Purification and characterization of a 22‐kDa protein in chloroplasts from green spores of the fern Osmunda japonica

Abstract

Changes in the polypeptide composition of chloroplasts were investigated during germination of green spores of the fern Osmunda japonica. The polypeptide composition of chloroplasts was appreciably changed during a germination time course of 48 h. Levels of five polypeptides with apparent molecular masses of 47, 44, 42, 22 and 18.5 kDa in the soluble fraction of chloroplasts and three polypeptides with molecular masses of 24, 22 and 15 kDa in the thylakoid membranes decreased during germination. In contrast, no decrease of chloroplast polypeptides was observed in the spores incubated with cycloheximide for 48 h. A new 22‐kDa protein was isolated from thylakoid membranes of spores and the amino‐terminal sequence of the purified protein was determined. High levels of alanine and glycine were found in the basic protein (pl > 10.3). This protein, with a native molecular mass of 80 kDa, was characterized by a subunit band observed at a molecular mass of 22 kDa on SDS‐PAGE and by the disappearance of the band during spore germination. Protease activity against the 22‐kDa protein was observed in an extract prepared from chloroplasts of quiescent spores. A hypothetical cytosolic proteinaceous factor is implicated in the regulation of protein degradation in chloroplasts.