Complexes between photoactivated rhodopsin and transducin: progress and questions

Abstract

Activation of GPCRs (G-protein-coupled receptors) leads to conformational changes that ultimately initiate signal transduction. Activated GPCRs transiently combine with and activate heterotrimeric G-proteins resulting in GTP replacement of GDP on the G-protein α subunit. Both the detailed structural changes essential for productive GDP/GTP exchange on the G-protein α subunit and the structure of the GPCR–G-protein complex itself have yet to be elucidated. Nevertheless, transient GPCR–G-protein complexes can be trapped by nucleotide depletion, yielding an empty-nucleotide G-protein–GPCR complex that can be isolated. Whereas early biochemical studies indicated formation of a complex between G-protein and activated receptor only, more recent results suggest that G-protein can bind to pre-activated states of receptor or even couple transiently to non-activated receptor to facilitate rapid responses to stimuli. Efficient and reproducible formation of physiologically relevant, conformationally homogenous GPCR–G-protein complexes is a prerequisite for structural studies designed to address these possibilities.

Regulators of G-Protein-Signaling Proteins: Negative Modulators of G-Protein-Coupled Receptor Signaling

“…In vitro studies confirmed that Ret-RGS1 act as a GAP for transducina (Ga t ), a heterotrimeric G protein that plays a central role in vertebrate phototransduction, in cell-free conditions (Faurobert and Hurley, 1997;Jastrzebska et al, 2010). Photoactivation of rhodopsin promotes Ga t binding to GTP and hydrolysis, switching to the active form.…”

The Significance of G Protein-Coupled Receptor Crystallography for Drug Discovery

“…In addition to G␣-controlled events, the G␤␥ subunits also can regulate their own effectors, including additional forms of adenylate cyclase as well as ion channels. The ramifications of signaling complexity implicit in the full range of combinatorial permutations within the heterotrimeric complex itself have yet to be fully examined (Jastrzebska et al, 2010).…”

“…Upon close examination, there is a lack of concordance in the degree of structural changes observed upon activation in the recently determined agonist-bound GPCR structures that probably reflects the structural plasticity needed for the varying levels of activity and regulation of the agonist induced response. Additional insight into the functional mechanics of these targets will likely be afforded by determining structures of important signaling states involving not only agonist-bound receptor but also G protein (Jastrzebska et al, 2010).…”

Differential Light-induced Responses in Sectorial Inherited Retinal Degeneration

“…In its basal resting state, it consists of a polypeptide chain, opsin, and its cognate chromophore, the vitamin A aldehyde derivative 11-cis-retinal, which acts as an inverse agonist preventing G-protein activation in the dark (3,11). The retinal chro-mophore is responsible for light absorption in the visual process (12), and, upon photon capture, it isomerizes to its all-trans configuration yielding the active Rh photointermediate metarhodopsin II (MetaII) capable of binding and activating the G-protein transducin (Gt) (13)(14)(15).…”