Proteolytic activities and proteases of plant chloroplasts

A concise overview on the current knowledge of the proteolytic activities in chloroplasts is presented, with an emphasis on the proteolytic events associated with thylakoid membranes. The Dl reaction centre protein of photosystem II undergoes rapid light-dependent turnover and chlorophyll a/b -binding proteins are effectively degraded upon acclimation of plants to higher irradiances. Insights into the partially characterized proteolytic systems in each case will be presented, but the proteases involved still remain unknown. It can be envisaged, however, that the proteolysis is probably an as highly regulated phenomenon as the various steps during biosynthesis of the photosynthetic multiprotein complexes. From the protease point of view, more progress has recently been made in characterization of processing proteases involved in protein import into chloroplasts and in C-terminal processing of the Dl protein. Moreover, there are an increasing number of proteases in chloroplasts which have been discovered and identified as bacterial homologues. These include a Clp-type protease, a homologue of the bacterial protease FtsH and the cyanobacterial PcrA protease, all of which have a specific location in the chloroplast but their definite physiological substrates are still missing. Attempts are made to bring together the recent progress in the identification of proteases and characterisation of proteolytic events in chloroplasts.