From the Max-Planck-Institut für Molekulare Physiologie, Abteilung Strukturelle Biologie, Rheinlanddamm 201, 44139 Dortmund, Germany
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ABSTRACT |
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The inability of the S17N mutant of Rap1A to
sequester the catalytic domain of the Rap guanine nucleotide exchange
factor C3G (van den Berghe, N., Cool, R. H., Horn, G., and
Wittinghofer, A. (1997) Oncogene 15, 845-850) prompted us
to study possible fundamental differences in the way Rap1 interacts
with C3G compared with the interaction of Ras with the catalytic domain
of the mouse Ras guanine nucleotide exchange factor
Cdc25Mm. A variety of mutants in both Ras and Rap1A were
designed, and both the C3G and Cdc25Mm catalyzed release of
guanine nucleotide from these mutants was studied. In addition, we
could identify regions in Rap2A that are responsible for the lack of
recognition by C3G and induce high C3G activity by replacement of these
residues with the corresponding Rap1A residues. The different Ras and
Rap mutants showed that many residues were equally important for both
C3G and Cdc25Mm, suggesting that they interact similarly
with their substrates. However, several residues were also identified
to be important for the exchange reaction with only C3G
(Leu70) or only Cdc25Mm (Gln61 and
Tyr40). These results are discussed in the light of the
structure of the Ras-Sos complex and suggest that some important
differences in the interaction of Rap1 with C3G and Ras with
Cdc25Mm indeed exist and that marker residues have been
identified for the different structural requirements.
Ras proteins are GTP-binding proteins that cycle between an active
GTP bound form and an inactive GDP bound form. Their nucleotide binding
state is tightly regulated by guanine nucleotide exchange factors
(GEFs),1 leading to GTP
binding, and by GTPase-activating proteins, inducing the transition to
the GDP bound state (5, 6). Ras functions as a growth stimulus, but can
also, depending on the cell type, induce cell differentiation (7). In
contrast to the extensive knowledge on Ras function, relatively little
is known of the function of Rap, another member of this GTPase
superfamily. Rap and Ras bind to the same set of effector proteins due
to their highly homologous effector region (8-10), and Rap has been
described to be a suppressor of Ras function, possibly by titrating out Ras effector molecules or localizing them to a different compartment (11-14). It is, however, more likely that Rap has a function
independently of Ras, since it has its own specific GEF, is activated
by growth factors receptors independent of Ras activation (15-17),
activates isoforms of Raf kinase (18-20), and recently Rap1B has been
proposed to be a conditional oncogene of its own (21).
The cascade in which Rap1 is functioning still has to be elucidated.
These studies are hampered by conflicting results concerning the
putative dominant negative mutant Rap1(S17N). One group has described dominant negative effects of Rap1(S17N) on B-Raf activation (19, 20). However B-Raf activation by Rap1A in other cell types could
not be shown in a reporter gene
assay2, and epidermal growth
factor-induced Rap activation could not be inhibited by
Rap(S17N).3 Furthermore, we
showed that the Rap1A(S17N) mutant is unlikely to act as a dominant
negative mutant on the C3G-mediated activation of Rap1 in
vivo (4), since it does not titrate away the Rap1-specific nucleotide exchange factor C3G in vitro, in clear contrast
to the sequestering ability of Ras(S17N) with RasGEF isoforms (22-24). These results suggested that something might be fundamentally different
between the interaction of Ras and the RasGEFs Cdc25/Sos and of Rap1A
and its exchange factor C3G.
Many studies made use of mutant proteins to show that three
regions in Ras are important for the recognition by its GEF: the switch
I region (amino acids 25-40), the switch II region (amino acids
57-75), and the stretch of amino acids at position 100-110 of Ras.
