Location and Possible Functions of Lipids

Fourteen lipids were found in the 3.0A structure of PSII, which is significantly more than the four lipids found in PSI [18]. The 14 lipids of PSII included 4 diga-lactosyldiacylglycerol (DGDG), 6 monogalactosyldiacylglycerol (MGDG), 3 sulfo-quinovosyldiacylglycerol (SQDG), and 1 phosphatidyldiacylglycerol (PG). Among these lipids, 11 constitute a belt surrounding the PSII-RC, separating it from the antenna and some LMM subunits. The remaining three lipids are found at the monomer - monomer interface.

A number of studies have suggested some specific functions for lipids in PSII, either using mutants defective in synthesizing the specific lipid molecules, or by examining the effects of treatments with lipases and phospholipases on isolated thylakoid membranes or PSII particles (for reviews, see [69, 70]). For example, loss of SQDG was found to not only increase the sensitivity of PSII to the herbicide DCMU, but also to inhibit the electron transfer from water to TyrZ due to a decreased stability of the Mn4Ca cluster and the oxygen-evolving machinery [7173]. In particular, several lines ofevidence have indicated that PG is required for maintaining the integrity of the QB-binding site. The loss of PG, either by muta-genesis in vivo, or by phospholipase treatment in vitro, has been shown to inhibit the electron transfer from QA to QB and to destabilize the dimeric structure of PSII [69, 74, 75] . Whether or not these functions of SQDG and PG can be correlated with the positions of specific molecules identified in the crystal structure of PSII is not clear. Since multiple functions have been suggested for PG, and also since the only PG molecule identified in the crystal structure is neither close enough to QA and QB nor located in the monomer- monomer interface, the previously suggested functions for PG may not be accomplished by this single PG molecule. This suggests that there may be additional PG molecules associated with PSII. Indeed, it has been found that there are 2-3 PG molecules that can not be decomposed by phospholipase A. in the purified PSII dimer used for crystallization (Shen and coworkers, unpublished results), suggesting the presence of more than one PG molecule in PSII.

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