Coat proteins in intracellular membrane transport.

Transport of newly synthesized material from the endoplasmic reticulum (ER) towards the Golgi complex, through the Golgi cisternae, and out of the trans-Golgi network (TGN) is thought to be mediated by vesicular carriers. Different types of vesicle are involved in this biosynthetic membrane traffic. All are coated with protein complexes on their cytoplasmic surface. COP-coated vesicles have recently been implicated in transport of cargo from the ER to the TGN, and clathrin-coated vesicles from the TGN to endosomes, but the carriers moving material to the cell surface are still unknown. Sequence homologies between subunits of the COP- and the clathrin-adaptor complexes suggest that coat proteins may belong to a family of proteins with related functions. The precise role of the coat proteins is not fully understood, although they have been implicated in clustering of cargo into buds and in budding of vesicles. In addition, coat proteins may play an essential role in targeting of transport intermediates and may serve to regulate membrane fusion.