GaGPCR Interactions

Although the mechanism by which receptors activate G proteins is beyond the scope of this review, some mention of Ga-GPCR recognition is in order. The structure of one GPCR, rhodopsin, has been reported, in an inverse-agonist-bound (i.e., resting) state [58]. The structural basis of the heterotrimer-receptor interaction is known only from extensive mutagenesis and cross-linking studies [3,59-61], particularly of the rhodopsin-Gat interface. These studies point to the a4-p6 loop and the C-terminal helix, a5, of Gat. Mutations of residues located at the inward face of the a5 helix of Gat dramatically increase receptor-independent rates of nucleotide release [62]. Mutation of a conserved alanine residue within the purine-contacting a4-p6 loop

Figure 6 The complex of Gp^Gpj is colored yellow, except for the second (from the N-terminus) WD repeat, which is rendered in orange. The four-stranded antiparallel p "blades" that comprise the propeller fold are numbered. Individual strands in one repeat are lettered (a) through (d), in order of sequence. The Gy subunit is green. The amino termini of both sub-units are labeled.

Figure 7 The complex of Ga^GDP with Gp1y2. GP1 and Gy2 are colored as in Figure 6. The sidechain of tryptophan 99 in GP., shown as a stick model, is prominent in the interface with Ga^, which is rendered in charcoal. The switch regions of Ga^ are red, and GDP is shown as a ball-and-stick model.

preceding a5 increases the intrinsic nucleotide exchange rate and also reduces the thermostability of Gaü [63,64]. Some residues that affect receptor coupling are located in the Ga-Gß interface but distant from the putative receptor binding surface, suggesting that Gßy plays a direct role in GPCR-mediated nucleotide exchange.

The mechanism by which GPCRs catalyze nucleotide exchange from Ga remains one of the more puzzling mysteries in the structural biology of signaling. Crystal structures of receptors bound to heterotrimeric G proteins, in a spectrum of functional states, will eventually be determined and will provide some, but probably not complete, insight into receptor function. Equally important is the need to understand the organization and dynamic behavior of G protein signaling complexes at the cell membrane. The tools required for such investigations are still being developed (see http://www.cellularsignaling.org/).

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Structure and Function of G-Protein-Coupled Receptors: Lessons from the Crystal Structure of Rhodopsin

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