Assembly of light-harvesting complexes

I. Introduction Possible Structural Role of Carotenoids in the Assembly of Light-harvesting Complexes... 124... 

E. Assembly of Light-harvesting Complexes into Larger Units. 131... 

Chlamydomonas mutants have been isolated that are deficient either in violaxanthin de-epoxidase or zeaxanthin epoxidase. Using these mutants, it could be demonstrated that only part of the non-photochemical quenching observed in Chlamydomonas is dependent on the formation of zeaxanthin (Niyogi etal., 1997a). It will be interesting to see whether the absence of xanthophyll epoxidase or de-epoxidase products has an effect on the assembly of light-harvesting complexes in these mutants. 

An involvement of carotenoids in the thylakoid insertion of light-harvesting proteins has also been concluded from a pulse-chase measurement of the rate of protein insertion into membranes that have reduced amounts of carotenoids, due to norflurazon treatment (Dahlin and Timko, 1997). It should be kept in mind that carotenoids may also control other biosynthetic steps in the synthesis and assembly of light-harvesting complexes such as the import of the protein precursors into plastids (Dahlin, 1993 Dahlin and Franz6n, 1997). 

Havaux M (1998) Carotenoids as membrane stabilizers in chloroplasts. Trends Plant Sci 3 147-151 Heinze 1, Pfiindel E, Huhn M and Dau H (1997) Assembly of light harvesting complexes 11(LHC 11) in the absence oflutein A study on the alpha carotenoid free mutant C 2a 34 of the green alga Scenedesmus obliquus. Biochim Biophys Acta 1320 188-194... 

Scherz A, Rosenbachbelkin V, and Fisher JRE. Distribution and Self-Organization of Photosynthetic Pigments in Micelles - Implication for the Assembly of Light-Harvesting Complexes and Reaction Centers in the Photosynthetic Membrane. Proc. Natl. Acad Sci. USA 1990 87 5430-5434. 

Paulsen H. Carotenoids and the assembly of light harvesting complexes. Advances in Photosynthesis and Respiration. The Photochemistry of Carotenoids (Frank HA, Young AJ, Britton G, Cogdell RJ, eds).1999 8 123-136. Kluwer Academic Publishers, Dordrecht. 

Fig. 12. Simple models of various photosystems (A) purple bacteria, (B) green bacteria, (C) cyanobacteria and red algae, (D) bro n algae and (E) green plants. RC=reaction center FCPA=fucoxanthin-chlorophyll-protein assembly LHC=light-harvesting complex PS ll=photosystem II PS l=photosystem I. See text for discussion.

Horn R, Paulsen H (2004) Early steps in the assembly of light-harvesting chlorophyll a/b complex. J Biol Chem 279(43) 44400-44406... 

Lang HP and Hunter CN. The relationship between carotenoid biosynthesis and the assembly of light-harvesting LH2 complex in Rhodobacter sphaeroides. Biochem. J. 1994 298 197-205. 

Booth PJ, and Paulsen H. Assembly of light harvesting chlorophyll a/b complex in vitro. Time-resolved fluorescence measurements. Biochem 1996 35 5103-5108. 

Layered materials are used to produce supramolecular assemblies, in which the properties of the assemblies differ from those of the individual members of the assembly, such that the whole is more than the sum of its components. Such a behavior may be used as a criterion to define a supramolecular system, and nature provides numerous examples. The light-harvesting complex of the photosynthetic apparatus is one such example of outstanding organization. 

The present work illustrates in some detail the result of a unique mutation, one that impaired the biosynthesis of Chi b and resulted in a truncated Chi antenna size for the photosystems [Tanaka et al. 1998]. In the present Chi Mess mutant, PSII contained 93 Chi a molecules and PSI contained 246 Chi a molecules (Table 3). These antenna sizes are significantly larger than the PSII-core and PSl-core antennae, suggesting that Chi b may not be absolutely essential for the assembly of all Chi a-b light-harvesting complexes (see also [Ghirardi et al. 1986]). This was especially true for PSI which, in the Chi Mess mutant, had a Chi antenna size almost as large as that of the control (Table 3). It may be concluded that the Chi Mless mutation can be overcome by a nearly quantitative substitution of Chi b with Chi a in the Chi antenna of PSI, and by a partial substitution by Chi a in the antenna of PSII. 

Morrissey PJ, Glick RE and Melis A. (1989). Supramolecular assembly and function of subunits associated with the chlorophyll a-b light-harvesting complex II (LHCII) in soybean chloroplast. Plant Cell Physiol. 30, 335-344. 

Fig. 1. Models for the light-harvesting complexes surrounding the PS-II reaction center in the thylakoid (A) Simplified model of the thylakoid in cross section, (B) More detailed model of PS II. (A) taken from Fig. 21 (A) in Chapter 1. The light-harvesting complex portion in (B) adapted from H-E Akerlund (1993) Function and organization of photosysfem II. In C Sundqvist and M Ryberg (eds) Pigment-Protein Complexes in Plastids Synthesis and Assembly, p 419. Acad Press.

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