The stromal processing peptidase activities from Chlamydomonas reinhardtii and Pisum sativum: unexpected similarities in reaction specificity

We have partially purified the stromal processing peptidase from Chlamydomonas reinhardtii and compared the properties of this activity with those of the pea counterpart. Whereas previous studies have suggested that the two enzymes may have significantly different reaction specificities, we find that they are in fact very similar. Both enzymes process precursors of two higher-plant thylakoid lumen proteins, and one C. reinhardtii lumenal protein, to similar intermediate-size forms. However, whereas the algal enzyme processes the precursor of C. reinhardtii Rubisco small subunit to the correct mature size, this precursor is cleaved only to an intermediate size by the pea enzyme. The small subunit precursor from pea appears to be cleaved by both enzymes in a similar manner. In terms of sensitivity to inhibitors, the two activities are notably different; the pea enzyme has previously been shown to be inhibited by several types of heavy-metal chelator, but we have found that none of these compounds affect the algal activity.