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Figure 1.
Fig. 1. Sequential organization of SOS and its four homology
segments. A, the hSOS1 sequence (10) contains a DbH domain (5),
followed by a PH domain (reviewed in Shaw (1)), an intervening
stretch (551-793), the CDC25 Ras-activating domain, and a
proline-rich segment (PP), associated with Grb2 binding (30).
The sequence^ expressed and structurally determined here is the
gray patch. B, structure-based alignment of PH domains using
pairwise superposition of the structures and direct calculation
of aligned RMSDs, based^ on elements of secondary structure,
optimized by addition of deletion of individual residue pairs.
Structures are hSOS1 (this work), GRK-2/  ARK-1 (D.
Fushman, T. Najmaabadi-Haske, S. Cahill, J. Zheng, H. LeVine
III, and D. Cowburn, J. Biol. Chem., in press), dynamin (31),
spectrin (18), pleckstrin (24), and PLC  (15). The^
color coding corresponds to the secondary structure elements in
Fig. 2 and the binding site are marked in red and underlined. C,
predicted helical segments of the DbH domain of hSOS1, using
programs DSC (17). The numbers in blocks below the sequence,
labeled P_H at the left, are the deciles of the probability that
the individual residue is in an  -helix. D,
alignment of the C-terminal segment of multiple DbH domains, and
the predicted C-terminal -helical
portion. The program CLUSTAL W (32) was used to perform the
multiple alignment. Sequences are (GI = GenBankTM accession no.)
CDC24 (GI1345705), DBL (GI118279), ECT2 (GI423597), FGD1
(GI1706789), LBC (GI458210), LFC (GI1582805), LSC (GI1389756),
OST (GI1083745), RasGRF (GI1083745), TIAM1 (GI897557), TIM
(GI484102), and VAV (GI586213) P[lwen]H, the decile of the^
probability that the individual residue in part of an -helix (16).
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