THE CRYSTALLOGRAPHIC STRUCTURE OF A G PROTEIN HETEROTRIMER. Stephen R. Sprang, Mark A. Wall, David E. Coleman, Ethan Lee, Jorge A. Iniguez-Lluhi, Bruce A. Posner, Alfred G. Gilman, Departments of Biochemistry and Pharmacology, Howard Hughes Medcial Institute, University of Texas Southwestern Medical Center, Dallas, TX, 75235, USA

We have determined the three-dimensional structure of the signal transduction complex comprising a heterotrimer of the GDP-bound ((Gi-(), (-1 and [[gamma]]-2 subunits at a resolution of 2.4 Å. In this state, the ( subunit and the (- [[gamma]] dimer are maintained in a non-signalling complex. Activated, heptahelical receptors promote the disassociation of this heterotrimeric complex by triggering the release of GDP from the ( subunit. The structure reveals two nonoverlapping regions of contact between ( and (, an extended interface between ( and nearly all of [[gamma]], and limited interaction of (with [[gamma]]. The major (/( interface covers a flexible helix (switch II) in the ( subunit which adopts different conformations in the GDP and GTP-bound states. The ( subunit stabilizes a conformation of the switch II helix that traps GDP in the binding site of the ( subunit. The (-( interface contains a large, negatively charged surface that may offer electrostatic complementarity to the cytoplasmic domains of the activated receptor. The helical amino terminus of the alpha subunit forms a second region of contact with the "side" of the ( subunit. Repeated "WD" motifs in ( form a circularized seven-fold beta propeller, which has been observed in a variety of unrelated proteins. In the WD family, the conserved cores of these motifs may serve as scaffolds for display of variable linkers on the exterior face of each propeller blade. The structure was determined by a combination of MIR and molecular replacement methods and has been refined to an R-factor of 0.24 (R_free=.32).