The Chaperone Protein SmgGDS Interacts with Small GTPases Entering the Prenylation Pathway by Recognizing the Last Amino Acid in the CAAX Motif

Background: SmgGDS-607 and SmgGDS-558 regulate GTPase movement through the prenylation pathway. Results: The specificity of SmgGDS for GTPases depends on the GTPase CAAX sequence and the cellular context. Conclusion: SmgGDS-607 binds to nonprenylated GTPases that end in a leucine and enter the geranylgeranylation pathway. Significance: The identification of SmgGDS-607 as a novel CAAX-binding protein will accelerate the development of more effective cancer therapeutics. Ras family small GTPases localize at the plasma membrane, where they can activate oncogenic signaling pathways. Understanding the mechanisms that promote membrane localization of GTPases will aid development of new therapies to inhibit oncogenic signaling. We previously reported that SmgGDS splice variants promote prenylation and trafficking of GTPases containing a C-terminal polybasic region and demonstrated that SmgGDS-607 interacts with nonprenylated GTPases, whereas SmgGDS-558 interacts with prenylated GTPases in cells. The mechanism that SmgGDS-607 and SmgGDS-558 use to differentiate between prenylated and nonprenylated GTPases has not been characterized. Here, we provide evidence that SmgGDS-607 associates with GTPases through recognition of the last amino acid in the CAAX motif. We show that SmgGDS-607 forms more stable complexes in cells with nonprenylated GTPases that will become geranylgeranylated than with nonprenylated GTPases that will become farnesylated. These binding relationships similarly occur with nonprenylated SAAX mutants. Intriguingly, farnesyltransferase inhibitors increase the binding of WT K-Ras to SmgGDS-607, indicating that the pharmacological shunting of K-Ras into the geranylgeranylation pathway promotes K-Ras association with SmgGDS-607. Using recombinant proteins and prenylated peptides corresponding to the C-terminal sequences of K-Ras and Rap1B, we found that both SmgGDS-607 and SmgGDS-558 directly bind the GTPase C-terminal region, but the specificity of the SmgGDS splice variants for prenylated versus nonprenylated GTPases is diminished in vitro. Finally, we present structural homology models and data from functional prediction software to define both similar and unique features of SmgGDS-607 when compared with SmgGDS-558.