D1 PROTEIN OF PHOTOSYSTEM-II - THE LIGHT SENSOR IN CHLOROPLASTS

Light, controls the ''blueprint'' for chloroplast development, but at high intensities is toxic to the chloroplast. Excessive light intensities inhibit primarily photosystem II electron transport. This results in generation of toxic singlet oxygen due to impairment of electron transport on the acceptor side between pheophytin and Q(B)-the secondary electron acceptor. High light stress also impairs electron transport on the donor side of photosystem II generating highly oxidizing species Z(+) and P680(+). A conformational change in the photosystem II reaction centre protein D1 affecting its Q(B)-binding site is involved in turning the damaged protein into a substrate for proteolysis. The evidence indicates that the degradation of D1 is an enzymatic process and the protease that degrades D1 protein has been shown to be a serine protease. Although there is evidence to indicate that the chlorophyll a-protein complex CP43 acts as a serine-type protease degrading D1, the observed degradation of D1 protein in photosystem II reaction centre particles in vitro argues against the involvement of CP43 in D1 degradation. Besides the degradation during high light stress of D1, and to a lesser extent D2-the other reaction centre protein, CP43 and CP29 have also been shown to undergo degradation. In an oxygenic environment, D1 is cleaved from its N- and C-termini and the disassembly of the photosystem II complex involves simultaneous release of manganese and three extrinsic proteins involved in oxygen evolution. It is known that protein with PEST sequences are subject to degradation; D1 protein contains a PEST sequence adjacent to the site of cleavage on the outer side of thylakoid membrane between helices IV and V. The molecular processes of ''triggering'' of D1 for proteolytic degradation are not clearly understood. The changes in structural organization of photosystem II due to generation of oxy-radicals and other highly oxidizing species have also not been resolved. Whether CP43 or a component of the photosystem II reaction centre itself (D1, D2 or cyt b559 subunits), which may be responsible for degradation of D1, is also subject to light modification to become an active protease, is also not known. The identity of proteases degrading D1, LHCII and CP43 and C29 remains to be established.