Characterization of Recombinant ELMOD ( Cell Engulfment and Motility Domain) Proteins as GTPase-activating Proteins ( GAPs) for ARF Family GTPases

Background: ELMOD proteins are atypical GAPs for the ARF family with links to deafness. Results: ELMOD proteins have distinct specific activities for ARF proteins and bind the non-opioid sigma-1 receptor. Conclusion: Different specific activities of ELMOD1-3 are predicted to underlie different cellular functions. Significance: Specificities and regulation of ELMODs contribute to understanding their roles in cell signaling and the inner ear. The ARF family of regulatory GTPases, within the RAS superfamily, is composed of approximate to 30 members in mammals, including up to six ARF and at least 18 ARF-like (ARL) proteins. They exhibit significant structural and biochemical conservation and regulate a variety of essential cellular processes, including membrane traffic, cell division, and energy metabolism; each with links to human diseases. We previously identified members of the ELMOD family as GTPase-activating proteins (GAPs) for ARL2 that displayed crossover activity for ARFs as well. To further characterize the GAP activities of the three human ELMODs as GAPs we developed new preparations of each after overexpression in human embryonic kidney (HEK293T) cells. This allowed much higher specific activities and enhanced stability and solubility of the purified proteins. The specificities of ELMOD1-3 as GAPs for six different members of the ARF family were determined and found to display wide variations, which we believe will reveal differences in cellular functions of family members. The non-opioid sigma-1 receptor (S1R) was identified as a novel effector of GAP activity of ELMOD1-3 proteins as its direct binding to either ELMOD1 or ELMOD2 resulted in loss of GAP activity. These findings are critical to understand the roles of ELMOD proteins in cell signaling in general and in the inner ear specifically, and open the door to exploration of the regulation of their GAP activities via agonists or antagonists of the S1R.