A new member of the small GTP-binding protein family in Arabidopsis thaliana.

In eukaryotes, GTP-binding proteins act as versatile molecular switches whose active or inactive state depends on the binding of GTP or GDP, respectively (Bourne et al., 1990). The rapidly growing family of small GTP-binding proteins (mo1 wt values between 20,000 and 36,000) is now generally subdivided into three major subfamilies: Ras-like, Rho-like, and Ypt/Rab-like proteins. Each group seems to play a distinct role in the cell: the Ras subfamily is involved mainly in cell growth and differentiation, the Rho subfamily is involved in cytoskeleton organization, and the Ypt/Rab subfamily has a role in intracellular transport and secretion. In plants, proteins have been identified that specifically bind GTP, and studies with nonhydrolyzable GTP analogs indicate a role for GTP-binding proteins in various processes, including plant hormone signal transduction and ion channel function (Terryn et al., 1993). A number of genes have now been isolated from a variety of plant species (Zea mays, Oyza sativa, Nicotiana tabacum, Nicotiana plumbaginifolia, Arabidopsis thaliana, Pisum sativum) by homology to members of the Ras superfamily (Nagano et al., 1993; Regad et al., 1993; Terryn et al., 1993; Bednarek et al., 1994). Here we report on a cDNA encoding a small GTP-binding protein from Arabidopsis thaliana ecotype Columbia that was isolated from a root cDNA library (Peterman and Goodman, 1991). We have designated the gene A.t.RAB11 because it belongs to the Ypt/Rab subfamily of GTP-binding proteins (Table I), showing 81% similarity to the mammalian Rabll protein (Chavrier et al., 1990) and 80% similarity to the Schizosaccharomyces pombe Ypt3 protein (Miyake and Yamamoto, 1990). Chavrier et al. (1990) had previously reported that Rabll and Ypt3 are probably homologs, with 80% similarity. Of the Arabidopsis small GTP-binding proteins that have been identified to date, the A.t.Rabl1 protein has the highest similarity (91%) to ARA2 (Anai et al., 1991). ,