The light harvesting complex of the red alga Porphyridium cruentum: Pigment binding, energy transfer, and functional relatedness to LHCs of other algae and higher plants

Previous discovery of a light harvesting complex (LHC) in the red alga Porphyridium cruentum established a link between cyanobacteria, the proposed progenitors of chloroplasts, and photosynthetic eukaryotes such as higher plants where the Chl a/b-binding (CAB) LHCs are nuclear-encoded. In vitro reconstitution of a recombinant red algal LHC polypeptide (LHCaR1) with pigments was chosen as a means of examining the pigment-binding characteristics of this presumably primitive LHC and possible functional relationships with the LHCs of other photosynthetic eukaryotes. LHCaR1, reconstituted with its own pigments, bound about 8.0 Chl a, 4.0 zeaxanthin, and 0.3 β-carotene. Pigments were functionally integrated, as indicated by fluorescence excitation and emission spectra. The reconstituted LHC was spectrally similar to the native red algal LHCI. The pigments were found attached to protein upon separation on sucrose gradients and on non-denaturing gels. LHCaR1 was also reconstituted with pigments from the diatom Thallasiosira fluviatilis (Chls a and c, fucoxanthin, xanthin and β-carotene) and separately with those of the dinoflagellate Prorocenimm micans (Chls a and c, peridinin, diadinoxanthin, and β-carotene). 7 Chl a, 1 Chl c, 8 fucoxanthin, and 1.9 diadinoxanthin were bound in the complex reconstituted with diatom pigments. Reconstitution with diatom and dinoflagellate pigments resulted in absorption and fluorescence excitation spectra with maxima characteristic of Chl a, Chl c, fucoxanthin and peridinin. Fluorescence emission and CD spectra showed functional binding and orientation for energy transfer to Chl a from Chl c and carotenoids. When LHCaR1 was reconstituted with spinach pigments, 6.2 Chl a, 1.8 Chl b, and 2.4 lutein were bound per polypeptide. Pigments were functionaIly integrated as indicated by fluorescence excitation and emission spectra where Chl a, b, and lutein participated in light absorption and energy transfer. Eight Chl per polypeptide were bound in all reconstitutions, even when two different ChIs (a/c and a/ b) were present, consistent with eight conserved Chl-binding sites. Eight Chl binding sites and the ability to bind different pigments appear to be ancestral characteristics of LHCs. The ancestral ability to bind different pigments could have facilitated the adaptive radiation of photosynthetic eukaryotes into different aquatic light environments and eventually into terrestrial habitats.