Nucleotide exchange on ARF mediated by yeast Geal protein

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Abstract
THE ADP-ribosylation factor ARF is a small GTP-binding protein that is involved in the transport of vesicles between the endo-plasmic reticulum (ER) and Golgi complex and within the Golgi complex itself1–4. ARF cycles between inactive and membrane-associated active forms as a result of exchange of bound GDP for GTP; the GTP-bound form is an essential participant in the formation of transport vesicles4. This nucleotide exchange is inhibited by the fungal metabolite brefeldin A (BFA) 5,6. Here we identify a protein (Gea1) from Saccharomyces cerevisiae that is a component of a complex possessing guanine-nucleotide-exchange activity for ARF. We show that the activity of the complex is sensitive to brefeldin A and that Geal function is necessary for ER–Golgi transport in vivo. Geal contains a domain that is similar to a domain of Sec7, a protein necessary for intra-Golgi transport. We propose that Geal and ARNO, a human protein with a homologous Sec7 domain7, are members of a new family of ARF guanine-nucleotide exchange factors.