Differences in the Phosphorylation-Dependent Regulation of Prenylation of Rap1A and Rap1B

Two isoforms of the small GTPase Rap1, Rap1A and Rap1B, participate in cell adhesion; Rap1A promotes steady state adhesion, while Rap1B regulates dynamic changes in cell adhesion. These events depend on the prenylation of Rap1, which promotes its membrane localization. Here, we identify previously unsuspected differences in the regulation of prenylation of Rap1A versus Rap1B, due in part to their different phosphorylation-dependent interactions with the chaperone protein SmgGDS-607. Previous studies indicate that the activation of Gα_s protein-coupled receptors (GPCRs) phosphorylates S-179 and S-180 in the polybasic region (PBR) of Rap1B, which inhibits Rap1B binding to SmgGDS-607 and diminishes Rap1B prenylation and membrane localization. In this study, we investigate how phosphorylation in the PBR of multiple small GTPases, including K-Ras4B, RhoA, Rap1A, and Rap1B, affects their binding to SmgGDS, with emphasis on differences between Rap1A and Rap1B. We identify the amino acids in SmgGDS-607 necessary for binding of Rap1A and Rap1B, and present homology models examining the binding between Rap1A or Rap1B and SmgGDS-607. Unlike Rap1B, phosphorylation in the PBR of Rap1A does not detectably inhibit its prenylation or its binding to SmgGDS-607. Activation of GPCRs suppresses Rap1A prenylation, but unlike this effect on Rap1B, the GPCR-mediated suppression of Rap1A prenylation can occur independently of Rap1A phosphorylation and does not detectably diminish Rap1A membrane localization. These data demonstrate unexpected evolutionarily conserved differences in the ability of GPCRs to regulate the prenylation of Rap1B compared to Rap1A.