Since nothing is known so far of the interaction between Rap1 and C3G,
we decided to make a comparative mutational analysis of
Ras-Cdc25Mm and Rap-C3G interactions. The aim of this study
was 1) to identify residues in Rap1A that are important for its
recognition by C3G and 2) to compare the Rap1A-C3G with the
Ras-Cdc25Mm interaction in order to find similarities and
more importantly possible differences. In line with our earlier
findings with Rap1A(S17N), our results show that clear differences
exist in the way Rap interacts with C3G compared with the
Ras-Cdc25Mm interaction. In addition, our study reveals
some previously unidentified novel residues as important for
the GEF interaction. These results are discussed in light of the recent
three-dimensional structure of the Ras-Sos complex (25).
Expression and Purification of Proteins--
Purification of
Rap1A, Rap2A, and Ras proteins was done as described before (4, 26),
Rap1A proteins being C-terminally truncated (residues 1-167). The
different mutations were introduced using the megaprimer polymerase
chain reaction method (27) and the appropriate mutagenesis primers. The
expression vector for full-length Rap2A (ptac) was kindly donated by
Dr. Jean de Gunzburg. The catalytic domain of C3G (C3G-CD, amino acids
830-1078) was cloned into pGEX and purified as described before (4).
Thrombin-cleaved C3G-CD or glutathione S-transferase-fused
C3G-CD were both used and gave similar results. The catalytic domain of
Cdc25Mm consisting of 285 amino acids was purified as
described before (2).
Guanine Nucleotide Exchange Reactions--
GTP-binding proteins
were loaded with the fluorescent analogue mGDP,
2',3'-bis(O)-(N-methylanthraniloyl)guanosine
diphosphate as described before (2). Nucleotide content was assayed by high performance liquid chromatography (28) and related to the protein
concentration determined by the Bradford assay (29). mGDP loading
efficiency usually was between 70 and 95%, except for the different
Rap2A proteins (30%). For the measurement of GEF activity, the
protein-nucleotide complex was incubated at 20 °C (Ras, 100 nM) or 25 °C (Rap1A and Rap2A, 200 nM) in
standard buffer (50 mM Tris-HCl, pH 7.5, 5 mM
dithioerythritol, and 5 mM MgCl2) with the
indicated amount of GEF protein and an approximately 200-fold excess of
unlabeled GDP. The decrease of fluorescence was monitored in a
Perkin-Elmer LS50 fluorescence spectrometer or Fluoromax-2 from Jobin
Yvon-Spex, with excitation and emission wavelengths of 366 and
450 nm, respectively. The kobs were obtained after fitting the data to a single exponential function using the
program Grafit (Erithakus Software). We observed that in the prolonged incubations with C3G the stimulation of the exchange rate
tended to decrease. We interpret this as a slow inactivation of C3G.
Analysis of these reactions by different fitting procedures resulted in
similar dissociation rates with approximately 20% deviations.
The use of Ras(S17N) as a dominant negative mutant of normal Ras
is well documented and is due to the low affinity of the mutant for
guanine nucleotide resulting in the relative (relative to nucleotide
binding) tight association with Cdc25-type of Ras guanine nucleotide
exchange factors. The Rap1A(S17N) mutation also has a low affinity for
guanine nucleotide but fails to inhibit the C3G-mediated guanine
nucleotide exchange reaction on wild-type Rap1A in vitro.
This lack of titration of C3G by Rap1A(S17N) in vitro (4)
prompted us to investigate whether the activation of GEF on Rap by C3G
might be different from the Ras-Cdc25Mm (or the Ras-Sos)
interaction. For that purpose we designed different mutations in
Ha-Ras, Rap1A, and Rap2A in those regions that have been described to
interact with Ras-specific GEFs containing the Ccd25 homology region
(30-40): the switch I, the switch II, and the region encoding residues
100-110 (Fig. 1). The recent structure determination of the Ras-Sos binary complex has confirmed that these
regions are indeed important for the interaction, with residues 57-62
encompassing switch II being the most important (25). The mutations
were chosen such that 1) highly conserved amino acids were mutated, or
2) residues in Ras were changed to those found in Rap1A and vice
versa, or 3) residues in Rap2A were converted into those of
Rap1A (see Fig. 1). After loading the mutant proteins with the
fluorescently labeled nucleotide mGDP, the dissociation of nucleotide
in the presence of a large amount of unlabeled GDP was measured using
the catalytic domains of Cdc25Mm (Cdc25Mm from
now on) and of C3G (C3G from now on). These domains were shown before
to be large enough to comprise the actual catalytic domain of the
Cdc25-like GEFs, in line with the structure determination (25). The
decrease of fluorescence was followed to
calculate the reaction rates which were
compared with the noncatalyzed reaction rates and scored as x-fold stimulation (Figs. 2, 3, and 4
and Tables I-III).
INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
MATERIALS AND METHODS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
RESULTS
TOP
ABSTRACT
INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
REFERENCES
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Fig. 1.
Sequence alignment of Ha-Ras, Rap1A and Rap2A
with respect to their switch I, switch II, and the
3 regions. The asterisk indicates
the different mutations used in this study.
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Fig. 2.
C3G-mediated guanine nucleotide exchange on
wild-type and mutant Rap1A. Interaction of 50 nM C3G
with either wild-type or mutant 200 nM Rap1A, mutated in
switch I (A) or in switch II (B), at 25 °C in
the presence of 100-fold excess of unlabeled GDP, was measured as the
decrease in fluorescence as described under "Materials and
Methods."
GEF-catalyzed guanine nucleotide exchange on Rap1A
1 × 10
5.
GEF-catalyzed guanine nucleotide exchange on Ha-Ras
Switch I-- The core effector region, defined as residues 32-40, overlaps with switch I and is identical between Ras and Rap1A, but residues outside this region such as 31, lysine in Rap1A and glutamic acid in Ras, have been shown to be responsible for the specificity with effectors (9, 26, 41-44). We made similar mutations in the effector region of both Ras and Rap1A and studied their effect on the interaction with the GEFs. K31E in Rap1A has a weak effect and appears to stimulate the reaction, whereas T35S in both Ras and Rap1A affected the GEF activity of C3G or Cdc25Mm 2-5-fold. Unlike the interaction with effectors (9, 26, 41-44) and with GTPase-activating protein (41-43), residue 31 is not responsible for the specificity of different GEFs. Other amino acids seem more important for both GEFs: D38A strongly reduces both C3G- and Cdc25Mm-stimulated mGDP release, whereas the E37G mutation shows a weaker effect (Fig. 2A). The Y40C mutation has dramatically different effects on the C3G-Rap versus Cdc25Mm-Ras interaction. Rap1A(Y40C) does not affect the GEF activity of C3G (Fig. 2A), but the same mutation in Ras induces a 9-fold higher activity of Cdc25Mm (Table II). The Y40C mutation also has a strong effect on the specificity of Cdc25Mm as shown by the 6-fold stimulation of Cdc25Mm on Rap1A(Y40C) (Fig. 3A), whereas nucleotide exchange on WT-Rap1A is not stimulated.
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Switch II-- It has been shown before that residues in the switch II region of Ras between Gln61 and Asn81 are crucial for the interaction with Ras-specific GEFs, since mutations such as E62H, E62K, E63H, and E63K are completely insensitive toward Sdc25 and Cdc25 (31, 33, 34). The structure of the Ras-Sos binary complex has also indicated that residues in the switch II region are responsible for the interactions disturbing the phosphate moiety of the nucleotide binding site. Here we designed most mutations within the switch II region of Rap1A or Ras such that the amino acid of Ras was introduced in Rap1A and vice versa. In general we find that residues in the switch II region of Rap1A such as Phe64 and Leu70 are similarly important for the catalytic interaction with C3G, whereas Thr61 is not.
Two amino acids were strikingly different in their interaction with C3G versus Cdc25Mm. Mutation of T61 of Rap1A to Q (as in Ras) did not influence the activity of C3G (Fig. 2B) but was found to introduce a recognition site for Cdc25Mm in Rap1A (Fig. 3A). This suggests that Gln61 is required for activation by Cdc25Mm, whereas C3G does not need Thr61. This idea was strengthened by the finding that Ras(Q61T) and Ras(Q61L) are both poor substrates for the Cdc25Mm-catalyzed mGDP release (Fig. 3B and Table II) in line with earlier observation on Sdc25 (31). Introduction of the corresponding Rap1A residue at position 61 to produce Ras(Q61T) did not induce C3G activity, again showing that this residue is unlikely to interact with C3G. When trying to shift the specificity of C3G and induce a reactivity toward Ras, we find that the introduction of Rap-to-Ras mutations in residues 63, 64, and 65 in Rap(T61Q) does not increase its sensitivity toward Cdc25Mm (Table I).
