Localization of the effector-specifying regions of G(i2 alpha) and G(q alpha)

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