diminish the high complementarity of the interactive surface seen in the crystal structure (30). Substitution of Arg for Pro31 increases side chain dimensions, introduces a positive charge, and perhaps precludes formation of the cis bond in Pro30. The obvious consequences would be steric hindrance of the bulkier Arg side chain with the cluster formed by mAChE residues Leu289, Ser293, and Tyr341, as well as Coulombic repulsion with Fas2-Lys32 and/or mAChE-Arg296 (30). To resolve these possible interactions, residue substitutions at positions 27, 30, and 31 with progressive changes in side chain properties may help delineate the respective contributions of the charge, volume, and stereochemical characteristics of these three residues to conformation and stability of the complex.
Both the substitutions of Thr for Arg24 and Ala for
Met33 resulted in an order of magnitude decrease in Fas2
activity (Table I). Arg24, located at the base of loop II
(Fig. 7), is a structurally important residue that stabilizes the
carboxyl-terminal and core regions of Fas2 through hydrogen bonding to
the carbonyl atom of Tyr61 and stacking interaction with
Arg37 (30, 41). None of these interactions should be
retained by the shorter, uncharged Thr side chain. Thus, a less stable
complex forms with mAChE because of indirect destabilization of the
mutated peptide molecule. The Met33 Ala mutation should
have a more direct influence. The loss of the sulfur atom and
shortening of the side chain preclude stacking interactions with
Trp286 and van der Waals contacts with Tyr341,
two key mAChE residues located at the peripheral anionic site (21, 58,
59). In addition, the smaller Ala side chain cannot retain secondary
hydrophobic interactions with mAChE residues Tyr72,
Tyr124, and Tyr341 (30). In the Fas2-mAChE
complex, most of the solvent-accessible surface area of
Met33 is lost; in fact, this residue contributes about 10%
of the Fas2 surface area buried at the interface (30), a value slightly greater than the individual contributions of neighboring critical Fas2
residues Arg27, Pro30, and Pro31.
On the other hand, of the several peripheral anionic site residues that
interact with Met33, a single one, Tyr124, does
not establish any other contact with Fas2 (30). A decrease in apparent
affinity for Fas2 of two orders of magnitude was observed for mAChE
mutant Y124Q (21). More than a single order of magnitude in the
decrease in Fas2 activity could therefore have been expected upon
mutation of Met33; however, the minimal change in the side
chain volume with an Ala substitution likely preserves the tight fit of
the other loop II residues with the peripheral anionic site.
Substitutions of Leu for Lys25, of Asp for Arg28, and of an Ala doublet for Val34-Leu35, resulted in unaltered Fas2 activity (Table I). These results are in accord with the crystallographic observations that show that the Lys25 and Arg28 side chains point away from the Fas2-mAChE interface, and that cavities exist between the van der Waals surfaces defined by the side chains of Fas2 residues Val34 and Leu35 and mAChE residues Leu76, Tyr72, and His287, respectively (30). Replacement of Arg28 by Asp probably induces structural rearrangements leading to a different network of internal charge compensation; surprisingly, this does not perturb the activity of the mutant toward the enzyme.
Substitution of Asp for His29 resulted in an apparent increase of two orders of magnitude in the Fas2 activity (Figs. 6 and 7 and Table I). In the crystalline complex, the side chain of His29 participates in van der Waals interactions with mAChE-Glu292 that the newly introduced Asp side chain, negatively charged, cannot establish because of charge repulsion (30). However, the Asp side chain may be sufficiently flexible to rotate toward the side chain of either Arg27 or Arg28 and undergo charge compensation. Also, an internal salt bridge could form with Arg27. Thus, an improved fit of the more rigid tip of loop II with the mAChE peripheral anionic site residues might result from stabilization of internal structure. However, as discussed below, a change in structure might also affect the immunoreactivity of this mutant.
