The beta6/alpha5 regions of Galphai2 and GalphaoA increase the promiscuity of Galpha16 but are insufficient for pertussis toxin-catalyzed ADP-ribosylation

Development of Generic Calcium Imaging Assay for Monitoring Gi-Coupled Receptors and G-Protein Interaction

G-protein-coupled receptors (GPCRs) are important therapeutic targets for many areas of drug research and development. Although chimeric Gα16 proteins are valuable tools for detecting the activation of Gαi/o-coupled receptors, the details of the activation process remain unclear. The authors introduce a series of chimeras that combine both Gα16 and Gα i/o (Gα16/o, Gα16/i2, and Gα 16/i3) into a well-established transient expression system to examine the ability of these chimeras to interact with D2 long-form (D 2L) dopamine and 5-HT1A serotonin receptors. The pEC 50 data obtained for known agonists were similar to results from previous studies that used other cell-based assays, thus indicating sufficient sensitivity for the assay. Moreover, quinpirole exhibited similar intrinsic activity to dopamine at the D2L receptor, whereas S-(—)-3-PPP displayed partial activity of dopamine and quinpirole in the presence of the Gα16/o chimera. The potency of dopamine for D2L receptors was similar among Gα16/o, Gα16/i2, and Gα 16/i3. In contrast, the 5-HT1A receptor exhibited a significantly preferential coupling for Gα16/i3 compared with Gα 16/i2 when serotonin was used as a ligand. This finding was in close agreement with the results of previous reports. The present system could therefore be used as a rapid functional assay for high-throughput screening and deorphanization. ( Journal of Biomolecular Screening 2009:781-788)

Identification of a stretch of six divergent amino acids on the alpha5 helix of Galpha16 as a major determinant of the promiscuity and efficiency of receptor coupling

A broad repertory of G-protein-coupled receptors shows effective coupling with the haematopoietic G16 protein. In the present study, individual residues along the C-terminal alpha5 helix of Galpha16 were examined for their contributions in defining receptor-coupling specificity. Residues that are relatively conserved within, but diverse between, the subfamilies of cloned Galpha subunits were mutated into the corresponding Galpha(z) residues. Six G(i)-linked receptors with different coupling efficiencies to Galpha16 were examined for their ability to utilize the various Galpha16 mutants to mediate agonist-induced inositol phosphate accumulation and Ca2+ mobilization. Co-operative enhancements of receptor coupling were observed with chimaeras harbouring multiple mutations at Glu350, Lys357 and Leu364 of Galpha16. Mutation of Leu364 into isoleucine appeared to be more efficient in enhancing receptor recognition compared with mutations at the other two sites. Mutation of a stretch of six consecutive residues (362-367) lying towards the end of the alpha5 helix was found to broaden significantly the receptor-coupling profile of Galpha16, and the effect was mediated partly through interactions with the beta2-beta3 loop. These results suggested that a stretch of six distinctive residues at the alpha5 helix of Galpha16 is particularly important, whereas other discrete residues spreading along the alpha5 helix function co-operatively for determining the specificity of receptor recognition.

Characterization of CHO Cells Stably Expressing a Gα16/zChimera for High Throughput Screening of GPCRs

G protein-coupled receptors (GPCRs) are important therapeutic targets for drug discovery. The identification and characterization of new ligands ideally requires the use of high throughput assays that are applicable to all GPCR subtypes. To circumvent the problem of different GPCRs coupling to distinct intracellular second messenger pathways, we describe a new method that uses the chimeric Galpha protein 16z25 to facilitate this process. Stably expressed in Chinese hamster ovary cells, 16z25 allows G(i/o)- and G(s)-coupled receptors to mobilize intracellular Ca(2+) upon agonist stimulation. We have generated nine cell lines each stably expressing 16z25 and a GPCR. All cell lines respond to appropriate agonist stimulation in fluorometric imaging plate reader (FLIPR) assays with robust and potent Ca(2+) mobilization. Several of these lines have been pharmacologically characterized using agonists and antagonists. We also demonstrate that the coexpression of GPCR and 16z25 does not interfere with the receptors' ability to activate endogenous signaling pathways. The ability of 16z25 to functionally mediate the agonist stimulation of a broad spectrum of GPCRs indicates that the use of cell lines stably coexpressing this chimera and GPCRs will simplify the drug screening process and aid in the deorphanization of new receptors.