Selective stabilization of the high affinity binding conformation of glucagon receptor by the long splice variant of Galpha(s)

To analyze functional differences in the interactions of the glucagon receptor (GR) with the two predominant splice variants of Galpha(s), GR was covalently linked to the short and the long forms Galpha(s)-S and Galpha(s)-L to produce the fusion proteins GR-Galpha(s)-S and GR-Galpha(s)-L. GR-Galpha(s)-S bound glucagon with an affinity similar to that of GR, while GR-Galpha(s)-L showed a 10-fold higher affinity for glucagon. In the presence of GTPgammaS, GR-Galpha(s)-L reverted to the low affinity glucagon binding conformation. Both GR-Galpha(s)-L and GR-Galpha(s)-S were constitutively active, causing elevated basal levels of cAMP even in the absence of glucagon. A mutant GR that failed to activate G(s) (G23D1R) was fused to Galpha(s)-L. G23D1R-Galpha(s)-L bound glucagon with high affinity, but failed to elevate cAMP levels, suggesting that the mechanisms of GR-mediated Galpha(s)-L activation and Galpha(s)-L-induced high affinity glucagon binding are independent. Both GR-Galpha(s)-S and GR-Galpha(s)-L bound the antagonist desHis(1)[Nle(9),Ala(11),Ala(16)]glucagon amide with affinities similar to GR. The antagonist displayed partial agonist activity with GR-Galpha(s)-L, but not with GR-Galpha(s)-S. Therefore, the partial agonist activity of the antagonist observed in intact cells appears to be due to GRs coupled to Galpha(s)-L. We conclude that Galpha(s)-S and Galpha(s)-L interact differently with GR and that specific coupling of GR to Galpha(s)-L may account for GTP-sensitive high affinity glucagon binding.     

Interdisciplinary biophysical studies of membrane proteins bacteriorhodopsin and rhodopsin

The centenary of the birth of H. Gobind Khorana provides an auspicious opportunity to review the origins and evolution of parallel advances in biophysical methodology and molecular genetics technology used to study membrane proteins. Interdisciplinary work in the Khorana laboratory in the late 1970s and for the next three decades led to productive collaborations and fostered three subsequent scientific generations whose biophysical work on membrane proteins has led to detailed elucidation of the molecular mechanisms of energy transduction by the light-driven proton pump bacteriorhodopsin (bR) and signal transduction by the G protein–coupled receptor (GPCR) rhodopsin. This review will highlight the origins and advances of biophysical studies of membrane proteins made possible by the application of molecular genetics approaches to engineer site-specific alterations of membrane protein structures.     

Genetically-encoded Molecular Probes to Study G Protein-coupled Receptors

To facilitate structural and dynamic studies of G protein-coupled receptor (GPCR) signaling complexes, new approaches are required to introduce informative probes or labels into expressed receptors that do not perturb receptor function. We used amber codon suppression technology to genetically-encode the unnatural amino acid, p-azido-L-phenylalanine (azF) at various targeted positions in GPCRs heterologously expressed in mammalian cells. The versatility of the azido group is illustrated here in different applications to study GPCRs in their native cellular environment or under detergent solubilized conditions. First, we demonstrate a cell-based targeted photocrosslinking technology to identify the residues in the ligand-binding pocket of GPCR where a tritium-labeled small-molecule ligand is crosslinked to a genetically-encoded azido amino acid. We then demonstrate site-specific modification of GPCRs by the bioorthogonal Staudinger-Bertozzi ligation reaction that targets the azido group using phosphine derivatives. We discuss a general strategy for targeted peptide-epitope tagging of expressed membrane proteins in-culture and its detection using a whole-cell-based ELISA approach. Finally, we show that azF-GPCRs can be selectively tagged with fluorescent probes. The methodologies discussed are general, in that they can in principle be applied to any amino acid position in any expressed GPCR to interrogate active signaling complexes.     

CXCR7/CXCR4 heterodimer constitutively recruits beta-arrestin to enhance cell migration

G protein-coupled receptor hetero-oligomerization is emerging as an important regulator of ligand-dependent transmembrane signaling, but precisely how receptor heteromers affect receptor pharmacology remains largely unknown. In this study, we have attempted to identify the functional significance of the heteromeric complex between CXCR4 and CXCR7 chemokine receptors. We demonstrate that co-expression of CXCR7 with CXCR4 results in constitutive recruitment of β-arrestin to the CXCR4·CXCR7 complex and simultaneous impairment of G(i)-mediated signaling. CXCR7/CXCR4 co-expression also results in potentiation of CXCL12 (SDF-1)-mediated downstream β-arrestin-dependent cell signaling pathways, including ERK1/2, p38 MAPK, and SAPK as judged from the results of experiments using siRNA knockdown to deplete β-arrestin. Interestingly, CXCR7/CXCR4 co-expression enhances cell migration in response to CXCL12 stimulation. Again, inhibition of β-arrestin using either siRNA knockdown or a dominant negative mutant abrogates the enhanced CXCL12-dependent migration of CXCR4/CXCR7-expressing cells. These results show how CXCR7, which cannot signal directly through G protein-linked pathways, can nevertheless affect cellular signaling networks by forming a heteromeric complex with CXCR4. The CXCR4·CXCR7 heterodimer complex recruits β-arrestin, resulting in preferential activation of β-arrestin-linked signaling pathways over canonical G protein pathways. CXCL12-dependent signaling of CXCR4 and its role in cellular physiology, including cancer metastasis, should be evaluated in the context of potential functional hetero-oligomerization with CXCR7.     

