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

Abstract

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. G Protein-coupled ReceptorsG protein-coupled receptors (GPCRs) 3 are by far the largest class of cell surface signal transducing receptors. These heptahelical integral membrane proteins are of immense importance in human physiology and health as evidenced by the fact that virtually every cell type expresses a subset of GPCRs. With the sequencing of the human genome complete, it is now abundantly clear that upwards of 950 genes encode GPCRs (1). The mammalian GPCRs are typically grouped by similar amino acid sequences into the three distinct families: A, B, and C. For most GPCRs, the external stimulus (agonist) leading to receptor activation is a small molecule that binds to a region of the transmembrane (TM) domain and triggers conformational changes that are transmitted to the cytoplasmic surface of the receptor to facilitate the activation of intracellular heterotrimeric guanine nucleotide-binding proteins (G proteins). Although it is widely believed that most GPCRs transduce their activating signals via heterotrimeric G proteins, recent studies have also highlighted a role for G protein-independent signaling by GPCRs (2). Because GPCRs modulate an extremely wide range of physiological processes, it is not surprising that mutations in the genes encoding many of these receptors have been implicated in numerous disease states. As such, these receptors also form the largest class of therapeutic targets.In vertebrate vision, rod cell rhodopsin, a Family A GPCR member, serves as the dim-light receptor. Remarkable advances in our understanding of rhodopsin structure, function, and signaling that remain at the forefront of GPCR research have been realized in recent years. Here, we review developments in rhodopsin research with regard to GPCR structure and activation, its propensity to form higher order oligomers, and the structural basis for the interaction with heterotrimeric G proteins. Rod Outer Segment and RhodopsinRetinal rod cells are specialized neurons that detect photons and communicate with secondary neurons about the presence of light. They are so exquisitely sensitive that even a single photon can be detected (3). Rods have highly differentiated outer segments (ROS) connected to their inner segments. In the ROS, hundreds of stacked disk membr...

Multiple Roles for the C-terminal Tail of the Chemokine Scavenger D6

“…7). An 8th helix is present in the crystal structure of bovine rhodopsin, anchored by the NPXXY motif at the end of the 7th helix (36,37) and undergoes a marked shift in location after rhodopsin activation (38,39). The recent crystal structures of the human ␤ 2 -adrenergic receptor have an 8th helix underlying the membrane perpendicular to the transmembrane helices which is stabilized by a hydrophobic surface (40 -43).…”

Inherent Instability of the Retinitis Pigmentosa P23H Mutant Opsin

“…4, D and E). These changes were attributed to a 9-cis to all-trans chromophore isomerization and deprotonation of the Schiff base linkage upon photoactivation (46). Changes in absorption at the maxima of 488 nm and 479 nm then were plotted as a function of time and fitted to an exponential decay formula, yielding halflives for WT isorhodopsin and P23H isorhodopsin of 5 and 15 s, respectively (Fig.…”

Thermal Properties of Rhodopsin

“…It is expressed in the disc membranes in the outer segment of rod photoreceptor cells (1)(2)(3). It has a seven-helical transmembrane structure and incorporates the 11-cis-retinal chromophore via a protonated Schiff base (SB) 6 in the transmembrane region (3,8,9).…”

“…Rhodopsin is a widely studied G protein-coupled receptor responsible for generating visual signals in dim-light environments (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11). It is expressed in the disc membranes in the outer segment of rod photoreceptor cells (1)(2)(3).…”