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A cDNA from Medicago sativa Encodes a Protein Homologous to Small GTP-Binding Proteins

The superfamily of monomeric small GTP-binding proteins comprises members of 20 to 30 kD that can switch between an inactive (GDP bound) and an active (GTP bound) conformation. They are divided into different groups according to their structure, function, and subcellular location. The Ras family interacts with plasma membrane receptors mediating growth and differentiation processes. Rho and Rac are also activated by membrane receptors but appear to be involved in the control of actin polymerization. The Rab and ARF subfamilies control intracellular vesicle transport. In mammals and yeast, Rab proteins serve as important regulators of endocytic and secretory pathways. Distinct members of the Rab family are localized at specific subcellular compartments and they are necessary for different steps of membrane transport (for review, see Pfeffer, 1992; Hall, 1993). Despite the crucial role of small GTP-binding proteins in diverse cellular processes, little is known about these proteins in plants (Terryn et al., 1993). Here we report the isolation of a cDNA potentially encoding a small GTPbinding protein from Medicago sativa, denoted as MsRabl (Table I). A homology search in current protein sequence data banks revealed up to 90% identity to GTP-binding proteins of various plant species (e.g. Nagano et al., 1993; Yi and Guerinot, Guerinot, 1994). Comparison to animal proteins revealed high homology to the Rab family of GTP-binding proteins: 70% identity to ORA3 from electric ray (Ngsee et al., 1991), 60% to Rabll, and 50% to Rab2 from human (GenBank accession numbers P24410 and B34323, respectively). The predicted MsRabl protein sequence shows a11 important features of GTP-binding proteins. The four GTPbinding domains as well as the effector regions, known to interact with GTPase-activating proteins, are conserved. The MsRabl sequence also contains two C-terminal Cys residues indispensible for membrane association. Southern analysis indicates that MsRab is a member of a gene family in M . sativa. Severa1 related Rab proteins are also present in other plant species (eg. Nagano et al., 1993). Recently, it has been demonstrated that two, small GTP-binding pro-