Proteolysis of the 22-KDA Protein in Thylakoid Membranes From Green Spores of the Fern Osmunda Japonica

Abstract

During germination of fern spores, many proteins are degraded and synthesized. Degradations of total proteins or cytoplasmic reserve proteins in spores of several ferns have been reported (1–5). Many polypeptides increased or decreased during germination and rhizoid elongation in green spores of Onoclea sensibilis (6). Protein synthesis in chloroplasts isolated from gametophytes (7) has been studied. However, there are few studies on protein degradation in chloroplasts of green spores during germination. Recently, we reported that 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 (8). We focused on thylakoid proteins. The 22-kDa protein almost completely disappeared during germination over 48 h. When germination was suppressed by cycloheximide, the 22-kDa protein did not disappear regardless of the presence of a protease for the 22-kDa protein in chloroplasts of the quiescent spores. An important step in understanding the molecular basis for the degradation mechanism of protein during germination is to isolate both the substrate protein and its specific protease. The 22-kDa protein and its specific protease release from thylakoid membranes by freezing and thawing. The 22-kDa protein was previously purified (8). As part of a more detailed analysis of the degradation of thylakoid proteins during spore germination, we are interested in characterizing the protease against the 22-kDa protein. The purpose of this research is to isolate a specific protease for the 22-kDa protein and to elucidate properties of the enzyme.