G protein-coupled receptors (GPCRs) comprise the most 'prolific' family of cell membrane proteins, which share a general mechanism of signal transduction, but greatly vary in ligand recognition and function. Crystal structures are now available for rhodopsin, adrenergic, and adenosine receptors in both inactive and activated forms, as well as for chemokine, dopamine, and histamine receptors in inactive conformations. Here we review common structural features, outline the scope of structural diversity of GPCRs at different levels of homology, and briefly discuss the impact of the structures on drug discovery. Given the current set of GPCR crystal structures, a distinct modularity is now being observed between the extracellular (ligand-binding) and intracellular (signaling) regions. The rapidly expanding repertoire of GPCR structures provides a solid framework for experimental and molecular modeling studies, and helps to chart a roadmap for comprehensive structural coverage of the whole superfamily and an understanding of GPCR biological and therapeutic mechanisms.
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