Multiple CCR5 conformations on the cell surface are used differentially by human immunodeficiency viruses resistant or sensitive to CCR5 inhibitors

Resistance to small-molecule CCR5 inhibitors arises when HIV-1 variants acquire the ability to use inhibitor-bound CCR5 while still recognizing free CCR5. Two isolates, CC101.19 and D1/85.16, became resistant via four substitutions in the gp120 V3 region and three in the gp41 fusion peptide (FP), respectively. The binding characteristics of a panel of monoclonal antibodies (MAbs) imply that several antigenic forms of CCR5 are expressed at different levels on the surfaces of U87-CD4-CCR5 cells and primary CD4(+) T cells, in a cell-type-dependent manner. CCR5 binding and HIV-1 infection inhibition experiments suggest that the two CCR5 inhibitor-resistant viruses altered their interactions with CCR5 in different ways. As a result, both mutants became generally more sensitive to inhibition by CCR5 MAbs, and the FP mutant is specifically sensitive to a MAb that stains discrete cell surface clusters of CCR5 that may correspond to lipid rafts. We conclude that some MAbs detect different antigenic forms of CCR5 and that inhibitor-sensitive and -resistant viruses can use these CCR5 forms differently for entry in the presence or absence of CCR5 inhibitors.     

Purinergic Receptors Crosstalk with CCR5 to Amplify Ca2+ Signaling

Cellular and Molecular Neurobiology - Many G protein-coupled receptors (GPCRs) signal through more than one subtype of heterotrimeric G proteins. For example, the C–C chemokine receptor type...     

Salivary alpha amylase and salivary cortisol response to fluid consumption in exercising athletes

The objective of the study was to examine salivary biomarker response to fluid consumption in exercising athletes. Exercise induces stress on the body and salivary alpha amylase (sAA) and salivary cortisol are useful biomarkers for activity in the sympathoadrenal medullary system and the hypothalamic pituitary adrenal axis which are involved in the stress response. Fifteen college students were given 150 ml and 500 ml of water on different days and blinded to fluid condition. The exercise protocol was identical for both fluid conditions using absolute exercise intensities ranging from moderate to high. Saliva was collected prior to exercise, post moderate and post high intensities and analyzed by Salimetrics assays. Exercise was significant for sAA with values different between pre-exercise (85 ± 10 U · ml⁻¹) and high intensity (284 ± 30 U · ml⁻¹) as well as between moderate intensity (204 ± 32 U · ml⁻¹) and high intensity. There was no difference in sAA values between fluid conditions at either intensity. Exercise intensity and fluid condition were each significant for cortisol. Cortisol values were different between pre-exercise (0.30 ± 0.03 ug · dL⁻¹) and high intensity (0.45 ± 0.05 ug · dL⁻¹) as well as between moderate intensity (0.33 ± 0.04 ug · dL⁻¹) and high intensity. Moderate exercise intensity cortisol was lower in the 500 ml condition (0.33 ± 0.03 ug · dL⁻¹) compared with the 150 ml condition (0.38 ± 0.03 ug · dL⁻¹). This altered physiological response due to fluid consumption could influence sport performance and should be considered. In addition, future sport and exercise studies should control for fluid consumption.     

Rhodopsin forms a dimer with cytoplasmic helix 8 contacts in native membranes

G protein-coupled receptors form dimers and higher-order oligomers in membranes, but the precise mode of receptor-receptor interaction remains unknown. To probe the intradimeric proximity of helix 8 (H8), we conducted chemical cross-linking of endogenous cysteines in rhodopsin in disk membranes. We identified a Cys316-Cys316 cross-link using partial proteolysis and liquid chromatography with mass spectrometry. These results show that a symmetric dimer interface mediated by H1 and H8 contacts is present in native membranes.     

Disruption of the alpha5 helix of transducin impairs rhodopsin-catalyzed nucleotide exchange

Photoactivated rhodopsin (R) catalyzes nucleotide exchange by transducin, the heterotrimeric G protein of the rod cell. Recently, we showed that certain alanine replacement mutants of the alpha5 helix of the alpha subunit of transducin (Galpha(t)) displayed very rapid nucleotide exchange rates even in the absence of R [Marin, E. P., Krishna, A. G., and Sakmar, T. P. (2001) J. Biol. Chem. 276, 27400-27405]. We suggested that R catalyzes nucleotide exchange by perturbing residues on the alpha5 helix. Here, we characterize deletion, insertion, and proline replacement mutants of amino acid residues in alpha5. In general, the proline mutants exhibited rates of uncatalyzed nucleotide exchange that were 4-8-fold greater than wild type. The proline mutants also generally displayed decreased rates of R-catalyzed activation. The degree of reduction of the activation rate correlated with the position of the residue replaced with proline. Mutants with replacement of residues at the amino terminus of alpha5 exhibited mild (<2-fold) decreases, whereas mutants with replacement of residues at the carboxyl terminus of alpha5 were completely resistant to R-catalyzed activation. In addition, insertion of a single helical turn in the form of four alanine residues following Ile339 at the carboxyl terminus of alpha5 prevented R-catalyzed activation. Together, the results provide evidence that alpha5 serves an important function in mediating R-catalyzed nucleotide exchange. In particular, the data suggest the importance of the connection between the alpha5 helix and the adjacent carboxyl-terminal region of Galpha(t).