SPIN90, an adaptor protein, mediates the proximity between Rab5 and Gapex5 and facilitates Rab5 activation during EGF endocytosis

Abstract

During ligand-mediated receptor endocytosis, the small GTPase Rab5 functions in vesicle fusion and trafficking. Rab5 activation is known to require interactions with its guanine nucleotide-exchange factors (GEFs); however, the mechanism regulating Rab5 interactions with GEFs has hardly ever been investigated. Here, I show that the SH3-adapter protein, SPIN90, participates in the activation of Rab5 through recruitment of both Rab5 and its GEF, Gapex5, to endosomal membranes during epidermal growth factor (EGF)-mediated endocytosis. SPIN90 strongly interacted inactive Rab5/GDI2 complex through its C-terminus. In response to EGF signaling, extracellular signal-regulated kinase (ERK)-mediated phosphorylation of SPIN90 on Thr 242 enabled SPIN90 to bind Gapex5 through its N-terminal SH3 domain. Gapex5 is a determinant for Rab5 membrane targeting, while SPIN90 mediates the interaction between Gapex5 and Rab5 depending on its phosphorylation status. Also, SPIN90 controls EGFR signaling-mediated cell proliferation. Collectively, our findings suggest that SPIN90 as the adaptor protein simultaneously binds inactive Rab5 and Gapex5, thereby their spatial proximity facilitates Rab5 activation and cell proliferation.