Acts as a GTPase activating protein for RAB5. Does not act on RAB4 or RAB11. Associated with lipid rafts. Expressed in liver, skeletal muscle, kidney, pancreas, spleen, testis, ovary, small intestine and peripheral blood leukocytes. Overexpressed in prostate cancers. Ubiquitinated by a CUL7-based E3 ligase, which leads to proteasomal degradation. Palmitoylation is required for membrane localization and protects TBC1D3 from ubiquitination. TBC1D3 is encoded by a collection of very similar paralogs with multiple copies of each paralog, some human genomes encoding well over 50 copies depending on ethnic origin of the donor.