THE NUCLEAR-ENCODED PSBW PROTEIN SUBUNIT OF PHOTOSYSTEM-II UNDERGOES LIGHT-INDUCED PROTEOLYSIS

The repair of photoinhibitory damage to photosystem II involves the ra pid degradation and turnover of the D1 reaction center subunit. Additi onal protein subunits which show a limited degradation at high light i ntensities are the complementary reaction center subunit, D2, and the two chlorophyll a binding proteins, CP 47 and CP 43. In this work, we provide the first evidence for light-induced degradation of a nuclear- encoded subunit of photosystem II, the recently discovered PsbW protei n. This 6.1 kDa protein is predicted to have a single membrane span an d was found to be closely associated with the photosystem II reaction center. The degradation of the PsbW protein was demonstrated by photoi nhibitory experiments, both in vitro, using thylakoid membranes and ph otosystem II core particles, and in vivo using leaf discs. The PsbW pr otein showed almost the same rate and extent of degradation as the D1 protein, and its degradation was more pronounced compared to the D2 an d CP 43 proteins. The degradation of the PsbW protein was shown to sha re many mechanistic similarities with the more well characterized D1 p rotein degradation, such as oxygen dependence, sensitivity to serine p rotease inhibitors, and high light triggering while the actual degrada tion could readily occur in total darkness. The degradation of the Psb W protein was impaired by protein phosphorylation, although this prote in was not itself phosphorylated. This impairment was correlated to th e phosphorylation of the D1 protein which has been shown to block its degradation during photoinhibitory conditions. It is concluded that th e PsbW protein is not degraded as a direct consequence of primary phot odamage but due to a general destabilization of the photosystem II com plex under conditions were the D1 protein becomes degraded in the abse nce of a sufficient repair system. The results are discussed in terms of a requirement for coordination between degradation and protein synt hesis/integration during the repair process of photodamaged photosyste m II reaction centers.