Light-dependent degradation of the QB -protein in isolated pea thylakoids

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Abstract
The 32 000‐dalton QB‐protein of photosystem II (PS II) is rapidly damaged and removed from isolated pea thylakoids during incubation in the light resulting in a loss of photosynthetic electron flow through PS II. This in vitro photoinhibition is similar to that previously reported with intact Chlamydomonas cells. The damage occurs at a faster rate in vitro, however, due to the inability of isolated thylakoids to synthesize replacement QB‐protein. The removal of the damaged QB‐protein does not require any soluble components of the chloroplast stroma and is unaffected by the protease inhibitors phenyl‐methylsulfonylfluoride or antipain. Unlike the effect of trypsin, no low mol. wt. membrane‐bound or soluble fragments of the labelled QB‐protein could be identified either by autoradiography or immunologically using polyclonal antibodies specific for the QB‐protein. The lightinduced damage to the QB‐protein (indicated by a loss of QB functional activity), preceded the removal of the protein from the membrane. We conclude that photodamage of the QB‐protein generates a conformational change which renders the protein susceptible to attack by a highly efficient, intrinsic membrane protease.

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