Regulation of membrane traffic in animal cells by COPI

Intracellular membrane transport is mediated predominantly by vesicles which bud from one compartment and fuse specifically with the next compartment in the pathway, resulting in delivery of cargo. COPI-coated vesicles were first identified as intermediates in intra-Golgi transport and subsequent work has shown that they are also involved in transport between the endoplasmic reticulum and the Golgi complex. The COPI coat components have been characterised in detail at the molecular level and a role for membrane proteins and lipids in membrane recruitment of COPI has been uncovered. However, precisely how these distinct membrane components regulate coat recruitment is still unclear and is currently a matter for debate. Furthermore, it is still not clear at exactly how many transport steps COPI is involved and whether it mediates secretory transport in the anterograde or retrograde direction or both. This review focuses on our understanding of COPI structure and function and describes recent findings on the sites of action of COPI in animal cells.