Arf GTPases and their regulators play important and conserved roles in protein trafficking in eukaryotic cells. Like all small G proteins, Arf GTPases are soluble and inactive when bound to GDP but relocate to membranes and become active when bound to GTP. To discover more about the molecular events underlying the complex membrane transformations involved in protein trafficking, Catherine Jackson and colleagues have studied yeast Gea2p, a guanine nucleotide exchange factor (GEF) that promotes exchange of GDP for GTP on Arf (see p. 711). Gea2p, which has been implicated in trafficking through the Golgi apparatus, associates peripherally with membranes but how this is achieved is unclear. The authors reveal that Drs2p, a multispan transmembrane protein, interacts directly with Gea2p in vitro and in vivo. They map the interaction domains in both proteins and then, by examining a yeast strain expressing a version of Gea2p that has a mutated Drs2p-interaction domain, provide evidence that the interaction between Gea2p and Drs2p is required for the correct formation of secretory granules/vesicles from the Golgi.