Isolation and characterization of cDNAs that encode eleven small GTP-binding proteins from Pisum sativum

Eleven cDNA clones (pra1 to pra9A, pra9B, and pra9C) were isolated from a pea (Pisum sativum) leaf cDNA library which were similar to small GTP-binding proteins. These cDNAs encoded proteins of 22-25 kDa, which exhibited 45-92% identity to one another at the amino acid level. The putative proteins included the characteristic sequences of ras-related small GTP-binding proteins: four conserved domains involved in binding of GTP/GDP; an effector domain; and cysteine residues at the COOH-terminus. Indeed, the pra6 protein, expressed in Escherichia coli, clearly showed GTP-binding activity. Phylogenetic analysis showed that these clones can be classified into two subgroups: proteins encoded by pra1 to pra7 being related to ypt3 and rab11 proteins; and proteins encoded by pra8, pra9A, pra9B, and pra9C being related to YPT1 and rab1 proteins. The effector sequences in these two subgroups were different. RNA gel blot analysis showed that most of the corresponding genes are differentially expressed in pea leaves and roots. Variations in expression were also observed for structurally related genes.