PROTEOLYTIC ACTIVITIES AND PROTEASES OF PLANT CHLOROPLASTS

A concise overview on the current knowledge of the proteolytic activit ies in chloroplasts is presented, with an emphasis on the proteolytic events associated with thylakoid membranes. The D1 reaction centre pro tein of photosystem II undergoes rapid light-dependent turnover and ch lorophyll a/b-binding proteins are effectively degraded upon acclimati on of plants to higher irradiances. Insights into the partially charac terized proteolytic systems in each case will be presented, but the pr oteases involved still remain unknown. It can be envisaged, however, t hat the proteolysis is probably an as highly regulated phenomenon as t he various steps during biosynthesis of the photosynthetic multiprotei n complexes. From the protease point of view, more progress has recent ly been made in characterization of processing proteases involved in p rotein import into chloroplasts and in C-terminal processing of the D1 protein. Moreover, there are an increasing number of proteases in chl oroplasts which have been discovered and identified as bacterial homol ogues. 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 physiologic al substrates are still missing. Attempts are made to bring together t he recent progress in the identification of proteases and characterisa tion of proteolytic events in chloroplasts.