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PDBsum entry 2ije
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Signaling protein
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PDB id
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2ije
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References listed in PDB file
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Key reference
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Title
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A ras-Induced conformational switch in the ras activator son of sevenless.
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Authors
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T.S.Freedman,
H.Sondermann,
G.D.Friedland,
T.Kortemme,
D.Bar-Sagi,
S.Marqusee,
J.Kuriyan.
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Ref.
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Proc Natl Acad Sci U S A, 2006,
103,
16692-16697.
[DOI no: ]
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PubMed id
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Abstract
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The Ras-specific guanine nucleotide-exchange factors Son of sevenless (Sos) and
Ras guanine nucleotide-releasing factor 1 (RasGRF1) transduce extracellular
stimuli into Ras activation by catalyzing the exchange of Ras-bound GDP for GTP.
A truncated form of RasGRF1 containing only the core catalytic Cdc25 domain is
sufficient for stimulating Ras nucleotide exchange, whereas the isolated Cdc25
domain of Sos is inactive. At a site distal to the catalytic site,
nucleotide-bound Ras binds to Sos, making contacts with the Cdc25 domain and
with a Ras exchanger motif (Rem) domain. This allosteric Ras binding stimulates
nucleotide exchange by Sos, but the mechanism by which this stimulation occurs
has not been defined. We present a crystal structure of the Rem and Cdc25
domains of Sos determined at 2.0-A resolution in the absence of Ras. Differences
between this structure and that of Sos bound to two Ras molecules show that
allosteric activation of Sos by Ras occurs through a rotation of the Rem domain
that is coupled to a rotation of a helical hairpin at the Sos catalytic site.
This motion relieves steric occlusion of the catalytic site, allowing substrate
Ras binding and nucleotide exchange. A structure of the isolated RasGRF1 Cdc25
domain determined at 2.2-A resolution, combined with computational analyses,
suggests that the Cdc25 domain of RasGRF1 is able to maintain an active
conformation in isolation because the helical hairpin has strengthened
interactions with the Cdc25 domain core. These results indicate that RasGRF1
lacks the allosteric activation switch that is crucial for Sos activity.
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Figure 5.
Fig. 5. The clamping of the helical hairpin. (a) View of
RasGRF1 showing the helical hairpin (red), flap1, and flap2
(both gray). (b) A cutaway view through the catalytic Ras
binding site of RasGRF1. A tight interface between flap1 and the
helical hairpin of RasGRF1 is formed by bulky, hydrophobic
residues (Phe-1052, Phe-1051, and Tyr-1048 in flap1, Ile-1214,
and Ile-1210 in the helical hairpin). A salt-bridge network and
hydrophobic interactions connect the helical hairpin with flap2
(Met-1181 and Phe-1188 bury Asp-1185 in the helical hairpin,
bridging to Arg-1160 and Arg-1165 in flap2). (c) In the active
conformation of Sos, the helical hairpin (dark blue) is similar
in position to that of RasGRF1, but the interface with flap1 is
not well packed (Val-805, Leu-804, and Pro-801 in flap1, Thr-964
and Val-968 in the helical hairpin). (d) In the absence of
allosteric Ras binding, the helical hairpin of uncomplexed Sos
(light blue) collapses inward to interact more closely with
flap1. Neither active nor inactive Sos helical hairpins form
close interactions with flap2 (Lys-939, Ile-932, and Asn-936 in
the helical hairpin do not form contacts with His-911 and
Leu-916 in flap2).
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Figure 6.
Fig. 6. Computational study of the effects of swapping
residues from RasGRF1 and Sos. The number of times a given
residue accumulated a conformation-stabilizing mutation in
low-energy sequences from 100 separate Monte Carlo simulations
is described by the substitution frequency. (a and b) C[  ]positions for buried
residues that are swapped with high frequency are indicated
(spheres) for Sos (a) and RasGRF1 (b). (c and d) Several Sos
residues that substitute with high frequency are located in the
flap1-helical hairpin interface (see also Fig. 5). (c) Wild-type
Sos. (d) Substitutions from RasGRF1.
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