PARENT SESSION
Posters P8B Supermolecular organization of the photosynthetic apparatus. Abstracts (592-611)

Participation of acidic lipids in structural and functional maintenance of the PSI and PSII complexes. NORIHORO SATO^*,1, MOTOHIDE AOKI¹, KUNIHIRO SUDA¹, MIKIO TSUZUKI¹, ¹ School of Life Science, Tokyo University of Pharmacy and Life Science, HACHIOJI, TOKYO, JAPAN

ABSTRACT- Chloroplasts and the postulated origin, cyanobacteria, contain sulfoquinovosyl diacylglycerol (SQDG) and phosphatidylglycerol (PG) as acidic membrane lipids. We have been investigating roles of the acidic lipids in cyanobacteria through characterization of respective mutants defective in the synthesis of SQDG or PG. The SQDG-mutant of Synechocystis sp. PCC6803 was impaired in PSII as represented by its lowered specific activity and enhancement in the sensitivity to DCMU. The mutant thus indicated involvement of SQDG in the functional and structural maintenance of the PSII complex, as was the case with an SQDG-defective mutant of a green alga Chlamydomonas reinhardtii. In the contrast, the SQDG-mutant of Synechococcus sp. PCC7942 was impaired neither in the PSII activity nor DCMU-sensitivity, indicating no contribution of SQDG to PSII and diversity of the role of SQDG in cyanobacteria. On the other hand, the PG-mutant of Synechocystis sp. PCC6803, distinct from its SQDG-mutant, failed to accumulate the PSI complex. Besides, the in vivo removal of PG detrimentally affected conformation of the PSI complex such that population of the PSI trimer, which normally exists to function in vivo, was decreased with an increase in the level of the PSI monomer. Despite such a drastic conformational change, the PSI activtiy measured with diaminodurene and methylviologen as the electron donor and acceptor, respectively, was little damaged. Similar results, i.e. dissociation of the PSI trimer into the monomer with little deleterious effect on the PSI activity, were observed also with isolated wild-type thylakoid membranes after in vitro depletion of PG by the action of a phospholipase. These results obtained through in vivo and in vitro deprivation of PG demonstrated that PG contributes to quantitative and conformational maintenance of the PSI complex.

KEY WORDS: PSII, PSI, phosphatidylglycerol, sulfoquinovosyl diacylglycerol