Mutations in Loop ILoop I of Fas2 encompasses 13 residues, located between Cys3 and Cys17 (Fig. 4). Both the mutations made on Thr8-Thr9 and Arg11 lead to apparent increases of one order of magnitude in the activities of the mutants toward mAChE (Fig. 6 and Table I). The Thr8-Thr9-Arg11 subset is fully exposed at the tip and external edge of loop I (Fig. 7), which fits in a crevice near the lip of the mAChE catalytic gorge and maximizes the surface area of contact of loop II at the gorge entry (30). In particular, the Thr8-Thr9 doublet potentially interacts with eight mAChE residues, Tyr70, Gln71, Tyr72, Val73, Leu92, Gln279, Val282, and Asp283 (30). Substitution of an Ala doublet eliminates hydrogen bonding of the Thr side chains with Gln279 and Asp283. The smaller size of the two Ala side chains might facilitate a rearrangement of the backbone at the tip of loop I, enabling it to come closer to loop II and establish a stabilizing internal interaction with Fas2-Leu35. In addition, the smaller side chain of Ala9 may establish more favorable interactions with mAChE residues Tyr70, Leu92, and Val282 and provide a tighter fit of the tip of loop I with the furrow that exists on the mAChE surface (30), possibly compensating for the loss of hydrogen bonding energy. On the other hand, Arg11 hydrogen bonds with mAChE-residues Glu84 and Asn87. Substitution of Gln for Arg11 likely preserves the capacity for hydrogen bonding, especially as the smaller size of the Gln side chain should favor a tighter fit between the tip of loop I and the enzyme. Internal stabilization of the Fas2 molecule and/or loop I anchorage at the enzyme surface could thus contribute to tighter occlusion, by the tip of loop II, of the catalytic site gorge of the enzyme. Incidentally, the apparent increase in Fas2 activity that results from the removal of the positive charge does not match with the 40-73% loss in the activity of Fas2 chemically modified at Arg11 (57). The modifying agent, however, produces a substantial enlargement in the effective side chain that is not reproduced by the Gln mutation.
Although infrequent, single residue substitutions have shown enhanced
affinity with the three-fingered peptidic toxins. The 35-fold higher
affinity of Fas3, compared to Fas1, for rat brain AChE likely results
from the Thr15 Lys substitution in loop I (20).
Substitution of Arg for Ile36 in erabutoxin a loop II led
to a 7-fold increase in this neurotoxin's affinity for the nicotinic
receptor (44). Two mutations of Bungarus AChE, near the
presumed peripheral site, resulted in up to a 14-fold increase in
affinity for Fas2 (22). Nevertheless, the uncertainties in the relative
activities of the Fas2 mutants T8A/T9A, R11Q, and H29D preclude
accurate determinations of specific activity without the availability
of homogeneous peptides.
In several three-fingered toxins, the edge of the first loop and the
tip and concave side of the second loop are domains immunoreactive to
monoclonal antibodies. In Fas2, the bulky side chains of
Arg11 and His29 could be particularly
immunogenic, as was found for Trp11 of a cardiotoxin, toxin
, and Lys15 of a neurotoxin, toxin
(54). Thus, apart
from the structural arguments that may explain increased inhibitory
activity, the Arg11
Gln and His29
Asp
mutations may also have altered dominant epitopes located on loop I and
loop II. Hence, a diminished, although not eliminated, recognition of
the mutant by the anti-Fas2 serum (Fig. 6) may result in an
overestimation of their relative inhibitory activities toward AChE.
Similar considerations could influence our results for the double
mutant T8A/T9A.
Despite the unique conservation of Arg11 in the fasciculins (42), the cationic side chain at this position does not appear to be a major contributor to the affinity nor to the inhibitory capacity of Fas2. In the crystalline complex, Arg11 associates with the region of Trp86, proposed as a second portal, or "back door," for substrate entry into the enzyme (60, 61). Should translation of the choline cation through the putative back door region be rate-limiting in catalysis in the Fas2-AChE complex, a greater influence of Arg11 on AChE inhibition by Fas2 would have been expected. The side chain of Arg11, however, displays high temperature factors (B factors) not only in the structures of unbound Fas1 and Fas2 (41, 62), but also in those of Fas2 bound to mAChE and Torpedo AChE (30, 31). In general, the highly flexible loop I may adopt an ensemble of conformations (41, 62); yet, a single conformation is selected in the formation of the high affinity complex (30, 31).
