Most plants have the ability to respond to fluctuations in light to minimize damage to the photosynthetic apparatus. A proteolytic activity has been discovered that is involved in the degradation of the major light-harvesting chlorophyll a/b-binding protein of photosystem II (LHCII) when the antenna size of photosystem II is reduced upon acclimation of plants from low to high light intensities. This ATP-dependent proteolytic activity is of the serine or cysteine type and is associated with the outer membrane surface of the stroma-exposed thylakoid regions. The identity of the protease is not known, but it does not correspond to the recently identified chloroplast ATP-dependent proteases Clp and FtsH, which are homologs to bacterial enzymes. The acclimative response shows a delay of 2 d after transfer of the leaves to high light. This lag period was shown to be attributed to expression or activation of the responsible protease. Furthermore, the LHCII degradation was found to be regulated at the substrate level. The degradation process involves lateral migration of LHCII from the appressed to the nonappressed thylakoid regions, which is the location for the responsible protease. Phosphorylated LHCII was found to be a poor substrate for degradation in comparison with the unphosphorylated form of the protein. The relationship between LHCII degradation and other regulatory proteolytic processes in the thylakoid membrane, such as D1-protein degradation, is discussed.The main focus of photosynthesis research has been the functional and structural aspects behind the energytransducing process. The progress has been significant, and the photosynthetic protein complexes can now be described at a very refined molecular level (Barber and Andersson, 1994). Conversely, relatively little is known about the acclimating, regulatory, and protective processes that maintain high photosynthetic efficiency during everfluctuating and even stressful environmental conditions.The identification of such auxiliary processes associated with thylakoid membranes and the characterization of the enzymes involved are therefore central tasks of current photosynthesis research. Examples of auxiliary enzymes or components are kinases and phosphatases responsible for reversible protein phosphorylation (Gal et al., 1997), desaturases controlling the lipid composition and dynamics of thylakoid membranes (Wada et al., 1990), stress-induced proteins such as the ELIPs (Adamska, 1997), and several enzymatic activities involved in various forms of regulatory proteolysis Andersson and Aro, 1997). In the latter case, D1-protein turnover during repair of damage caused by photoinhibition is the most illustrative example (Prasil et al., 1992; Andersson and Aro, 1997). The knowledge of chloroplast proteases involved in regulatory proteolysis is still very limited. The presence of homologs to bacterial proteases in chloroplasts, such as the Clp and FtsH proteins, has provided new insights and possibilities (Gray et al., 1990;Moore and Keegstra, 1993; Lindahl et...
