Acts as a GTPase activating protein for RAB5. Does not act on RAB4 or RAB11 (By similarity). Associated with lipid rafts. 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.