Acts as a GTPase activating protein for RAB7A. Does not act on RAB4, RAB5 or RAB6. Interacts with non-phosphorylated form of RAB8A; phosphorylation of RAB8A at 'Thr-72' disrupts this interaction (By similarity). Interacts with ARMC12 (By similarity). Ubiquitous, with highest expression in heart, liver and testis and lower expression in brain, spleen, lung, kidney and skeletal muscle. PubMed:16055087 showns that TBC1D15 can also functions as GTPase activating for RAB11 at a lower extent than for RAB7A, however this function is not confirmed by PubMed:20363736.