Mutations of residue 70 point to another difference in the recognition sites for C3G and Cdc25Mm. Rap1A(L70Q) completely blocks the GEF activity of C3G (Fig. 2B, Table I), indicating that Leu70 is important for the interaction of Rap1A with C3G, and Gln is unable to substitute the function. This was supported by the finding that Ras(Q70L) is a much better substrate for C3G than wild-type Ras (Fig. 3B, Table II). In contrast, mutation of residue 70 does not affect the interaction of Cdc25Mm with Rap1A and even increases the activity toward Ras(Q70L) almost 2-fold, indicating that residue 70 has an important role in the interaction with C3G but not as much with Cdc25Mm. Remarkably, a leucine at position 70 appears to be favorable for both reactions. Mutations of other switch II residues in Ras showed that, unlike the mutations E63H and E63K, E63A shows only a 2-fold decrease of the stimulatory action of Cdc25Mm on Ras. Furthermore the E63D mutation in Ras induces a 2-fold higher activity in the Cdc25Mm reaction than wild-type, similar to Q70L.
Mutations in the 100-110 Region--
Mutations of residues in the
100-110 region have similar effects on the Rap1A or Ras exchange
reaction: mutation of the variable residue at position 101 had no
obvious effect, whereas mutation of the conserved Arg102 to
leucine blocked the GEF activity with either C3G or
Cdc25Mm. Sequence alignment shows the most obvious sequence
difference lies in the Rap-DTED motif versus DSDD in Ras
(residues 105-108, Fig. 1). Introduction of DTED into Ras had no
effect on catalysis by either Cdc25Mm or C3G. However, this
same stretch of amino acids introduced into Rap2A did have a
major impact on recognition by C3G (see below and Fig.
4).
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Rap2A Mutants--
Previously we had shown that despite the high
level of homology between Rap1A and Rap2A, C3G specifically recognizes
Rap1 as a substrate and not Rap2, even with high concentrations of the
GEF (4). As expected Rap2A is also not recognized by
Cdc25Mm. Therefore we made mutations in Rap2A, in which we
replaced residues in Rap2A in the switch II and 100-110 regions, by
the corresponding residues of Rap1A. Due to the small amount of
variable residues, we could design Rap2A to almost completely look like
Rap1A and study whether C3G was now able to recognize the chimeric
Rap2A. Interestingly, a double mutation of residues 65 and 66 (switch II region) already resulted in the 11-fold stimulation of the nucleotide release by 1 µM C3G (Table III). A similar
effect was observed when the differing residues in region 100-110 were
mutated (105 + 106 + 108 = DTED). Even more dramatic was the
combination of these two regions: the guanine nucleotide exchange of
this Rap2A mutant could now be stimulated 83-fold by 1 µM
C3G. These results clearly show that both the Switch II and the
100-110 regions are necessary for efficient activation by C3G and that
they synergistically contribute to the catalytic reaction.
Surprisingly, this Rap2A mutant is now also a weak substrate for
Cdc25Mm, in contrast to wild-type Rap1A or Rap2A.
Considering the alignment of residues in the three regions studied no
easy explanation for this observation is available. The most drastic
difference between Rap1A and Ras versus Rap2A appears to be
residue 39, which is Ser in Ras and Rap1A and Phe in Rap2A (Fig. 1),
and is located adjacent to the Tyr40 shown to be crucial
for the Ras-Cdc25Mm reaction.
