Light-harvesting complex II pigments and proteins in association with Cbr,a homolog of higher-plant early light-inducible proteins in the unicellular green alga Dunaliella

Like higher plants, unicellular green algae of the genus Dunaliella respond to light stress by enhanced de-epoxidation of violaxanthin and accumulatio n of Cbr, a protein homologous to early light-inducible proteins (Elips) in plants. Earlier studies indicated that Cbr was associated with the light-h arvesting complex of photosystem II (LHCII) and suggested it acted as a zea xanthin-binding protein and fulfilled a photo-protective function (Levy et al. 1993, J. Biol. Chem. 268: 20892-20896). To characterize the protein-pig ment subcomplexes containing Cbr in greater detail than attained so far, th ylakoid membranes from Dunaliella salina grown in high light or normal ligh t were solubilized with dodecyl maltoside and fractionated by isoelectric-f ocusing. Analysis of the resolved LHCII subcomplexes indicated preferred as sociations among the four LHCIIb polypeptides and between them and Cbr: sub complexes including Cbr contained one or two of the more acidic of the four LHCIIb polypeptides as well as large amounts of lutein and zeaxanthin rela tive to chlorophyll a/b. After sucrose gradient centrifugation, Cbr free of LHCIIb polypeptides was detected together with released pigments; this Cbr possibly originated in subcomplexes dissociated in the course of the analy sis. These results agree with the conclusion that Cbr is part of the networ k of LHCIIb protein-pigment complexes and suggest that the role played by C br involves the organization and/or stabilization of assemblies highly enri ched in zeaxanthin and lutein. Such assemblies may function to protect PSII from photodamage due to overexcitation.