Detection of a Precursor Polypeptide of the Rapidly-Synthesized 32,000-Dalton Thylakoid Protein in Spinach Chloroplasts

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Spinach chloroplast RNA was translated in a wheat germ cell-free system in the presence of [^<35>S]methionine or [^3H]lysine, and the products were analyzed by SDS polyacrylamide gel electrophoresis and fluorography. A polypeptide with molecular mass of 2,000-Da larger than the 32,000-Da thylakoid protein was detected as a major product labeled by [^<35>S]methionine but not by [^3H]lysine. Peptide mapping of this polypeptide showed a pattern very close to that of the 32,000-Da protein synthesized in isolated chloroplasts. A better separation of this polypeptide from the 32,000-Da protein was observed in the electrophoresis on polyacrylamide gel including urea at 8_M. Pulse-labeling of the isolated chloroplasts showed the occurrence of the larger molecular weight form, which was converted to the mature size by a chasing incubation with cold methionine. These results suggested that the 32,000-Da protein of spinach is translated primarily as a high molecular weight precursor in the chloroplasts, as has been reported for other plant species.
日本植物生理学会の論文