NOVEL PLANT CA2-BINDING PROTEIN EXPRESSED IN RESPONSE TO ABSCISIC-ACID AND OSMOTIC-STRESS()

A cDNA corresponding to an mRNA which accumulates in germinating rice seeds in response to the phytohormone abscisic acid was isolated by di fferential hybridization. Northern blotting indicated that the mRNA al so accumulates in vegetative tissues in response to treatment with abs cisic acid and to osmotic stress. Sequencing identified a major open r eading frame encoding a novel protein of 27.4 kDa. The identity of the open reading frame was confirmed by comparing the translation product s of cellular, hybrid selected, and in vitro transcribed RNAs and by i mmunoprecipitation. Western blotting of cellular extracts indicated th at the protein is associated with microsomal or membrane fractions. Da ta base searches indicated that it contains a conserved Ca2+-binding, EF-hand motif and that related proteins are similarly expressed in Ara bidopsis thaliana. A fusion protein purified from Escherichia coli con taining the putative EF-hand region was shown to bind Ca2+ in blot bin ding assays. These data identify a novel gene family encoding proteins involved in the response of plants to abscisic acid and osmotic stres s.