A RECOMBINANT INWARDLY RECTIFYING POTASSIUM CHANNEL COUPLED TO GTP-BINDING PROTEINS

GTP-binding (G) proteins have been shown to mediate activation of inwa rdly rectifying potassium (K+) channels in cardiac, neuronal and neuro endocrine cells. Here, we report functional expression of a recombinan t inwardly rectifying channel which we call KGP (or <(hp)under bar Kir 3.4>), to signify that it is <(K)under bar(+)> selective, <(G)under ba r-protein-Pated> and isolated from <(h)under bar uman> <(p)under bar a ncreas>. KGP expression in Xenopus oocytes resulted in sizeable basal (or agonist-independent) currents while coexpression with a G-protein- linked receptor, yielded additional agonist-induced currents. Coexpres sion of KGP and <(h)under bar GIRK1> (a human brain homolog of GIRK1/K ir3.1) produced much larger basal currents than those observed with KG P or hGIRK1 alone, and upon coexpression with receptor, similarly larg e agonist-induced currents could be obtained. Pertussis toxin treatmen t significantly diminished agonist-dependent currents due to either KG P or KGP/hGIRK1 expression. Interestingly, PTX also significantly redu ced basal KGP or KGP/hGIRK1 currents, suggesting that basal activity i s largely the result of G-protein gating as well. When the two channel s were coexpressed with receptor, the relative increase in current eli cited by agonist was similar whether KGP and hGIRK1 were expressed alo ne or together. When in vitro translated or when expressed in Xenopus oocytes or CHO mammalian cells, KGP gave rise to a nonglycosylated 45- kD protein. Antibodies directed against either KGP or hGIRK1 coprecipi tated both proteins coexpressed in oocytes, providing evidence for the heteromeric assembly of the two channels and suggesting that the curr ent potentiation seen with coexpression of the two channel subunits is due to specific interactions between nous oocyte protein similar in s ize to KGP was also coprecipitated with them. An endogenous oocyte pro tein similar in size of KGP was also coprecipitated with hGIRK1.