A novel system that reports the G-proteins linked to a given receptor: a study of type 3 somatostatin receptor.

SSTR3, a somatostatin (SST) receptor, is an adenylyl cyclase (AC)-inhibiting receptor. To assign the G-protein alpha-subunit (G alpha) linked to this receptor, we created a novel reporter system which utilizes the well-established facts that the C-terminal 5 residues of G alpha are the receptor contact site and G alpha(s) stimulates all subtypes of AC. We constructed chimeric G alpha(s) the C-terminal 5 residues of which were replaced with the corresponding C-terminus of each known G alpha, and examined which chimera confers SSTR3-induced activation of AC. Cellular transfection of SSTR3 and measurement of SST-dependent AC activity through co-transfected chimeric G alpha(s) revealed that SSTR3 recognizes the C-termini of G alpha(i1/2) but not of G alpha(o) or G alpha(z), and those of G alpha(14) and G alpha(16), but not of G alpha(q) or G alpha(11). As predicted by the chimeric G alpha(s), SST-bound SSTR3 stimulated polyphosphoinositide turnover only when G alpha(16) or G alpha(14) was co-transfected. We conclude that the chimeric G alpha(s) system provides a new approach towards the assignment of G-proteins linked to a given receptor.