Structure and mechanism of COPI vesicle biogenesis
Section snippets
Introduction to the COPI coat
Most vesicles are surrounded by protein coats, which interact with specific membranes [1]; initiate, promote, and/or stabilize curvature through protein interactions [2, 3, 4, 5]; select and capture cargo [6, 7]; and ultimately drive vesicle formation. Heptameric COPI (α/β/β′/ɛ/γ/δ/ζ), or coatomer, is essential [8] in organisms from yeast to humans and is recruited en bloc onto Golgi membranes [9], but is often considered conceptually as the B-subcomplex (α/β′/ɛ) and F-subcomplex (β/δ/γ/ζ).
Dilysine-based cargo binding by the B-subcomplex
Efficient COPI coat assembly depends upon both membrane and cargo binding [9, 11, 31], but a molecular explanation for any sort of cargo recognition by COPI has long proven elusive. COPI sorts a variety of important retrograde and recycling cargoes, including the KDEL/HDEL receptor [36, 37]; SNARE proteins [38, 39] required for fusion in the anterograde and retrograde pathways; p24 family proteins with diphenylalanine motifs (FFxx) [32]; and type I transmembrane proteins bearing linear
F-subcomplex recruitment by Arf1-GTP
The small GTPase, Arf1 (reviewed in [51]), plays a central role in recruiting COPI to Golgi membranes through interactions with the F-subcomplex. Briefly, inactive Arf1-GDP is located in the cytosol. Arf1 becomes active when GDP is replaced with GTP by a guanine nucleotide exchange factor (GEF). In COPI vesicle biogenesis, active Arf1-GTP [52] is first recruited to the Golgi membrane following exposure of a myrisotylated, amphipathic N-terminal helix, and Arf1-GTP subsequently recruits the
Electron microscopy and tomography reveal COPI coat heterogeneity
Two groups have published reconstructions of heptameric COPI: the first used single-particle electron microscopy on native COPI isolated from yeast [15], and the second employed cryo-electron tomography on reconstituted COPI coated vesicles [55••]. Both studies highlight intrinsic heterogeneity in COPI coats, in contrast to clathrin or COPII coats. In the first study, a ‘globular’ F-subcomplex and ‘extended’ B-subcomplex were assigned in the heptameric COPI electron density [15]. The second
A structural perspective of COPI vesicle biogenesis
New structural data (summarized in Table 1), combined with published biochemistry and experiments in yeast and mammalian cells, allow us to form an updated picture of COPI membrane recruitment, cargo binding, and coat polymerization. These data also highlight key differences between COPI-based and clathrin-based or COPII-based coats.
X-ray structures have provided a molecular explanation about how multiple reinforcing interactions enable en bloc recruitment of COPI to Golgi membranes. γ-COP and
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
The author would like to thank David Owen for his insight, support, and encouragement over the years; and David Owen, Todd Graham, Richard Suckling, and Bernard Kelly for helpful discussion and critical comments on the manuscript.
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