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

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 remains unclear. Here, we show that the SH3-adapter protein SPIN90 participates in the activation of Rab5 through the recruitment of both Rab5 and its GEF, Gapex5, to endosomal membranes during epidermal growth factor (EGF)-mediated endocytosis. SPIN90 strongly interacts with the inactive Rab5/GDI2 complex through its C-terminus. In response to EGF signaling, extracellular signal-regulated kinase (ERK)-mediated phosphorylation of SPIN90 at Thr-242 enables SPIN90 to bind Gapex5 through its N-terminal SH3 domain. Gapex5 is a determinant of Rab5 membrane targeting, while SPIN90 mediates the interaction between Gapex5 and Rab5 in a phosphorylation-dependent manner. Collectively, our findings suggest that SPIN90, as an adaptor protein, simultaneously binds inactive Rab5 and Gapex5, thereby altering their spatial proximity and facilitating Rab5 activation. Intracellular transport: a new SPIN A key protein required for incorporating substances into cells has been identified by researchers in South Korea. During this process, called endocytosis, materials are engulfed by the cell membrane, then packaged into vesicles by organelles called endosomes for transport around the cell. The small enzyme Rab5 is involved in fusing and transporting vesicles through interactions with a protein called Gapex5. It has been unclear how these interactions occur, but Woo Keun Song and Yun Hyun Huh at Gwangju Institute of Science and Technology and co-workers suspected a role for SPIN90, an ‘adapter protein’ that helps connect other molecules. They found that Rab5 and Gapex5 did not come close in mutant cells lacking SPIN90. In normal cells, SPIN90 interacted strongly with both Rab5 and Gapex5, recruiting them to endosomes where they operated together on vesicle formation.