COPI budding within the Golgi stack

Abstract

The Golgi serves as a hub for intracellular membrane traffic in the eukaryotic cell. Transport within the early secretory pathway, that is within the Golgi and from the Golgi to the endoplasmic reticulum, is mediated by COPI-coated vesicles. The COPI coat shares structural features with the clathrin coat, but differs in the mechanisms of cargo sorting and vesicle formation. The small GTPase Arf1 initiates coating on activation and recruits en bloc the stable heptameric protein complex coatomer that resembles the inner and the outer shells of clathrin-coated vesicles. Different binding sites exist in coatomer for membrane machinery and for the sorting of various classes of cargo proteins. During the budding of a COPI vesicle, lipids are sorted to give a liquid-disordered phase composition. For the release of a COPI-coated vesicle, coatomer and Arf cooperate to mediate membrane separation.

Figure 1.

The heptameric complex coatomer is compared with the clathrin/adaptor complex AP2. The stable complex coatomer can be dissociated in vitro into subcomplexes: a trimer (αβ′ε) and a tetramer (βγδζ). Structural similarities exist between the trimeric coatomer subcomplex (Lee and Goldberg 2010) and the triskelion of clathrin heavy and light chains (Xing et al. 2010). Within the tetrameric subcomplex of coatomer structural similarities are reported for ζ-COP and AP2σ2 (Yu et al. 2009), and for the γ-COP and the β₂ appendage domains (Watson et al. 2004). Thus, the trimeric COPI-subcomplex is thought to resemble the clathrin part, and the tetrameric COPI-subcomplex the adaptor part of a clathrin-coated vesicle.

Figure 2.

Individual steps in the formation of a COPI vesicle. (Scheme adapted from Beck et al. [2009b] and reprinted, with permission, from Elsevier © 2009.) For details see text.