Localization of the effector-specifying regions of G(i2α) and G(qα)

Abstract

Heterotrimeric G proteins transmit hormonal and sensory signals received by cell surface receptors to effector proteins that regulate cellular processes. Members of the highly conserved family of a subunits specifically modulate the activities of a diverse array of effector proteins. To investigate the determinants of subunit-effector specificity, we localized the effector-specifying regions of α(i2), which inhibits adenylyl cyclase, and α(q) which stimulates phosphoinositide phospholipase C using chimeric α subunits. The chimeras were generated using an in vivo recombination method in Escherichia coli. The effector-specifying regions of both α(i2) and α(q) were localized within the GTPase domain. An α(q)/α12/α(q) chimera containing only 78 α12 residues within the GTPase domain robustly inhibited adenylyl cyclase. This α(i2) segment includes regions corresponding to two of the three regions of α(s) that activate adenylyl cyclase. But does not include any of the α subunit regions that switch conformation upon binding GTP. Replacement of the α(q) residues that comprise the helical domain with the homologous α(i2) residues resulted in a chimeric α subunit that activated phospholipase C. Combined with previous studies of the effector-specifying residues of α(s) and α(t), our results suggest that the effector specificity of α subunits is generally determined by the GTPase and not the helical domain.