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

Abstract

The 32 000‐dalton Q_B‐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 Q_B‐protein. The removal of the damaged Q_B‐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 Q_B‐protein could be identified either by autoradiography or immunologically using polyclonal antibodies specific for the Q_B‐protein. The lightinduced damage to the Q_B‐protein (indicated by a loss of Q_B functional activity), preceded the removal of the protein from the membrane. We conclude that photodamage of the Q_B‐protein generates a conformational change which renders the protein susceptible to attack by a highly efficient, intrinsic membrane protease.