Mutations in Loop IIILoop III of Fas2 encompasses 10 residues, located between Cys41 and Cys52 (Fig. 4). The unaltered Fas2 activities of the D45K and K51S mutants (Figs. 6 and 7 and Table I) are consistent with the lack of interaction of residues Asp45 and Lys51 with mAChE, as observed in the crystalline complex. Yet, in the fasciculins, compared to the other members of the three-fingered toxin family, the Asp45-Asp46 doublet confers to loop III an exclusive anionic locus which dictates the orientation of the dipole vector of the molecule (18, 42). An internal structural rearrangement in the D45K mutant molecule may occur, since the Lys side chain introduced at position 45 should repel Arg28, which stabilizes Asp45 in Fas2 (41, 42). In the mutant, however, it may be compensated by the neighboring side chain of Asp46.
Changes in the Fas2 structure have been proposed earlier to explain the 57% decrease in Fas2 activity upon acetylation of Lys51 (63). Two residues of loop III, Asn47 and Leu48, interact with mAChE-His287, but at distances of ~4 Å (30); however, Leu48 does not interact with Torpedo AChE (31). Loop III residues, therefore, likely play no critical functional role other than internal stabilization of loop II conformation.
Numerous aromatic residues reside at the Fas2-mAChE complex interface: Fas2-Tyr4, His6, His29, and Tyr61, and mAChE-Tyr70, Tyr72, Tyr77, Tyr124, Trp286, His287, Tyr341, as well as several nonaromatic residues which undergo hydrophobic interactions: Fas2-Pro30, Pro31, and Met33, and mAChE-Pro78 (30). Mutation of Fas2-Pro30 and Pro31, which are central to the interface, resulted in two to three orders of magnitude decreases in Fas2 activity. Single-site substitutions of mouse AChE peripheral anionic site residues Tyr72, Tyr124, and Trp286, also central to the interface, reduced the affinity of Fas2 by two to six orders of magnitude (21). Thus, patches of high hydrophobicity at the interface dominate the binding free energy of the Fas2-AChE complex, consistent not only with the tight intermolecular packing areas observed in the crystal structures (30, 31), but also with general structural patterns observed at interfaces of high affinity protein-protein complexes (64-69).
Site-directed mutagenesis of the Fas2 molecule and expression of the mutants from transiently transfected mammalian cells have provided substantial information on the respective contributions of 16 residues of Fas2 positioned for interaction with mouse AChE. The determinants identified by the structural and the functional approaches do coincide, but only a few of the many residues which make up the overall interactive site of the Fas2 molecule provide the strong interactions required for high affinity binding. Mutant cycle analyses, coupled with kinetic and crystallographic studies, should further delineate the contribution of individual residues to the affinity and inhibitory capacity of the fasciculin-AChE complexes.
We are grateful to Drs. Tyler White, John Lewicki, Barbara Cordell, and Andy Lin (Scios Nova, Inc.) for the gift of the pGS expression vector, to Dr. Daniel Donoghue (UCSD) for synthesis of the 130-base pair oligonucleotides used in construction of the expression plasmid, to Siv Garod (UCSD) for amino-terminal sequencing of rFas2, to Dr. Igor Tsigelny (UCSD) for three-dimensional superimposition and sequence analysis of the toxins, and to Dr. Yves Bourne (CNRS) for help in structural analysis of the mutants and fruitful discussions. Assistance from Kael Duprey and Jonathan Eads (UCSD) in inhibition and radioimmunoassays and from Maryse Alvitre (CNRS) in rabbit serum production is much appreciated.