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 α subunits specifically modulate the activities of a diverse array of effector proteins. To investigate the determinants of α subunit-effector specificity, we localized the effector-specifying regions of αi2, which inhibits adenylyl cyclase, and αq, 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 αi2 and αq were localized within the GTPase domain. An αq/αi2/αq chimera containing only 78 αi2 residues within the GTPase domain robustly inhibited adenylyl cyclase. This αi2 segment includes regions corresponding to two of the three regions of αs that activate adenylyl cyclase, but does not include any of the α subunit regions that switch conformation upon binding GTP. Replacement of the αq residues that comprise the helical domain with the homologous αi2 residues resulted in a chimeric α subunit that activated phospholipase C. Combined with previous studies of the effector-specifying residues of αs and αt, our results suggest that the effector specificity of α subunits is generally determined by the GTPase and not the helical domain.
This work was supported by grants from the National Institutes of Health and the Patrick and Catherine Weldon Donaghue Medical Research Foundation. The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
