The role of FtsH proteins in photosystem biosynthesis and turnover in Arabidopsis and Synechocystis

Abstract

Both Arabidopsis thaliana and Synechocystis PCC 6803 genomes encode several homologues of Escherichia coli FtsH. These proteins are AAA-family proteases (ATPases associated with a variety of cellular activities), which are widely distributed in prokaryotes and eukaryotes. Among their diverse roles, FtsH proteins are important in photosynthesis. Of four found in Synechocystis, one (encoded by open-reading frame slr0228) appears to be involved in photosystem I (PSI) biosynthesis, and it has been suggested that it may also participate in photosystem II (PSII) D1-protein turnover (see S8 poster by P. Silva et al.). In Arabidopsis, mutation of the `Var2¿-ftsH leads to severe leaf-variegation [1]. Whereas fluorescence-emission spectra of Synechocystis show a higher PSI:PSII ratio in wild-type (WT) than in slr0228¿ cells, in Arabidopsis, WT and Var2 have similar PSI:PSII peaks under normal growth conditions. High-light treatment, however, results in greater photoinhibition of Var2-FtsH¿ plants, measured by Fv/Fm. Fluorescence emission spectra also show reduced recovery from photoinhibitory high light in Var2 compared with WT. These experiments and Western blots indicate that the D1 repair cycle is impeded in the Arabidopsis FtsH mutant as in the cyanobacterium. Sequence similarities in the slr0228 and Var2 ftsH genes indeed suggest they may be closely-related proteins. To study the role of slr0228 in reaction centre biosynthesis, site-directed mutants have been made in PSII¿ and chlorophyll-synthesis-deficient Synechocystis strains. Further work characterising the role of plant and cyanobacterial FtsH proteins is presented. 1. Chen M, Choi Y, Voytas DF, Rodermel S: Mutations in the Arabidopsis VAR2 locus cause leaf variegation due to the loss of a chloroplast FtsH protease. Plant J 2000, 22:303.