9Å structure of the COPI coat reveals that the Arf1 GTPase occupies two contrasting molecular environments

Abstract

COPI coated vesicles mediate trafficking within the Golgi apparatus and between the Golgi and the endoplasmic reticulum. Assembly of a COPI coated vesicle is initiated by the small GTPase Arf1 that recruits the coatomer complex to the membrane, triggering polymerization and budding. The vesicle uncoats before fusion with a target membrane. Coat components are structurally conserved between COPI and clathrin/adaptor proteins. Using cryo-electron tomography and subtomogram averaging, we determined the structure of the COPI coat assembled on membranes in vitro at 9 Å resolution. We also obtained a 2.57 Å resolution crystal structure of βδ-COP. By combining these structures we built a molecular model of the coat. We additionally determined the coat structure in the presence of ArfGAP proteins that regulate coat dissociation. We found that Arf1 occupies contrasting molecular environments within the coat, leading us to hypothesize that some Arf1 molecules may regulate vesicle assembly while others regulate coat disassembly.DOI: http://dx.doi.org/10.7554/eLife.26691.001

ArfGAPs; Depletion or Overexpression?

“…There are experimental data that the membrane dissociation rate of Arf1 is more rapid than that of coatomer in living cells [20], indicating that coatomer remains on the membrane after GTP hydrolysis on Arf1. Furthermore, recent studies involving structural analyses have suggested that the binding site of -COP to Arf1•GTP might be transmitted to KDEL cargo receptor after GTP hydrolysis on Arf1 [21]. These studies support the model by which GTP hydrolysis on Arf is related to cargo sorting.…”

A tale of short tails, through thick and thin: investigating the sorting mechanisms of Golgi enzymes

“…It remains to be seen whether this interaction occurs in vivo with the full coat complex and the tail in the context of a membrane. Various structures of coatomer suggest there may be sufficient flexibility in δ‐COP and the COPI coat as a whole to accommodate for a direct interaction with the cytoplasmic tails . Another curiosity is that Wx(n1‐6)W/F motifs often feature amongst highly acidic stretches ; however, this does not appear to be the case for the motif in the tail of GalNAc‐T4.…”

“…Residues in bold highlight a φ(K/R)XLX(K/R) motif corresponding to a d-COP/f1-COP consensus binding motif [63]. Highlighted residues correspond to a WX(n1-6)(W/F) motif shown to bind to d-COP [ structures of coatomer suggest there may be sufficient flexibility in d-COP and the COPI coat as a whole to accommodate for a direct interaction with the cytoplasmic tails [19,77,78]. Another curiosity is that Wx(n1-6) W/F motifs often feature amongst highly acidic stretches [65]; however, this does not appear to be the case for the motif in the tail of GalNAc-T4.…”

Formation of COPI-coated vesicles at a glance

“…Three mammalian ArfGAPs (ArfGAP1, ArfGAP2 and ArfGAP3) have been implicated in the regulation of COPI function (Weimer et al, 2008) and have been shown to interact with COPI-coated vesicles produced in vitro (Weimer et al, 2008;Yang et al, 2002). COPI catalyses the ArfGAP-dependent GTP hydrolysis of Arf1 (Ahmadian et al, 1997;Dodonova et al, 2017;Goldberg, 1999;Luo et al, 2009).…”