The gene encoding alsin, a protein with three guanine-nucleotide exchange factor domains, is mutated in a form of recessive amyotrophic lateral sclerosis

RasGAP mediates neuronal survival in Drosophila through direct regulation of Rab5-dependent endocytosis

The GTPase Ras can either promote or inhibit cell survival. Inactivating mutations in Drosophila RasGAP (encoded by vap), a Ras GTPase-activating protein, lead to age-related brain degeneration. Genetic interactions implicate the epidermal growth factor receptor (EGFR)-Ras pathway in promoting neurodegeneration but the mechanism is not known. Here, we show that the Src homology 2 (SH2) domains of RasGAP are essential for its neuroprotective function. By using affinity purification and mass spectrometry, we identify a complex containing RasGAP together with Sprint, which is a Ras effector and putative activator of the endocytic GTPase Rab5. Formation of the RasGAP-Sprint complex requires the SH2 domains of RasGAP and tyrosine phosphorylation of Sprint. RasGAP and Sprint colocalize with Rab5-positive early endosomes but not with Rab7-positive late endosomes. We demonstrate a key role for this interaction in neurodegeneration: mutation of Sprint (or Rab5) suppresses neuronal cell death caused by the loss of RasGAP. These results indicate that the long-term survival of adult neurons in Drosophila is crucially dependent on the activities of two GTPases, Ras and Rab5, regulated by the interplay of RasGAP and Sprint.