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...

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.

No abstract

There is growing evidence that severe acute respiratory syndrome coronavirus 2 can affect the CNS. However, data on white matter and cognitive sequelae at the one-year follow-up are lacking. Therefore, we explored these characteristics in this study. We investigated 22 recovered coronavirus disease 2019 (COVID-19) patients and 21 matched healthy controls. Diffusion tensor imaging, diffusion kurtosis imaging and neurite orientation dispersion and density imaging were performed to identify white matter changes, and the subscales of the Wechsler Intelligence scale were used to assess cognitive function. Correlations between diffusion metrics, cognitive function, and other clinical characteristics were then examined. We also conducted subgroup analysis based on patient admission to the intensive care unit. The corona radiata, corpus callosum and superior longitudinal fasciculus had lower volume fraction of intracellular water in the recovered COVID-19 group than in the healthy control group. Patients who had been admitted to the intensive care unit had lower fractional anisotropy in the body of the corpus callosum than those who had not. Compared with the healthy controls, the recovered COVID-19 patients demonstrated no significant decline in cognitive function. White matter tended to present with fewer abnormalities for shorter hospital stays and longer follow-up times. Lower axonal density was detected in clinically recovered COVID-19 patients after one year. Patients who had been admitted to the intensive care unit had slightly more white matter abnormalities. No significant decline in cognitive function was found in recovered COVID-19 patients. The duration of hospital stay may be a predictor for white matter changes at the one-year follow-up.