Proteins synthesised on the ribosome and processed in the endoplasmic reticulum are transported from the Golgi apparatus to the trans-Golgi network (TGN), and from there via small carrier vesicles to their final destination compartment. This traffic is bidirectional, to ensure that proteins required to form vesicles are recycled. Vesicles have specific coat proteins (such as clathrin or coatomer) that are important for cargo selection and direction of transfer [1]. While clathrin mediates endocytic protein transport, and transport from ER to Golgi, coatomers primarily mediate intra-Golgi transport, as well as the reverse Golgi to ER transport of dilysine-tagged proteins [2]. For example, the coatomer COP1 (coat protein complex 1) is responsible for reverse transport of recycled proteins from Golgi and pre-Golgi compartments back to the ER, while COPII buds vesicles from the ER to the Golgi [3]. Coatomers reversibly associate with Golgi (non-clathrin-coated) vesicles to mediate protein transport and for budding from Golgi membranes [4]. Activated small guanine triphosphatases (GTPases) attract coat proteins to specific membrane export sites, thereby linking coatomers to export cargos. As coat proteins polymerise, vesicles are formed and budded from membrane-bound organelles. Coatomer complexes also influence Golgi structural integrity, as well as the processing, activity, and endocytic recycling of LDL receptors. In mammals, coatomer complexes can only be recruited by membranes associated to ADP-ribosylation factors (ARFs), which are small GTP-binding proteins. Coatomer complexes are hetero-oligomers composed of at least an alpha, beta, beta', gamma, delta, epsilon and zeta subunits.

This entry represents the WD-associated region found in coatomer subunits alpha, beta and beta' subunits. The alpha-subunit (RET1P) of the coatomer complex in Saccharomyces cerevisiae (Baker's yeast), participates in membrane transport between the endoplasmic reticulum and Golgi apparatus. The protein contains six WD-40 repeat motifs in its N-terminal region [5].

More information about these proteins can be found at Protein of the Month: Clathrin [6].

1. COP and clathrin-coated vesicle budding: different pathways, common approaches. Curr. Opin. Cell Biol. 16, 379-91
2. Gamma-COP appendage domain - structure and function. Traffic 5, 79-88
3. Traffic COPs of the early secretory pathway. Traffic 1, 371-7
4. COPI-mediated transport. J. Membr. Biol. 211, 65-79
5. HEP-COP, a novel human gene whose product is highly homologous to the alpha-subunit of the yeast coatomer protein complex. Gene 169, 223-7
6. Protein of the Month: Clathrin. null

Proteins synthesised on the ribosome and processed in the endoplasmic reticulum are transported f
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