The Arf1 GTPase and vesicle coat proteins catalyze distinct steps in the budding and release of COPI vesicles from the Golgi, Beck et al. reveal.
Arf1 recruits several types of vesicle coats to membranes, including the coatomer complex that forms COPI vesicles at the Golgi. Both the GTPase and the vesicle coat can deform membranes. Arf1 is thought to accomplish this by inserting an amphipathic helix into the outer leaflet of the lipid bilayer. Beck and colleagues previously found that Arf1 dimerizes on membranes and that a mutant version unable to pair up couldn't bend membranes or support vesicle formation, even though it could still recruit coatomer.
To find out why Arf1 dimerization is required for COPI vesicle biogenesis, Beck et al. analyzed Arf1's effects on lipid membranes by cryo-electron microscopy. In the presence of coatomer, both wild-type and dimerization-deficient Arf1 prompted the formation of coated vesicle buds. But these buds were unable to separate from their donor membrane in the presence of mutant Arf1, suggesting that coatomer drives the initial budding of COPI vesicles before dimerized Arf1 pinches them off. Vesicle scission was restored if the mutant Arf1 was artificially dimerized using chemical cross-linking reagents.
The mechanism of membrane separation is not entirely clear. It is more difficult for Arf1 to insert its amphipathic helix at the bud neck, where lipid headgroups are more tightly packed. If dimeric Arf1 is kept in place via its interactions with coatomer, an energetically unfavorable situation would arise, which could be relieved by vesicle scission. Similar mechanisms may also apply to other types of GTPase-dependent vesicles.