Molecular characterization of a cDNA encoding a small GTP-binding protein, SYPT, in soybean.

ras-related genes, a ubiquitous eukaryotic gene family, encode small GTP-binding proteins (21-25 kD) that play a key role in the regulation of diverse cellular processes (Lowy and Willumsen, 1993). Depending on the amino acid sequence of encoded proteins and their cellular localization, the genes can be grouped into several subfamilies, such as ras, rul, rap, rho, rab, and ypt (Chavrier et al., 1990). Although their physiological functions have not been fully characterized, it was generally proposed that one group of ras-related proteins, including ral and rap, is involved in the receptormediated signal-transduction pathway, and the other yptlrab protein group, including ryh and SEC 4, plays a role in the unidirectional vesicular transport (Zahraoui et al., 1989). They are attached to the inner plasma membrane through a lipid that is covalently linked to a carboxy-terminal Cys residue and possess an inherent GTPase activity that is stimulated by GTPase activating protein and neurofibromin (Gibbs et al., 1990). Like the signal-transducing heterotrimeric G proteins, the small GTP-binding proteins are biologically active when bound to GTP and inactivated when the bound GTP is hydrolyzed to GDP. They exhibit a high degree of homology, especially in the first 165 amino acids, and share the common structural domains of small GTP-binding proteins. Since their basic structure and biological function are highly conserved, the mammalian ras-related genes under the appropriate control of yeast promoters can complement nonviable yeast mutants (Haubruck et al., 1989). Contrary to the extensive studies on ras-related proteins in nonplant organisms, only several reports for small GTPbinding proteins in different plant species have been published in the last few years (Terryn et al., 1993), and a few cDNAs encoding small GTP-binding proteins in plants have been cloned (Matsui et al., 1989; Anuntalabhochai et al., 1991; Palme et al., 1992). To identify other small GTP-binding protein genes in plants, we screened a soybean (Glycine mux L.) cDNA library using a probe of the human rab 2 gene. Among the several putative small GTP-binding protein cDNAs, the longest cDNA insert (SYPT) was isolated and sequenced. The deduced amino acid sequence of the SYPT showed an extensive homology to the rab 1 gene in rat (73%) and the ypt 1 gene