Regulatory Proteolysis of the Major Light-Harvesting Chlorophyll a/b Protein of Photosystem II by a Light-Induced Membrane-Associated Enzymic System

Abstract

An endogenous proteolytic activity associated with spinach chloroplast thylakoid membranes has been identified. This enzymic activity is involved in the degradation of the major light-harvesting chlorophyll a/b protein of photosystem II (LHCII) in response to exposure of leaves to increased irradiance. This proteolysis of LHCII requires an induction period and can only be detected 48-72 hours after transfer of the plants from low-intensity to high-intensity light. Once initiated by high-intensity light, the degradation of LHCII can readily occur in complete darkness. The proteolysis can, after induction in vivo, be experimentally followed in vitro, both in isolated intact chloroplasts and thylakoid membranes. The proteolytic process is strictly dependent on ATP and the protease involved is of the serine or cysteine type. The activity can be released from isolated thylakoid membranes by washing with high concentrations of NaCl and reconstituted by readdition of the desalted wash supernatant. It is concluded that the protease is extrinsically bound to the outer surface of the stroma-exposed regions of the stacked thylakoid membrane. The mechanism for the induction of the proteolytic process as well as its relation to previously described thylakoid proteases will be discussed.

AtFtsH6 is involved in the degradation of the light-harvesting complex II during high-light acclimation and senescence

“…In vivo, this trigger must in some way sense the ''surplus'' of antenna size. Both our data (24) and those of other authors (14,20,21) indicate that the substrate availability (or structure), rather than the presence of the protease per se, determines the proteolytic rate in chloroplasts.…”

“…However, the kinetics of FtsH6-catalyzed, HL-dependent degradation of LHC II seem to be quicker than those of the activity reported in ref. 20, in which was observed ATP-dependent degradation of the LHC II monomer only after a lag phase of 48 -72 h. These data suggest that FtsH6-mediated proteolysis is the primary step, but it is also possible that there are multiple proteolytic activities that can act on LHC II. If, in the future, mutants lacking other proteolytic activities are isolated, the relative importance of these proteases in LHC II degradation can be addressed.…”

“…These proteases are localized peripherally at either the stroma thylakoid membranes (19) or at the outer surface of the grana thylakoids (14,20,38). Their main substrate during chl deficiency conditions (17,18) and during acclimation to HL (14,20,38) was found to be the LHC II monomer. LHC II trimer (19) or LHC II N-terminal peptide (22) preaccumulated in mature chloroplasts.…”

“…It has been suggested that a light-activated serine͞cysteine-type protease is involved in LHC II degradation during bean chloroplast development (17,18) and in the degradation of LHC II trimers that have preaccumulated in mature bean thylakoid membranes (19). Another serine͞cysteine-type proteolytic enzyme has been implicated in the ATP-dependent degradation of LHC II apoproteins during high-light acclimation of spinach plants (14,20,21). Both enzymes are claimed to be peripherally attached to the stromal side of the thylakoid membrane.…”

Overexpression of chlorophyllide a oxygenase (CAO) enlarges the antenna size of photosystem II in Arabidopsis thaliana

“…These results are apparently not consistent with the observation in many Chl-b de®cient mutants lacking most of the Lhc proteins, except for Lhcb5 (Harrison and Melis, 1992;Bossman et al, 1997;Kro  l et al, 1995). A mechanism involving Lhc-speci®c proteases (Lindahl et al, 1995;Yang et al, 1998) and/or a vesiclemediating transport system of Lhcb proteins (Hoober and Eggink, 1999) Genetic modi®cation of the antenna size might be a useful tool for future improvement in crop productivity. Plants that are distributed both in sun and shade conditions tend to change their a : b ratio more drastically upon change in the light environment than those distributed exclusively in sun or shade conditions (Murchie and Horton, 1997).…”