MOLECULAR CHARACTERIZATION OF A RAB-RELATED SMALL GTP-BINDING PROTEINCDNA FROM RICE (ORYZA-SATIVA L IR-36)

To study physiological roles of plant small GTP binding proteins, we i solated a cDNA clone (ORrabZ) encoding the rab-related small GTP bindi ng protein from rice (Oryza sativa L. IR-36) by using human cDNA rab2 as a probe. The deduced amino acid sequence of the ORrab2 gene shared all the conserved regions, important for GTPase/GTP binding activities , with those of other small GTP binding proteins. ORrab2 is a 1028 bp long cDNA, encodes a 23.2 kDa protein which shows 85.2% similarity on the amino acid sequence level to the Hrab2 protein, and was used as a probe. Through Southern and Northern blot analyses, we found that ORra b2 is a single copy gene and actively expressed at the stages of cell division and elongation. We investigated GTP binding abilities by a fi lter assay procedure. Deletion of a binding motif, GDTGVGKS, within an ORrab2 protein showed a significant decrease of GTP binding affinity, suggesting its important role in nucleotide binding.