17.5:
Coat Assembly and GTPases
Vesicle coats are composed of coat protein subunits and adaptor proteins. Adaptor proteins use their PIP-binding domains to interact with PIPs and bend the cytosolic side of the plasma membrane. This allows the coat protein subunits to bind the membrane.
GTP-binding proteins such as Sar1, a small GTPase in the ER, regulate coat protein recruitment to control the location and timing of COPII vesicle formation and fusion. They flip between a GDP-bound inactive state and a GTP-bound active state.
Guanine nucleotide exchange factors or GEFs catalyze the exchange of GDP for GTP in the protein, while GTPase activating proteins or GAPs catalyze the hydrolysis of GTP, thus deactivating the proteins.
The inactive, cytoplasmic Sar1-GDP complex binds Sar1-GEF, located in the ER membrane. As a result, GTP replaces GDP on Sar1, exposing an amphiphilic helix that interacts with the ER membrane.
The ER-bound Sar1-GTP complex recruits COPII adaptor proteins—the Sec23/24 subcomplex. This allows the coat proteins to assemble and initiate vesicle budding.
17.5:
Coat Assembly and GTPases
Vesicles incorporate different coat protein subunits in different cell locations, which changes the properties of the coat, such as the shape and geometry of the transport vesicles. Thus, vesicle coat proteins also play a significant role in cargo selection.
Coat assembly depends on the local availability of phosphatidylinositol phosphates or PIPs and GTP-binding proteins. Adaptor proteins, which link the coat proteins to the membrane, bind to these PIPs and play a crucial role in controlling vesicular traffic by identifying compartments and determining the time and place of coat assembly. The adaptor protein binds transmembrane receptors involved in cargo capture, thus indirectly playing a role in cargo selectivity. Adaptor proteins are specific for the type of cargo receptor. Hence, distinct adaptor proteins are involved in the budding of clathrin-coated vesicles from different membranes.
Vesicular traffic is a tightly regulated process. So, coat proteins assemble and initiate bending the membrane budding of the vesicle only when vesicle formation is initiated locally at the membrane. Different small GTPases control the assembly of coat proteins by switching between GTP and GDP-bound states. ARF, a monomeric GTPase, regulates COPI and clathrin coat assembly in the Golgi membrane, while another small GTPase, Sar1 protein, regulates COPII coat assembly in the ER.
Leitura Sugerida
- Pucadyil, T. J., & Schmid, S. L. (2009). Conserved functions of membrane active GTPases in coated vesicle formation. Science, 325(5945), 1217-1220.
- Sato, K. (2004). COPII coat assembly and selective export from the endoplasmic reticulum. Journal of biochemistry, 136(6), 755-760.