Binds predominantly GDP, and also GTP (PubMed:17448446). Acts as a dynein adapter protein that activates dynein-mediated transport and dynein-dynactin motility on microtubules (PubMed:30814157). Homodimer (PubMed:17448446). Interacts with the dynein-dynactin complex (PubMed:30814157). Down-regulated in cutaneous melanoma cells but not in breast cancer cells. Belongs to the small GTPase superfamily. Rab family.