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Rab GTPase family 27a (Rab27a) The Rab27a subfamily consists of Rab27a and its highly homologous isoform, Rab27b. Unlike most Rab proteins whose functions remain poorly defined, Rab27a has many known functions. Rab27a has multiple effector proteins, and depending on which effector it binds, Rab27a has different functions as well as tissue distribution and/or cellular localization. Putative functions have been assigned to Rab27a when associated with the effector proteins Slp1, Slp2, Slp3, Slp4, Slp5, DmSlp, rabphilin, Dm/Ce-rabphilin, Slac2-a, Slac2-b, Slac2-c, Noc2, JFC1, and Munc13-4. Rab27a has been associated with several human diseases, including hemophagocytic syndrome (Griscelli syndrome or GS), Hermansky-Pudlak syndrome, and choroidermia. In the case of GS, a rare, autosomal recessive disease, a Rab27a mutation is directly responsible for the disorder. When Rab27a is localized to the secretory granules of pancreatic beta cells, it is believed to mediate glucose-stimulated insulin secretion, making it a potential target for diabetes therapy. When bound to JFC1 in prostate cells, Rab27a is believed to regulate the exocytosis of prostate- specific markers. 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. |
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