Ypt Proteins in Yeast and Their Role in Intracellular Transport

Members of the superfamily of guanine nucleotide binding proteins (GTPases) mediate diverse cellular processes that are important for both prokaryotic and eukaryotic organisms. The monomeric Ras oncoproteins constitute one GTPase family, which like heterotrimeric GTPases are thought to be involved in signal transduction. The identification of the yeast Saccharomyces cerevisiae YPT1 gene in the actin-β-tubulin gene cluster as belonging to the Ras superfamily initiated new studies on the cellular functions of small GTPases (Gallwitz et al. 1983). Since the initial isolation of Ypt1p, the number of related proteins in S. cerevisiae and the corresponding Rab proteins in mammalian systems has grown so dramatically that now they constitute their own family, called Ypt in yeast and Rab in mammals. There are now six Ypt proteins in S. cerevisiae and more than 20 Rab proteins identified in mammals. Their small size and high degree of sequence conservation facilitates the isolation of new family members using the polymerase chain reaction and low stringency DNA hybridization techniques (Touchot et al. 1987; Haubruck et al. 1987, 1990; Hengst et al. 1990; Miyake and Yamamoto 1990; Chavrier et al. 1992; Elferink et al. 1992).