Rab GTPase family 21 (Rab21)The localization and function of Rab21 are not clearly defined, with conflicting data reported. Rab21 has been reported to localize in the ER in human intestinal epithelial cells, with partial colocalization with alpha-glucosidase, a late endosomal/lysosomal marker. More recently, Rab21 was shown to colocalize with and affect the morphology of early endosomes. In Dictyostelium, GTP-bound Rab21, together with two novel LIM domain proteins, LimF and ChLim, has been shown to regulate phagocytosis. GTPase activating proteins (GAPs) 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. Due to the presence of truncated sequences in this CD, the lipid modification site is not available for annotation.