Ras:SOS GEF complex
(1bkd)

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I. Introduction

info on Ras:SOS GEF complex.

For more information see Biochemistry Online: Chapter 9C-7 - Small G proteins, GAPs and GEFs

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II. General Structure

The superfamily of small G proteins have a common 20K molecular weight catalytic domain with 5 alpha helices, 6 beta strands and connecting loops.  The G1 loop binds phosphate groups as is often called the P-loop (as in the case of protein tyrosine phosphatases).  Its binds GTP and GDP and has GTPase activity.  The small G proteins are "active" in the GTP bound form.  Hydrolysis of GTP to GDP causes the protein to become inactive in signaling pathways.

cartoon: Ras (yellow); SOS (green)

Important parts of Ras necessary for GTP binding:

- phosphate-binding (P loop), residues 10 to 16;

- switch regions I (30 to 37) and II (60 to 76), which are flexible loops  which sandwich GTP;

P loop (red); Switch I (orange); Switch II (blue)