REFINED STRUCTURES OF HETEROTRIMERIC G PROTEIN COMPLEXES AT HIGH RESOLUTION. John Sondek^*, Andrew Bohm^*, David G. Lambright^#, Nikolai P. Skiba^{[[daggerdbl]]}, Joseph P. Noell^{[[paragraph]]}, Heidi E. Hamm^{[[daggerdbl]]}, Paul B. Sigler^*, ^*Dept. of Molecular Biophysics & Biochem. and Howard Hughes Medical Institute, Yale University, New Haven, CT 06510, ^{[[daggerdbl]]}Dept. of Physiology & Biophysics, U. of Illinois, Chicago IL 60612 ^#Prog. in Mol. Med., U. of Massachusetts Med. Center, Worchester MA ^{[[paragraph]]}Structural Bio. Lab., The Salk Institute, La Jolla, CA 92037

Many signaling cascades use heterotrimeric G proteins coupled to seven-helical transmembrane receptors to convert extracellular signals into intracellular responses. Upon nucleotide exchange catalyzed by activated receptors, heterotrimers dissociate into G[[alpha]].GTp subunits and G_{t[[beta]][[gamma]]} dimers, either of which can modulate numerous downstream effectors. The crystal structures of the trimeric (G_{t[[alpha]][[beta]][[gamma]]}), dimeric (G_{t[[beta]][[gamma]]}), and three monomeric forms (G_t[[alpha]].GDP, G_t[[alpha]].GTP[[gamma]]S, G_t[[alpha]].GDP.AlF₄^-) of transducin, the heterotrimeric G protein involved in the visual pathway, have been solved and refined to high resolution.

The heterotrimeric form of transducin, G_{t[[alpha]][[beta]][[gamma]]}, reveals the mechanism of the nucleotide dependent engagement/disengagement between the [[alpha]] and [[beta]][[gamma]] subunits that regulates their interaction with receptor and effector molecules. The interaction involves two distinct interfaces and dramatically alters the conformation of the [[alpha]] but not the [[beta]][[gamma]] subunits. The location of the known sites for post-translational modification and receptor coupling suggest a plausible orientation for the heterotrimer with respect to both the membrane surface and the activated heptahelical receptor.

Multiwavelength anomalous diffraction data were used to solve the crystal structure of G_{t[[beta]][[gamma]]} . The [[beta]] subunit is primarily a seven-bladed [[beta]]-propeller that is partially encircled by an extended [[gamma]] subunit. The [[beta]]-propeller, which contains seven structurally similar WD-repeats, defines the stereochemistry of the WD-repeat and the probable architecture of all WD-repeat containing domains. The structure details interactions between G protein [[beta]] and [[gamma]] subunits and highlights regions implicated in effector modulation for the conserved family of G protein [[beta]][[gamma]] dimers.