Dynamics of photosystem II: a proteomic approach to thylakoid protein complexes.

Oxygenic photosynthesis produces various radicals and active oxygen species with harmful effects on photosystem II (PSII). Such photodamage occurs at all light intensities. Damaged PSII centres, however, do not usually accumulate in the thylakoid membrane due to a rapid and efficient repair mechanism. The excellent design of PSII gives protection to most of the protein components and the damage is most often targeted only to the reaction centre D1 protein. Repair of PSII via turnover of the damaged protein subunits is a complex process involving (i) highly regulated reversible phosphorylation of several PSII core subunits, (ii) monomerization and migration of the PSII core from the grana to the stroma lamellae, (iii) partial disassembly of the PSII core monomer, (iv) highly specific proteolysis of the damaged proteins, and finally (v) a multi-step replacement of the damaged proteins with de novo synthesized copies followed by (vi) the reassembly, dimerization, and photoactivation of the PSII complexes. These processes will shortly be reviewed paying particular attention to the damage, turnover, and assembly of the PSII complex in grana and stroma thylakoids during the photoinhibition-repair cycle of PSII. Moreover, a two-dimensional Blue-native gel map of thylakoid membrane protein complexes, and their modification in the grana and stroma lamellae during a high-light treatment, is presented.

A SecY homologue is involved in chloroplast-encoded D1 protein biogenesis.

We have used the photosystem II reaction center D1 protein as a model to study the mechanisms of targeting and insertion of chloroplast-encoded thylakoid membrane proteins. The unusually high turnover rate and distinct pausing intermediates during translation make the D1 protein biogenesis particularly suitable for these purposes. Here we show that cpSecY, a chloroplast homologue of bacterial essential translocon component SecY, interacts tightly with thylakoid membrane-bound ribosomes, suggesting its involvement in protein translocation and insertion. Co-immunoprecipitation and cross-linking experiments indicated that cpSecY resides in the vicinity of D1 elongation intermediates and provided evidence for a transient interaction of cpSecY with D1 elongation intermediates during the biogenesis of D1. After termination of translation, such interactions no longer existed. Our results indicate that, in addition to a well characterized role of cpSecY in posttranslational translocation of nuclear-encoded proteins, it seems to be also involved in cotranslational membrane protein translocation and insertion in chloroplasts.