The vesicle coat protein COPI is at the heart of trafficking through the Golgi, but its precise roles are still the subject of some debate. The recruitment of COPI involves ADP-ribosylation factors (ARFs), which cycle from inactive to active states with the help of ARF-activating guanine nucleotide exchange factors (GEFs). On , Elizabeth Sztul and colleagues investigate the role of COPI in Golgi structure and function by knocking down the ARF-GEF GBF1. They show that GBF1 is the major recruiter of COPI in the cell. Surprisingly, preventing recruitment of COPI to membranes does not cause the complete collapse of the secretory pathway but does cause selective tubulation of the cis-Golgi. Equally unexpected is the finding that, although depletion of GBF1 blocks the transport of transmembrane proteins, it does not affect the trafficking of soluble secretory proteins. The authors therefore suggest that GBF1-mediated COPI recruitment is required for trafficking of select types of cargo – transmembrane proteins in particular – through specific stages of the secretory pathway.
