TBC1D15 functions as an Arl4D GAP and promotes the mitochondrial translocation of Arl4D for organelle homeostasis.

ADP-ribosylation factor-like 4D (Arl4D), a Ras small GTPases superfamily member, plays crucial roles in membrane trafficking, cytoskeletal remodeling and cell migration. GDP-bound Arl4D has previously been shown to locate at the mitochondria and alter mitochondrial morphology and activity; however, how the nucleotide-binding state and mitochondrial targeting of Arl4D is regulated had remained unclear. We now discover that TBC1D15, a well-known Rab7 GTPase-activating protein (GAP), functions also as an Arl4D GAP to promote Arl4D mitochondrial targeting. We initially show that GDP-bound Arl4D translocates to the mitochondria under serum starvation and affects mitochondrial homeostasis. We also show that TBC1D15 interacts with Arl4D through the TBC domain and promotes GTP hydrolysis of Arl4D. Knockdown of TBC1D15 leads to an increase in Arl4D activity and decreased Arl4D mitochondrial translocation under serum starvation. These findings support the hypothesis that TBC1D15 acts as an Arl4D GAP and reveal a new role for this GAP in modulating mitochondrial homeostasis.