Transport of proteins into chloroplasts. The effect of incorporation of amino acid analogues on the import and processing of chloroplast polypeptides

Abstract

We have synthesized abnormal precursors of imported chloroplast proteins by incorporating amino acid analogues during translation in a cell‐free wheat germ system. Incorporation of analogues of either proline, arginine or leucine markedly inhibits both the import of Pisum sativum ribulosebisphosphate carboxylase small subunit precursor by isolated chloroplasts and processing to the mature size by the purified processing enzyme. One effect of the arginine analogue is to remove a positive charge(s) in the precursor essential for efficient recognition by the processing enzyme. Incorporation of a lysine analogue results in moderate inhibition of the import of small subunit precursor but complete inhibition of import of the chlorophyll a/b‐binding polypeptide precursor. The effect of carboxymethylation on the import of chloroplast proteins is also analyzed. The results indicate that residues essential for transport of the imported proteins by the chloroplast vary among different protein precursors.