A novel plastid-targeted J-domain protein in Arabidopsis thaliana

Arabidopsis cDNAs encoding ATJ11, the smallest known J-domain protein, have been isolated and characterized. The precursor protein of 161 amino acid r esidues was synthesized in vitro and imported by isolated pea chloroplasts where it was localized to the stroma and cleaved to a mature protein of 125 amino acid residues. The mature protein consists of an 80 amino acid J-dom ain, and N- and C-terminal extensions of 24 and 21 amino acid residues, res pectively, which show no similarity to regions in other DnaJ-related protei ns. ATJ11 produced in Escherichia coli stimulated the weak ATPase activity of E. coli DnaK, but was unable to stimulate refolding of firefly luciferas e by DnaK, and inhibited refolding by DnaK, DnaJ and GrpE. ATJ11 is encoded by a single-copy gene on chromosome 4, and is expressed in all plant organ s examined. A paralogue of ATJ11, showing 72% identity, is encoded in a 4.5 Mb duplication of chromosome 4 on chromosome 2. These proteins represent a novel class of J-domain proteins.