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DISCUSSION |
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In the past years, the interaction of Ras and its nucleotide exchange factors Cdc25/Sos/Sdc25 has been extensively studied by mutagenesis (30-40). From these studies three regions in Ras were found to contribute to the GEF-mediated nucleotide release. The three-dimensional structure of the Ras-Sos complex, which we assume to be similar to the Ras-Cdc25 complex in its basic features, has principally confirmed the involvement of these three regions in complex formation, with residues 60-73 supplying the most important contacts (25). In this study we make a mutational analysis of these three regions and compared Ha-Ras and Rap1A residues that are needed for the interaction with their GEF. On a global scale we find that C3G, for which a fold similar to Sos/Cdc25 can be assumed due to the albeit limited but significant sequence conservation, is likely to interact with Rap in a similar way as Ras with Sos or Cdc25Mm, since most mutations in these three regions affect the catalytic reaction.
Although only limited sequence specific contacts between the switch I area of Ras and Sos have been identified in the structure, residues of switch I such as Thr35 and Asp38 are found biochemically to be important both for the Ras-Cdc25Mm and the Rap1-C3G catalytic reaction. Ras residues Tyr32, Pro34, Glu37, and Tyr40 were found to directly interact with Sos, Gln25, Val29, Glu31, Asp33, Thr35, and Ser39 were positioned at the interface between Ras and Sos (25). These data fit with our observations. Studying the effect of switch I mutations is additionally important since T35S, E37G, or Y40C are extensively used as partial loss-of-function mutations in the study of Ras pathways (45-50), and their interactions with GEF have only partially been analyzed.
In this paper we show that the T35S and E37G mutations similarly reduce
the interaction of Ras and Rap1A with their respective GEFs by a factor
of 2-4. As noticed before,4
Y40C proved to be a particularly interesting mutation in that it
strongly increases the Cdc25Mm-mediated effect on Ras,
decreased the specificity of Cdc25Mm toward Rap1A by
inducing partial reactivity, but did not influence the Rap1A-C3G
interaction itself. Tyr40, which is conserved between Ras,
Rap1A and Rap2A, contacts His911 in the Ras-Sos structure
and forms an interaction mediated between the amino and the aromatic
group (Fig. 5) (52). Sequence alignment between Sos, Cdc25Mm, and C3G (Fig.
6) shows that His911 is
homologous to Arg1158 in Cdc25Mm and
Ser966 in C3G. Whereas Arg could still form an
amino-aromatic interaction with Tyr40 in a similar way, the
nature of a Tyr-Ser interaction would appear to be different,
indicating a structural difference in the interface of the
Ras-Sos/Cdc25Mm versus the Rap-C3G interaction.
How the replacement of Tyr40 by Cys leads to an increase in
the reaction rate needs to be analyzed in more detail, since the
overall reaction involves a number of partial reactions. A possible
explanation could be a decrease in the affinity of the binary
Ras-Sos/Cdc25 complex. The use of Ras(Y40C), which is believed to
interact mostly with PI (3)kinase but not with Raf and RalGEF, thus
introduces an additional complication into unraveling signal
transduction pathways via Ras, as it has a drastically increased
reactivity toward Cdc25Mm, and possibly also Sos, and would
thus be expected to be activated preferentially over wild-type Ras or
other Ras mutants. Our study clearly show that the exchange factor
interaction is, depending on the mutation, strongly or weakly disrupted
by the partial loss-of-function mutations. Consequently, the GTP
loading of Ras effector mutants in vivo is likely to be
variable between the partial loss-of-function mutations, which should
be taken onto account considering the observed biological effects,
which may need a different threshold concentration of activated
Ras-GTP.
