The Functional Significance of the Monomeric and Trimeric States of the Photosystem II Light Harvesting Complexes

Abstract

The main light harvesting complex of photosystem II in plants, LHCII, exists in a trimeric state. To understand the biological significance of trimerization, a comparison has been made been LHCII trimers and LHCII monomers prepared by treatment with phospholipase. The treatment used caused no loss of chlorophyll, but there was a difference in carotenoid composition, together with the previously observed alterations in absorption spectrum. It was found that, when compared to monomers, LHCII trimers showed increased thermal stability and a reduced structural flexibility as determined by the decreased rate and amplitude of fluorescence quenching in low-detergent concentration. It is suggested that LHCII should be considered as having two interacting domains: the lutein 1 domain, the site of fluorescence quenching [Wentworth et al. (2003) J. Biol. Chem. 278, 21845-21850], and the lutein 2 domain. The lutein 2 domain faces the interior of the trimer, the differences in absorption spectrum and carotenoid binding in trimers compared to monomers indicating that the trimeric state modulates the conformation of this domain. It is suggested that the lutein 2 domain controls the conformation of the lutein 1 domain, thereby providing allosteric control of fluorescence quenching in LHCII. Thus, the pigment configuration and protein conformation in trimers is adapted for efficient light harvesting and enhanced protein stability. Furthermore, trimers exhibit the optimum level of control of energy dissipation by modulating the development of the quenched state of the complex.

On the regulation of photosynthesis by excitonic interactions between carotenoids and chlorophylls

“…It has long been known, that LHCII fluorescence quenching in vitro, for example upon acidification, has similarities with nonphotochemical quenching of Chl fluorescence observed in entire plants under high-light conditions (27). Also similarly as in plants, the extent of fluorescence quenching in vitro is influenced by the presence of the carotenoid Zea (28). We therefore first determined Coupling Car S1ϪChl for LHCII as a function of different conditions that can lead to fluorescence quenching.…”

“…2B demonstrates the direct, linear correlation between Coupling Car S1ϪChl and fluorescence quenching (NPQ), as quantified in this manner. For LHCII preparations that do not contain Zea significantly less fluorescence quenching can be observed in otherwise identical experimental conditions (28). It is, however, important to note, that Zea is not absolutely mandatory to achieve quenching.…”

“…2D). The occurrence of such red shifted bands in LHCII is also related to oligomerization (28). It might be surprising that the combination of a relatively small amount of 0.14 Zea molecules per monomer of LHCII with the acidification leads to a reduction of the fluorescence of LHCII by Ͼ50% in comparison to the LHC II samples without Zea.…”

The lack of LHCII proteins modulates excitation energy partitioning and PSII charge recombination in Chlorina F2 mutant of barley

“…As the regulated ∆pH-and zeaxanthin-dependent NPQ is believed to occur within the LHCII antenna and depend on its oligomerization state (Horton et al 1996;Wentworth et al, 2004) the strongly reduced abundance of Lhcb proteins in the F2 mutant ( Fig. 1) (Bossmann et al, 1997;Krol et al, 1995Krol et al, , 1999 can easily explain the lower NPQ in the F2 mutant.…”

“…The role of pH-and zeaxanthin-dependent shifts in the oligomerization state of LHCII in developing the rapidly relaxing energy dependent component (qE) of NPQ is well established with qE representing the major protective mechanism against photoinhibitory damage of PSII (Horton et al, 1996; Niyogi, 1999). It has been demonstrated that trimers of LHCII exhibit the optimum level of non-photochemical energy dissipation by modulating the development of the quenched state of the complex (Wentworth et al, 2004).…”