Visual Rhodopsin Sees the Light: Structure and Mechanism of G Protein Signaling*

The availability of crystal structures for the dark, inactive, and several light-activated photointermediate states of vertebrate visual rhodopsin has provided important mechanistic and energetic insights into the transformations underlying agonist-dependent activation of this and other G protein-coupled receptors (GPCRs). The high natural abundance of rhodopsin in the vertebrate retina, together with its specific localization to the disk membranes of the rod cell, has also enabled direct imaging of rhodopsin in its native environment. These advances have provided compelling evidence that rhodopsin, like many other GPCRs, forms highly organized oligomeric structures that, in all likelihood, are important for receptor biosynthesis, optimal activation, and signaling.