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The structure of the Ras-Sos complex and the number of mutations and their effect on catalysis have shown that the switch II region of Ras is the most crucial site of the Ras-GEF interaction (25, 31, 33, 34; this paper). Sos interacts directly with Asp57, Ala59, Gly60, Gln61, Glu63, Tyr64, Ala66, Met67, Gln70, Tyr71 of the switch II region and Thr58, Glu62, Ser65, Arg68, and Asp69 are close to the interface (25). The data presented here confirm and extend the importance of some of these residues for both the Ras and the Rap system and also show significant differences. The nature of residue 61 in Rap1A was found to be important Cdc25Mm but only marginally so for C3G (Fig. 5). The Ras-Sos structure shows that Gln61 is involved in a donor hydrogen bond to Thr935 and the main chain CO of Gly931 of Sos and accepts a H-bond from Tyr71 from Ras and that Tyr71 is embedded in a hydrophobic pocket formed by residues in Ras and Sos and also forms a direct hydrogen bond to Tyr912. Thr935 is highly conserved in RasGEFs (Fig. 6) such as human RasGRF1 and 2, Sos, and in the Ral-specific GEFs RalGDS and Rgl (but not Rlf) and is found as Ser in CDC25 and SDC25 from yeast. The residue analogous to Thr935 in the Rap-specific GEF C3G is Gln690, and in Rap Gln61 is replaced by Thr, producing an analogous exchange in the interacting pair of Gln and Thr. Since Tyr71 is conserved between Ras and Rap proteins, the hydrogen bond network seen in the Ras-Sos structure would be similar in the Rap-C3G binary complex. The mutational data suggest, however, that either this network does not exist in the Rap-C3G complex or that its contribution to catalysis is distinctly different from the Ras-GEF interaction, maybe due to the different nature of the hydrophobic pocket surrounding Tyr71.
Mutation of residue 63 in Ras from Glu to His or Lys has been reported to abolish the interaction with yeast exchange factors SDC25 and CDC25 and with SOS (31, 33, 34, 40). Our results here show that E63A only slightly affects the interaction of Cdc25Mm, whereas E63D even weakly increases the activity. The Ras-Sos structure showed that Arg826 from Sos directly interacts with Glu63, and the single and double mutants in Rap1A suggest that residues 63 and 64 are both important for the Rap1A-C3G interaction. No mutational analysis of residue 64 in Ras has been performed, but the Ras-Sos structure shows a hydrophobic interaction of Tyr64 with residues Ile825 and Phe929 from Sos. From our Rap2A mutant proteins we can conclude that at least one of the residues Thr65 or Ala66 is important for recognition by C3G. In the Ras-Sos structure these residues are located in the interface without making any direct contact pointing at an indirect effect of the mutations or at a difference with the Rap-C3G interaction. Finally, we also studied mutants of amino acid 70 in both Ras and Rap1A. This proved to be very interesting, since the nature of residue 70 was found to be very important for C3G, but not for Cdc25Mm. In the Ras-Sos structure, Gln70 interacts with Ser876 in Sos, a conserved residue in Sos, Cdc25Mm, and C3G (Fig. 6). Hence, an explanation for this deviation is not easily found.
For the region encoding amino acids 100-110, we show that the single mutations of residue 101 in both Ras and Rap1A does not interfere with the GEF activity. This mutation had only been tested in combination with other mutations in Ras before (37, 38). Residue 102 is, however, important for recognition both by the Ras- and Rap1A-specific GEFs, supporting the structural observation where Arg102 directly interacts with Sos (25). The structure also revealed that Asp105 was located in the interface and that other amino acids from this region did not contribute to the interaction. Our mutant analysis showed residues 105, 106, and 108 to be important for the C3G-Rap1A, but not equally important for the Ras-Cdc25Mm interaction, although point mutations have not been tested.
We thus obtained evidence that many mutations have similar effects on the Ras and Rap exchange reaction, whereas others are important for C3G, but not for Cdc25Mm or vice versa. Ras and Rap have the same three-dimensional structure (9, 53), and C3G and Cdc25 most likely have a fold very similar to Sos (25). When the Ras and Rap mutations affecting catalysis are plotted onto the surface of Ras and Rap, they are all localized in what would be the interface of the Ras-Sos structure, as shown in Fig. 5. The color coding pattern, which is related to the importance of residues for the exchange reaction, indicates that the different GTP-binding proteins and their respective GEFs interact with each other using the same overall interface but different specific interactions which supports the notion that residues on Sos involved in the interaction with Ras are not conserved in all proteins with this topology, not even between Sos and Cdc25Mm, much less even between Sos and C3G. We thus would expect that differences exist in the kinetic and molecular mechanism of the GEF-catalyzed nucleotide exchange reaction of Ras with either Sos and Cdc25Mm and between Cdc25Mm and C3G. Our analysis has only measured the overall multiple turnover reaction which consists of several kinetic steps (2, 31, 54). A definite model for the mechanism of guanine nucleotide exchange can thus only be derived by solving the structures of the respective binary (or possibly ternary) complexes and from a more detailed kinetic analysis.
