PARENT SESSION

Symposium S6B Photosynthetic acclimation: Mechanisms and gene expression
Thursday September 2nd, 2004 10:20 AM-12:40 PM Room 210A
Chair: Norio Murata
Co-Chair: Eva-Mari Aro

FtsH metalloproteases VAR1 and VAR2 involved in the repair cycle of Photosystem II. Wataru Sakamoto^*,1, ¹ Research Institute for Bioresources, Okayama University, Kurashiki, Okayama, Japan

ABSTRACT- Photooxidation causes irreversible damages in Photosystem II and other photosynthetic complexes, and the mechanisms to avoid such a photodamage have been extensively investigated. One of the important ways is to recognize and degrade photodamaged proteins and to replace it with newly synthesized subunit proteins. Such a repair mechanism may require a protease and chaperone activity. Recent studies by our group and others imply that several proteases are involved in degradation of protein components in thylakoid membranes. Among these, FtsH is one of the best-characterized proteases in thylakoid membranes. We have recently shown that VAR1 and VAR2 are the major Arabidopsis FtsHs involved in a repair cycle of photosystem II (Sakamoto et al. 2003, Plant Cell 15:2843). Arabidopsis VAR1 and VAR2 are separate loci that encode similar chloroplast FtsH proteases. We have cloned these genes by characterizing a category of leaf-variegated mutants termed yellow variegated (var1 and var2). Mutations in either VAR1 or VAR2 cause typical leaf variegation and sensitivity to photoinhibiton. In contrast, these phenotypes were never observed in any of the other FtsH genes, suggesting that VAR1and VAR2 predominantly act on the photoprotection mechanism. By generating VAR1- and VAR2-specific antibodies, we showed that loss of either VAR1 or VAR2 protein results in decreased accumulation of the other. Thus, the genetic non-redundancy between VAR1 and VAR2 could be attributed to their coordinated regulation at the protein level. These observations led us to examine whether VAR1 and VAR2 form a hetero-complex in thylakoid membranes. Our data from sucrose density gradient, gel filtration chromatography, and co-immunoprecipitation analyses indicate that VAR1 and VAR2 constitute a hetero-complex in Arabidopsis thylakoid membranes and play a major role in the repair cycle of Photosystem II.

KEY WORDS: Arabidopsis thaliana, leaf variegation, FtsH metalloprotease, thylakoid membrane