Carotenoids and the Assembly of Light-harvesting Complexes

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SummaryCarotenoids are constitutive components of all light-harvesting complexes in plants and many such complexes in bacteria. In the crystal structures of several light-harvesting complexes, carotenoids are seen to span the lipid bilayer and connect components of the complex on both membrane surfaces and/or to mediate the interaction of transmembrane protein helices. This important stabilizing function suggests that these pigments are also actively involved in the assembly of light-harvesting complexes. Verification of this notion appears too ambitious a goal at present, as the question of how the pigment-protein complexes of the photosynthetic apparatus are assembled is still open. However, information is emerging about which light-harvesting complexes depend on the presence of carotenoids during their assembly, and which carotenoids are specifically required. This information comes from experiments in which allor some carotenoids are missing during biogenesis of the photosynthetic apparatus, due either to inhibitors of carotenoid biosynthesis or mutations in carotenoid biosynthesis pathways. Further information comes from reconstitution experiments in vitro in which light-harvesting complexes are assembled from their apoproteins and a pigment mixture containing a restricted or heterologous selection of carotenoids.One important result of such studies is that the peripheral light-harvesting complex in bacteria, LH2, but not the core antenna LH1, is dependent on carotenoids for stable assembly. The chlorophyll-a/b light-harvesting complexes in plants also assemble only when carotenoids are present. In some organisms, lutein appears to play a particularly important role: when lutein is missing, at least some of the chlorophyll-a/b complexes do not assemble. On the other hand, in other organisms the lack of lutein has no obvious effect on the chlorophyll-a/b antenna. An explanation may come from in-vitro reconstitution experiments: light-harvesting complexes in which luteins presumably are replaced with other carotenoids exhibit lower stabilities than those reconstituted with the authentic pigment, and different organisms may vary in their ability to tolerate this reduced complex stability.