It was interesting to observe that certain mutations such as Y40C or Q70L increased the overall rate of the exchange reaction, indicating that the GEF-catalyzed exchange reaction is not optimized for fast exchange and that slight modifications of the interface can considerably increase the overall catalytic rate. We have observed before that the affinity of Cdc25Mm for Ras-GDP is in the 400 µM range (2), which is probably low enough to prevent Cdc25 Mm from accidentally activating Ras and inducing futile GTPase cycles. The GEF reaction on Ras is initiated by the recruitment of GEF to the plasma membrane where Ras is located, which would mean that the catalytic efficiency in solution is kept low and explains the likeliness of finding mutants in either the GEF (55) or in the GTP-binding proteins (this paper) that increase the overall rate of the reaction.
Finally our results show that the mechanistic details of nucleotide
exchange might be sufficiently different between the Ras and the Rap
system such that extrapolating from one system to the other is not
suitable. This was demonstrated by the finding that contrary to
Ras(S17N) on Cdc25 Mm and Sos, Rap(S17N) is not inhibitory
to the C3G-mediated guanine nucleotide exchange reaction on Rap1A (4).
Since Rap(S17N) has been used to delineate the effect of Rap1 on B-Raf
and MAP kinase activation in PC12 cells, this biological activity of
Rap1 is unlikely be mediated via the C3G pathway (20, 21). Two new
Rap1-specific GEFs have very recently been cloned (56, 57), but the
effect of Rap(S17N) on these still needs to be tested. For the
C3G-dependent pathway it will be necessary to find dominant negative mutants of Rap by an approach, unbiased by the findings for
Ras and other GTP-binding proteins, with either site-directed mutagenesis in the regions outlined here or by a random approach. Such
a mutant would be extremely important to outline the signal transduction pathway of Rap1 via C3G, in a similar way as Ras(S17N) has
been used to map Ras signal transduction pathways.
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ACKNOWLEDGEMENT |
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We thank Doro Vogt, Jochen Linz, Astrid Kramer, and Christiane Theiss for purification of proteins, Michael Hess for help with graphical presentation, and Rita Schebaum for secretarial assistance. We are grateful to Jean de Gunzburg for the Rap2A expression vectors and John Kuriyan for kindly supplying the coordinates of the Ras-Sos complex.
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FOOTNOTES |
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* This work was supported by a european community BioTech Grant Bio4CT961110.The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Both authors contributed equally to this work.
§ Received a Training and Mobility of Researchers-Stipend of the european community.
¶ To whom correspondence should be addressed: Max-Planck-Institut für molekulare Physiologie, Abteilung Strukturelle Biologie, Rheinlanddamm 201, 44139 Dortmund, Germany. Tel.: 49-231-1206280; Fax: 49-231-1206230; E-mail: alfred.wittinghofer{at}mpi-dortmund.mpg.de.
2 C. Block, unpublished results.
3 J. L. Bos, unpublished results.
4 R. H. Cool, G. Schmidt, C. Lenzen, H. Prinz, D. Vogt, and A. Wittinghofer, submitted for publication.
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ABBREVIATIONS |
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The abbreviation used is: GEF, guanine nucleotide exchange factor. Cdc25Mm is the catalytic domain of the mouse Cdc25 Ras guanine nucleotide exchange factor (1) consisting of 285 residues as described earlier (2). C3G is the catalytic domain of the human Rap GEF C3G (3) consisting of 249 residues as described earlier (4). Mutations are indicated with the original and modified amino acid using the single letter code, such that Ras(Y40C) designates Ras where tyrosine 40 has been replaced by cysteine. mGDP is the fluorescent analogue of GDP analogue carrying the N-methylanthraniloyl (m) group on the 2'- or 3'-hydroxy group of the ribose.
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