G(I-ALPHA-1) SELECTIVELY COUPLES SOMATOSTATIN RECEPTOR SUBTYPE-3 TO ADENYLYL-CYCLASE - IDENTIFICATION OF THE FUNCTIONAL DOMAINS OF THIS A SUBUNIT NECESSARY FOR MEDIATING THE INHIBITION BY SOMATOSTATIN OF CAMP FORMATION

A major cellular action of the neuropeptide somatostatin (SRIF) is the inhibition of adenylyl cyclase activity. SRIF induces this effect aft er its interaction with membrane-bound receptors. Five SRIF receptors (SSTRs), which differ in their functional coupling to adenylyl cyclase , have recently been cloned. The third SSTR cloned, SSTR3, effectively mediates the inhibition of adenylyl cyclase by SRIF. The molecular me chanism by which SRIF modulates intracellular cAMP synthesis via SSTR3 was investigated by initially identifying which G(alpha) subunits are involved in coupling SSTR3 to adenylyl cyclase. SRIF did not inhibit cAMP formation in Chinese hamster ovary cells stably expressing SSTR3 and G(i alpha 2) or C-i alpha 3 but lacking G(i alpha 1). However, SRI F did inhibit forskolin-stimulated cAMP formation in Chinese hamster o vary cells stably expressing SSTR3 and G(i alpha 1), indicating that G (i alpha 1) selectively couples SSTR3 to adenylyl cyclase. To investig ate the functional domains of G(i alpha 1) necessary for interaction w ith SSTR3, a chimeric cc subunit (G(i alpha 2)/G(i alpha 1)) was const ructed, consisting of the amino-terminal two thirds of G(i alpha 2) li gated to the carboxyl-terminal third of G(i alpha 1). SRIF inhibited c AMP formation in cells expressing SSTR3 and the G(i alpha 2)/G(i alpha 1) chimera. These findings indicate that the carboxy-terminal third o f G(i alpha 1) interacts with SSTR3 and is important in transmitting t he signal of SSTR3 activation to adenylyl cyclase. In contrast, a simi lar G(i alpha 2)/G(i alpha 3) chimera did not couple SSTR3 to adenylyl cyclase, further indicating that G(i alpha 3) does not contribute to SRIF inhibition of adenylyl cyclase activity. These findings demonstra te that G(i alpha 1) selectively couples SSTR3 to adenylyl cyclase, an d they indicate that the carboxyl-terminal region of this alpha subuni t is involved in mediating SRIF inhibition of adenylyl cyclase activit y.