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 vesicula r carriers. Different types of vesicle are involved in this biosynthet ic membrane traffic. All are coated with protein complexes on their cy toplasmic surface. COP-coated vesicles have recently been implicated i n transport of cargo from the ER to the TGN, and clathrin-coated vesic les 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 prote ins may belong to a family of proteins with related functions. The pre cise role of the coat proteins is not fully understood, although they have been implicated in clustering of cargo into buds and in budding o f vesicles. In addition, coat proteins may play an essential role in t argeting of transport intermediates and may serve to regulate membrane fusion.