Common and distinct mechanisms of activation of rhodopsin and other G protein-coupled receptors

Detailed and systematic examination of high-resolution structural data is a rational strategy for understanding the function of biological macromolecules. G protein-coupled receptors (GPCRs) are an exceptionally valuable superfamily of proteins for such analysis. The most intriguing question is how a variety of extracellular stimuli evoke structural changes in the intracellular surface of the receptors. The recent active-like crystal structures of GPCRs provide information for uncovering common and distinct mechanisms of light-induced and ligand-induced activation. Based on systematic structural alignment, we have analyzed 3 receptors (rhodopsin, β 2 adrenergic receptor, adenosine A 2A receptor) and demonstrate that the extracellular movement of helix VI is significantly different between rhodopsin and the other 2 receptors and that the extracellular side of helix III exhibits distinct features in the 3 receptors. These findings not only emphasize the specialization of rhodopsin as a photoreceptor but also provide insights into the mechanism leading to rearrangement of helix VI.