Cloning of a cDNA encoding the 22-kDa protein in chloroplasts of spores of Osmunda japonica

Abstract

Five polypeptides in the soluble fraction of chloroplasts and three polypeptides in the thylakoid membranes in chloroplasts from green spores of the fern Osmunda japonica decreased during spore germination. The 22-kDa protein in thylakoid membranes almost completely disappeared during germination over 48 h. The 22-kDa protein had been purified (Physiologia Plantarum 95: 465. 1995). A protease for the 22-kDa protein also had been separated from chloroplasts. Several fragments appeared after proteolysis of the 22-kDa protein by the protease (Physiologia Plantarum 109: 129. 2000). We have attempted to determine a gene of the 22-kDa protein by PCR using the information of amino-terminal sequences of proteolytic products. The cDNA obtained from the mRNA in the spores shows that the precursor protein is composed of the signal peptide having 16 amino acid residues and the mature protein having 180 amino acid residues. A computer search of the protein database revealed similar features between the 22-kDa protein and LEA 3 homologs of Deinococcus radiodurans, Drosophila melanogaster, Caenorhabditis elegans, and cDNA corresponding to cold hardening-induced Chlorella genes. It seems possible that the 22-kDa protein is a homolog of ABA-inducible proteins, desiccation-related proteins, or a LEA protein. The presence of a chloroplast transit peptide in a sequence of the cDNA supports that the 22-kDa protein is one of the thylakoid membranes. The 22-kDa protein may be a desiccation-tolerant protein in quiescent spores, and a role of the protease is a scavenger for the protein during germination.