Chloroplasts of the green alga Chlamydomonas reinhardtii possess at least four distinct stromal processing proteases

The majority of the proteins in the chloroplast are encoded in the nucleus and synthesised in the cytoplasm as precursors with N-terminal extensions. These targeting sequences guide the precursor proteins into the chloroplast where they are immediately cleaved off by a stromal processing protease (S PP). It is commonly assumed that in higher plant chloroplasts one general S PP processes almost all imported precursor proteins. In the green alga Chla mydomonas, however, there exist several different SPPs which process the va rious Chlamydomonas precursor proteins. The seven precursor proteins invest igated here, which were all correctly imported into isolated chloroplasts, could be divided into two groups: Four precursor proteins were cleaved corr ectly when processed in vitro with an extract of stromal proteins. Four dif ferent SPPs were found in Chlamydomonas chloroplasts to be responsible for the processing of this class of precursors and these four activities were s eparated chromatographically, characterised and further distinguished by th eir sensitivity to different inhibitors. The three precursors of the second group were degraded completely by unidentified enzyme(s) present in the st romal extract. Degradation of these precursors was dependent on their confo rmational integrity as well as on the redox state in the stroma.