Gi alpha 1 selectively couples somatostatin receptor subtype 3 to adenylyl cyclase: identification of the functional domains of this alpha 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 after 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 mechanism 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 Gi alpha 2 or Gi alpha 3 but lacking Gi alpha 1. However, SRIF did inhibit forskolin-stimulated cAMP formation in Chinese hamster ovary cells stably expressing SSTR3 and Gi alpha 1, indicating that Gi alpha 1 selectively couples SSTR3 to adenylyl cyclase. To investigate the functional domains of Gi alpha 1 necessary for interaction with SSTR3, a chimeric alpha subunit (Gi alpha 2/Gi alpha 1) was constructed, consisting of the amino-terminal two thirds of Gi alpha 2 ligated to the carboxyl-terminal third of Gi alpha 1. SRIF inhibited cAMP formation in cells expressing SSTR3 and the Gi alpha 2/Gi alpha 1 chimera. These findings indicate that the carboxy-terminal third of Gi alpha 1 interacts with SSTR3 and is important in transmitting the signal of SSTR3 activation to adenylyl cyclase. In contrast, a similar Gi alpha 2/Gi alpha 3 chimera did not couple SSTR3 to adenylyl cyclase, further indicating that Gi alpha 3 does not contribute to SRIF inhibition of adenylyl cyclase activity. These findings demonstrate that Gi alpha 1 selectively couples SSTR3 to adenylyl cyclase, and they indicate that the carboxyl-terminal region of this alpha subunit is involved in mediating SRIF inhibition of adenylyl cyclase activity.