Key components of the fission machinery are interchangeable.

Brefeldin-A ADP-ribosylated substrate (BARS) and dynamin function in membrane fission in distinct intracellular transport pathways, but whether their functions are mechanistically similar is unclear. Here, we show that ARFGAP1, a GTPase-activating protein (GAP) for ADP-ribosylation factor 1 (ARF1), couples to either BARS or endophilin B for vesicle formation by the coat protein I (COPI) complex - a finding that reveals an unanticipated mechanistic flexibility in mammalian COPI transport. Because dynamin is coupled to endophilin A in vesicle formation by the clathrin-coat complex, our finding also predicts that dynamin and ARF GAPs are likely to be functional counterparts in membrane fission among different transport pathways that connect intracellular membrane compartments.