A NaCl‐regulated plant gene encoding a brain protein homolog that activates ADP ribosyltransferase and inhibits protein kinase C

Abstract

A cDNA clone pCZ1, with a 1.1 kb insert, was isolated from a NaCI‐adapted tobacco cell cDNA library that encodes an apparently full‐length 29 kDa protein (251 amino acids) with a calculated pl of 5.7. The encoded peptide had a high amino acid sequence identity with bovine 14‐3‐3 protein which was originally found as an abundant protein in the animal central nervous system. Recently, proteins with sequence identity to 14‐3‐3 protein have also been found in plants, insects and yeast, and appear to have diverse physiological functions. Similar to the bovine brain 14‐3‐3 protein, the recombinant pCZ1 protein stimulated ADP‐ribosylation of protein substrate by ADP‐ribosyl‐transferase from the plant and animal pathogenic bacterium Pseudomonas aeruginosa. This recombinant protein also inhibited protein kinase C activity in vitro. Southern blot analyses indicated that most likely five genes encoding 14‐3‐3‐like proteins are present in tobacco. The pCZ1 cDNA insert hybridized to a single mRNA of 1.1 kb from cultured tobacco cells. The level of this mRNA transcript in tobacco cells was downregulated upon adaptation to NaCl but was unaffected by short‐term treatment with NaCl, ABA or ethylene. In tobacco plants, expression of transcript that hybridized to pCZ1 was tissue specific, and was most abundant in roots and flower parts. Monoclonal antibody raised against GF14 protein, a maize protein with substantial sequence identity with 14‐3‐3 protein detected two bands on SDS‐PAGE of total proteins from unadapted tobacco cells and only a single band from cells adapted to NaCl. The GF14 antibody was also used to illustrate that the G‐box element of a salt‐induced gene is associated with a 14‐3‐3‐type protein.