Abstract

Regions of basic amino acids in proteins can promote membrane localization through electrostatic interactions with negatively charged membrane lipid head groups. Previous work showed that the heterotrimeric G protein subunit α_q contains a polybasic region in its N terminus that contributes to plasma membrane localization. Here, the role of the N-terminal polybasic region of α_q in signaling was addressed. For α_q mutants, loss of plasma membrane localization correlated with loss of signaling function, as measured by the ability to couple activated G protein-coupled receptors (GPCRs) to stimulation of inositol phosphate production. However, recovery of plasma membrane localization of α_q polybasic mutants by introduction of a site for myristoylation or by coexpression of βγ failed to recover signaling, suggesting a role for N-terminal basic amino acids of α_q beyond simple plasma membrane localization. It is noteworthy that an α_q4Q mutant, containing glutamine substitutions at arginines 27, 30, 31, and 34, was identified that failed to mediate signaling yet retained plasma membrane localization. Although α_q4Q failed to couple activated receptors to inositol phosphate production, it was able to bind βγ, bind RGS4 in an activation-dependent manner, stimulate inositol phosphate production in a receptor-independent manner, and productively interact with a GPCR in isolated membranes. It is noteworthy that α_q4Q showed a differing localization to plasma membrane nanodomains compared with wild-type α_q. Thus, basic amino acids in the N terminus of α_q can affect its lateral segregation on plasma membranes, and changes in such lateral segregation may be responsible for the observed signaling defects of α_q4Q.