Processing of a Chloroplast‐Translated Membrane Protein in vivo

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Abstract
The 32,000-dalton (Da) shield protein regulating electron transport in S. oligorrhiza is an integral chloroplast membrane polypeptide. It is rapidly synthesized, constituting a major chloroplast-translation product in vivo. Following in vitro translation of Spirodela chloroplast RNA in a wheat germ system, a 33,500-Da polypeptide is produced. Synthesis of a 33,500-Da protein, associated with the chloroplast membrane, is also seen in vivo, within 2 min of pulse-labeling Spirodela with radioactive amino acids. Comparative analyses among these polypeptides reveal that all 3 are deficient in lysine residues; the two 33,500-Da species have indistinguishable partial proteolytic digestion patterns while that for the 32,000-Da protein differs only slightly from them; and radioactivity from the 33,500-Da polypeptide is rapidly chased in vivo into the 32,000-Da protein, even in the presence of protein synthesis inhibitors. The 33,500-Da proteins synthesized in vitro and in vivo are the precursor form of the 32,000-Da shield protein in Spirodela, with processing commencing only after completion of the precursor polypeptide chain and insertion into the membrane.