The formation of transport vesicles is initiated by small G proteins that insert into the donor membrane and assemble coat proteins. The coat proteins are COPI, COPII or clathrin, depending on the pathway, and the coat includes adaptor proteins that link the coat to the vesicle and its cargo. Epsins and dynamin are involved in the budding process.
The vesicle cargo depends on the adaptor proteins, sorting proteins and receptors that are assembled in the vesicle.
After vesicles have moved to the target membrane, GTP is hydrolysed and the coat proteins depolymerise to uncoat the vesicle.
The initial tethering of vesicles with the target membrane is mediated by TRAPPs, and docking is mediated by SNAREs, but other proteins and ion channels are involved in triggering fusion.
Rab GTPases act as master regulators of vesicle trafficking, by binding to membranes and, in association with Rab effectors, assembling appropriate groups of proteins to mediate transport to and fusion with the target membrane. There are numerous Rabs, each selectively localised in particular membranes or organelles.
Membrane components and proteins involved in controlling vesicular traffic and vesicle coating are recycled to their original compartment for re-use. Misdirected cargo proteins are returned to their correct compartment, by a process involving their signal sequences. Misfolded proteins are returned to the endoplasmic reticulum.
Enveloped viruses must also fuse with cell membranes in order for the virus to enter the cytoplasm of the cell. Viral fusion proteins have structural similarities with eukaryotic fusion proteins.