Molecular architecture of the rapidly metabolized 32-kilodalton protein of photosystem II. Indications for COOH-terminal processing of a chloroplast membrane polypeptide.

The molecular architecture of the rapidly metabolized 32-kilodalton chloroplast protein was investigated. Modified proteolytic techniques were applied to construct a cleavage map for this photosynthetic membrane component. The portion which anchors the protein to the membrane was shown to be rich in hydrophobic amino acid residues, while the surface-exposed portion was richer in polar residues. The amino to carboxy polarity of the map was established by comparing oligopeptide radiolabeling data with the polypeptide sequence decoded from the gene. This showed that the anchor domain contains the NH2 terminus, and the exposed domain the COOH terminus. The 32-kilodalton protein was previously demonstrated to derive from a 33.5-kilodalton precursor. We now show that the precursor molecule has a COOH-terminal oligopeptide extension. A model relating precursor cleavage to membrane integration is presented. COOH-terminal processing is proposed to ensure a correct sequence of events for assembly of the 32-kilodalton protein into the photosynthetic membrane.