Coat Assembly and GTPases

Vesicle coats consist of coat protein subunits and adaptor proteins that work together to form the vesicle structure. Adaptor proteins bind to phosphatidylinositol phosphates (PIPs) on the membrane and bend the cytosolic side, allowing coat protein subunits to attach. This coordinated assembly initiates membrane budding and vesicle formation at specific cellular locations.

Small GTPases like Sar1 control coat assembly by switching between inactive GDP-bound and active GTP-bound states. GEFs catalyze GDP-to-GTP exchange to activate GTPases, while GAPs promote GTP hydrolysis to deactivate them. This molecular switch ensures coat proteins assemble only when and where vesicle formation is needed.

Adaptor proteins link coat proteins to the membrane and control vesicular traffic by identifying specific compartments and determining when and where coat assembly occurs. They bind to transmembrane cargo receptors and are specific for particular cargo types, thus playing a crucial role in cargo selectivity and ensuring distinct adaptor proteins regulate budding from different membranes.

Inactive Sar1-GDP binds Sar1-GEF at the ER membrane, triggering GTP exchange. The resulting Sar1-GTP exposes an amphiphilic helix that inserts into the ER membrane. This activated complex then recruits COPII adaptor proteins, specifically the Sec23/24 subcomplex, which allows coat proteins to assemble and begin membrane budding.

Different coat protein subunits are incorporated at different cellular locations, which changes coat properties such as shape and geometry of transport vesicles. This variation allows cells to tailor vesicle characteristics to specific transport needs. ARF regulates COPI and clathrin coats at the Golgi, while Sar1 regulates COPII coats at the ER.

Guanine nucleotide exchange factors (GEFs) catalyze GDP-to-GTP exchange, activating GTPases and promoting coat protein recruitment. GTPase activating proteins (GAPs) catalyze GTP hydrolysis, deactivating GTPases and terminating coat assembly. This cycle ensures precise temporal and spatial control of vesicle formation and subsequent membrane fusion events.

Adaptor proteins bind transmembrane receptors involved in cargo capture, indirectly controlling cargo selectivity. Since distinct adaptor proteins are specific for different cargo receptor types, they ensure appropriate cargo is packaged into vesicles budding from particular membranes. This specificity is essential for maintaining proper protein and lipid distribution throughout the cell.