A mammalian guanine-nucleotide-releasing protein enhances function of yeast secretory protein Sec4

Small GTP-binding proteins of the ras superfamily are important for exocytosis from eukaryotic cells. GTP-binding proteins can exist in two different conformations depending on whether they are bound to GDP or GTP, and are thought to function as molecular switches that regulate a variety of cellular processes. The GTP-GDP cycle is controlled by accessory proteins that promote the exchange of bound GDP or the hydrolysis of GTP. The protein Sec4, a member of the Sec4/Ypt1/Rab branch of the Ras superfamily, is involved in a late stage of the secretory pathway in yeast. Here we report the isolation of a mammalian complementary DNA, mss4, encoding a GDP-releasing protein that enhances Sec4 function. The Mss4 protein also stimulates GDP release from Ypt1 and from the mammalian protein Rab3a, but not from Ras2. Mss4 shows sequence similarity to Dss4, a yeast protein with similar biochemical properties.