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Abstract

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 (SPP). It is commonly assumed that in higher plant chloroplasts one general SPP processes almost all imported precursor proteins. In the green alga Chlamydomonas, however, there exist several different SPPs which process the various Chlamydomonas precursor proteins. The seven precursor proteins investigated here, which were all correctly imported into isolated chloroplasts, could be divided into two groups: Four precursor proteins were cleaved correctly when processed in vitro with an extract of stromal proteins. Four different SPPs were found in Chlamydomonas chloroplasts to be responsible for the processing of this class of precursors and these four activities were separated chromatographically, characterised and further distinguished by their sensitivity to different inhibitors. The three precursors of the second group were degraded completely by unidentified enzyme(s) present in the stromal extract. Degradation of these precursors was dependent on their conformational integrity as well as on the redox state in the stroma.

Disassembly and Degradation of Photosystem I in an in Vitro System Are Multievent, Metal-dependent Processes

“…This has led to the idea that the chloroplast possesses a "quality control" system that is responsible for assessing the integrity of these complexes and targeting the aberrant ones for degradation. Not much is known about the mechanisms used to recognize aberrant membrane proteins in the chloroplast; however, the identities of several chloroplasts proteases (ClpP, DegP, FtsH, SppA, and Lon) have been revealed recently by genomic sequencing and biochemical and molecular techniques (11)(12)(13)(14)(15)(16)(17)(18). Furthermore, several reports indicate that some of these proteases are involved in the degradation of thylakoid membrane protein complexes.…”