Metarhodopsin-II stabilization by crosslinked Gtalpha C-terminal peptides and implications for the mechanism of GPCR-G protein coupling

Signal protein-derived peptides as functional probes and regulators of intracellular signaling

The functionally important regions of signal proteins participating in their specific interaction and responsible for transduction of hormonal signal into cell are rather short in length, having, as a rule, 8 to 20 amino acid residues. Synthetic peptides corresponding to these regions are able to mimic the activated form of full-size signal protein and to trigger signaling cascades in the absence of hormonal stimulus. They modulate protein-protein interaction and influence the activity of signal proteins followed by changes in their regulatory and catalytic sites. The present review is devoted to the achievements and perspectives of the study of signal protein-derived peptides and to their application as selective and effective regulators of hormonal signaling systems in vitro and in vivo. Attention is focused on the structure, biological activity, and molecular mechanisms of action of peptides, derivatives of the receptors, G protein α subunits, and the enzymes generating second messengers.

Complexes between photoactivated rhodopsin and transducin: progress and questions

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 alpha subunit. Both the detailed structural changes essential for productive GDP/GTP exchange on the G-protein alpha 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.

Conserved waters mediate structural and functional activation of family A (rhodopsin-like) G protein-coupled receptors

G protein-coupled receptors with seven transmembrane alpha-helices (GPCRs) comprise the largest receptor superfamily and are involved in detecting a wide variety of extracellular stimuli. The availability of high-resolution crystal structures of five prototypical GPCRs, bovine and squid rhodopsin, engineered A(2A)-adenosine, beta(1)- and beta(2)-adrenergic receptors, permits comparative analysis of features common to these and likely all GPCRs. We provide an analysis of the distribution of water molecules in the transmembrane region of these GPCR structures and find conserved contacts with microdomains demonstrated to be involved in receptor activation. Colocalization of water molecules associating with highly conserved and functionally important residues in several of these GPCR crystal structures supports the notion that these waters are likely to be as important to proper receptor function as the conserved residues. Moreover, in the absence of large conformational changes in rhodopsin after photoactivation, we propose that ordered waters contribute to the functional plasticity needed to transmit activation signals from the retinal-binding pocket to the cytoplasmic face of rhodopsin and that fundamental features of the mechanism of activation, involving these conserved waters, are shared by many if not all family A receptors.

Different properties of the native and reconstituted heterotrimeric G protein transducin

Visual signal transduction serves as one of the best understood G protein-coupled receptor signaling systems. Signaling is initiated when a photon strikes rhodopsin (Rho) causing a conformational change leading to productive interaction of this G protein-coupled receptor with the heterotrimeric G protein, transducin (Gt). Here we describe a new method for Gt purification from native bovine rod photoreceptor membranes without subunit dissociation caused by exposure to photoactivated rhodopsin (Rho*). Native electrophoresis followed by immunoblotting revealed that Gt purified by this method formed more stable heterotrimers and interacted more efficiently with membranes containing Rho* or its target, phosphodiesterase 6, than did Gt purified by a traditional method involving subunit dissociation and reconstitution in solution without membranes. Because these differences could result from selective extraction, we characterized the type and amount of posttranslational modifications on both purified native and reconstituted Gt preparations. Similar N-terminal acylation of the Gtalpha subunit was observed for both proteins as was farnesylation and methylation of the terminal Gtgamma subunit Cys residue. However, hydrogen/deuterium exchange experiments revealed less incorporation of deuterium into the Gtalpha and Gtbeta subunits of native Gt as compared to reconstituted Gt. These findings may indicate differences in conformation and heterotrimer complex formation between the two preparations or altered stability of the reconstituted Gt that assembles differently than the native protein. Therefore, Gt extracted and purified without subunit dissociation appears to be more appropriate for future studies.

Isolation and functional characterization of a stable complex between photoactivated rhodopsin and the G protein, transducin

Transitory binding between photoactivated rhodopsin (Rho* or Meta II) and the G protein transducin (Gt-GDP) is the first step in the visual signaling cascade. Light causes photoisomerization of the 11-cis-retinylidene chromophore in rhodopsin (Rho) to all-trans-retinylidene, which induces conformational changes that allow Gt-GDP to dock onto the Rho* surface. GDP then dissociates from Gt, leaving a transient nucleotide-empty Rho*-Gt(e) complex before GTP becomes bound, and Gt-GTP then dissociates from Rho*. Further biochemical advances are required before structural studies of the various Rho*-Gt complexes can be initiated. Here, we describe the isolation of n-dodecyl-beta-maltoside solubilized, stable, functionally active, Rho*-Gt(e), Rho(e)*-Gt(e), and 9-cis-retinal/11-cis-retinal regenerated Rho-Gt(e) complexes by sucrose gradient centrifugation. In these complexes, Rho* spectrally remained in its Meta II state, and Gt(e) retained its ability to interact with GTPgammaS. Removal of all-trans-retinylidene from Rho*-Gt(e) had no effect on the stability of the Rho(e)*-Gt(e) complex. Moreover, opsin in the Rho(e)*-Gt(e) complex with an empty nucleotide-binding pocket in Gt and an empty retinoid-binding pocket in Rho was regenerated up to 75% without complex dissociation. These results indicate that once Rho* couples with Gt, the chromophore plays a minor role in stabilizing this complex. Moreover, in complexes regenerated with 9-cis-retinal/11-cis-retinal, Rho retains a conformation similar to Rho* that is stabilized by Gt(e) apo-protein.