Rab24 is distinct from other Rabs in several ways. It exists primarily in the GTP-bound state, having a low intrinsic GTPase activity; it is not efficiently geranyl-geranylated at the C terminus; it does not form a detectable complex with Rab GDP-dissociation inhibitors (GDIs); and it has recently been shown to undergo tyrosine phosphorylation when overexpressed in vitro [, ]. The specific function of Rab24 still remains unknown. It is found in a transport route between ER-cis-Golgi and late endocytic compartments. It is putatively involved in an autophagic pathway, possibly directing misfolded proteins in the ER to degradative pathways [].Rabs are regulated by GTPase activating proteins (GAPs), which interact with GTP-bound Rab and accelerate the hydrolysis of GTP to GDP. Guanine nucleotide exchange factors (GEFs) interact with GDP-bound Rabs to promote the formation of the GTP-bound state. Rabs are further regulated by guanine nucleotide dissociation inhibitors (GDIs), which facilitate Rab recycling by masking C-terminal lipid binding and promoting cytosolic localization. Most Rab GTPases contain a lipid modification site at the C terminus, with sequence motifs CC, CXC, or CCX. Lipid binding is essential for membrane attachment, a key feature of most Rab proteins [, ].
