page_title: Rab (G-protein)
section_title: Function
hierarchical_section_title: Rab (G-protein) / Function
caption_attribution_description: English: This image depicts the 4 steps in RAB protein vesicle transport.
context_page_description: The Rab family of proteins is a member of the Ras superfamily of small G proteins. Approximately 70 types of Rabs have now been identified in humans. Rab proteins generally possess a GTPase fold, which consists of a six-stranded beta sheet which is flanked by five alpha helixes. Rab GTPases regulate many steps of membrane trafficking, including vesicle formation, vesicle movement along actin and tubulin networks, and membrane fusion. These processes make up the route through which cell surface proteins are trafficked from the Golgi to the plasma membrane and are recycled. Surface protein recycling returns proteins to the surface whose function involves carrying another protein or substance inside the cell, such as the transferrin receptor, or serves as a means of regulating the number of a certain type of protein molecules on the surface.
context_section_description: Rab proteins are peripheral membrane proteins, anchored to a membrane via a lipid group covalently linked to an amino acid.  Specifically, Rabs are anchored via prenyl groups on two cysteines in the C-terminus.  Rab escort proteins (REPs) deliver newly synthesized and prenylated Rab to its destination membrane by binding the hydrophobic, insoluble prenyl groups and carrying Rab through the cytoplasm.  The lipid prenyl groups can then insert into the membrane, anchoring Rab at the cytoplasmic face of a vesicle or the plasma membrane. Because Rab proteins are anchored to the membrane through a flexible C-terminal region, they can be thought of as a 'balloon on a string'.
  Rabs switch between two conformations, an inactive form bound to GDP (guanosine diphosphate), and an active form bound to GTP (guanosine triphosphate). A guanine nucleotide exchange factor (GEF) catalyzes the conversion from GDP-bound to GTP-bound form, thereby activating the Rab.  The inherent GTP hydrolysis of Rabs can be enhanced by a GTPase-activating protein (GAP) leading to Rab inactivation.  REPs carry only the GDP-bound form of Rab, and Rab effectors, proteins with which Rab interacts and through which it functions, only bind the GTP-bound form of Rab.  Rab effectors are very heterogeneous, and each Rab isoform has many effectors through which it carries out multiple functions. The specific binding of the effector to the Rab protein allows the Rab protein to be effective, and conversely, the conformation shift of the Rab protein to the inactive state leads to effector dissociation from the Rab protein.
  Effector proteins have one of four different functions. 
  Cargo budding, selection, and coating
  Vesicle Transport
  Vesicle Uncoating and Tethering
  Vesicle Fusion
  After membrane fusion and effector dissociation, Rab is recycled back to its membrane of origin. A GDP dissociation inhibitor (GDI) binds the prenyl groups of the inactive, GDP-bound form of Rab, inhibits the exchange of GDP for GTP (which would reactivate the Rab) and delivers Rab to its original membrane.
caption: The 4 Steps of Vesicle Transport
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