Characterization of cDNAs encoding small GTP-binding proteins from maize.

Abstract

To date, more than 100 genes encoding small GTP-binding proteins (21-25 kD) have been identified from various eukaryotic organisms, including mammals, insects, yeasts, and higher plants (Hall, 1990; Bourne et al., 1991; Terryn et al., 1993). These proteins, which contain highly homologous regions that have been conserved during evolution, are conveniently classified into three major groups; ras, rho, and rab/ypt families (Kahn et al., 1992). Although the physiological functions of these proteins are not fully understood, increasing evidence suggests that whereas the ras-related proteins are involved in the signal transduction pathways, the rab/ypt-related proteins are associated with intracellular transport and cell growth (Bednarek and Raikhel, 1992). From the plant kingdom, about 30 small GTP-binding protein genes have been identified in Arabidopsis thaliana, rice, tobacco, maize, pea, and others (Terryn et al., 1993). Here, we report the isolation and characterization of two new cDNAs, mgpl and mgp2, that encode small GTPbinding proteins in maize (Table I). The mgpl sequence of 941 bp contains a 633-bp open reading frame that encodes a protein of 211 amino acids with a calculated M, of 22,400. The mgp2 sequence consists of 769 bp with a 648-bp open reading frame encoding a protein of 216 amino acids and a calculated M, of 23,800. Both proteins contain four conserved regions necessary for GTP binding and an effector loop that interacts with specific effector proteins such as GAP. The mgpl and mgp2 proteins show the highest sequence similarities with rabll and the marine ray ora3 proteins followed by various rab/ypt family proteins. This indicates that the mgpl and mgp2 proteins belong to the rab/ypt subfamily, although they are rather distantly related to the only other maize small GTP-binding proteins, YPTml and YPTm2, reported to date (Palme et al., 1992). Northern hybridization analyses revealed that, although the mgpl and mgp2 mRNAs were detected in a11 tissues examined, the highest transcript levels of mgpl and mgp2 were in the tassel and ear tissues, respectively. The mRNA levels of mgpl and mgp2 were also examined during the various developmental stages of the plant at 14, 24,50, 70, and 90 d after emergence. Both mgpl and mgp2 mRNAs