Synthesis, Transport, and Assembly of Chloroplast Proteins
Intact chloroplasts of higher plants have three separate compartments: envelope, stroma, and thylakoids. These three subchloroplastic compartments may be purified, with minimal cross-contamination, by fractionation of a lysed chloroplast preparation on discontinuous sucrose gradients (cf. Douce and Joyard 1979). Electrophoretic analysis by SDS polyacrylamide gels reveals that each subchloroplastic fraction contains a distinctive set of constituent polypeptides (Pineau and Douce 1974; Joy and Ellis 1975; Morgenthaler and Mendiola-Morgenthaler 1976). During the last decade, studies carried out in several laboratories have demonstrated that only three envelope polypeptides, 1–3 stromal polypeptides, and 10–15 thylakoid membrane polypeptides are synthesized inside the organelle (cf. Ellis 1977). The great majority of chloroplast polypeptides are synthesized on cytosolic ribosomes and must be imported into the organelle in order to reach their final location. Our laboratory is interested in the biosynthetic pathways of this particular group of polypeptides. Here, we summarize recent results on the sequence of events surrounding the cytosolic synthesis of chloroplast polypeptides and their subsequent transport into the organelle. Results of earlier work can be found in a previous review dealing with the same topic (Chua and Schmidt 1979).
KeywordsSucrose Chlorophyll Fractionation Polypeptide Assimilation
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