Light-Harvesting Complexes in the Dinoflagellate Symbiodinium

Symbiodinium, the most commonly found endosymbionts of corals, are key constituents of coral reef ecosystems. Their photosynthesis is essential for the maintenance of the algal-host relationship, but is susceptible to various factors such as increased temperature and/or irradiance. However, the molecular level understanding of Symbiodinium photosynthesis is limited. In this work, the light-harvesting complexes (LHCs) in Symbiodinium were studied. There are two major LHCs, namely, the water-soluble peridinin-chlorophyll a-proteins (PCPs) and the thylakoid-intrinsic chlorophyll a-chlorophyll c2-peridinin-protein (apcPC). PCP is not related to any other LHCs, while acpPC is a member of the CAB family of light-harvesting complexes. They are heterogeneous at both protein and gene levels, which could contribute to the eco-physiological adaptations of Symbiodinium. They contain the carotenoid peridinin, unique to certain dinoflagellates, as the primary light-harvester. In each complex, the major energy transfer channel is the peridinin S1/ICT state to Chl a Qy pathway with the overall efficiency close to unity. Energetically, excitations from PCP and acpPC transfer to photosystems, and acpPC is an intermediate between PCP and PSII. Symbiodinium sp. CS-156 possesses a major PCP, which is acidic and of the long form (32.7 kDa), in spite of the existence of multiple PCPs. We sequenced several PCPs at gene and protein levels via molecular cloning and LC-MS/MS analysis, revealing that the intronless PCP gene cassette is composed of 1,095-bp coding regions and spacers in between, and the mature PCP apoprotein is 312 amino acids in length. Pigment analysis showed a peridinin-to-Chl a ratio of 4. Size-exclusion chromatography (SEC) and small angle neutron scattering (SANS) analysis indicated PCP exists as monomers in solution. Native mass spectrometry (native MS) demonstrated two oligomeric states of PCP, monomer and trimer. The trimeric PCP had a relative abundance of less than 5 % compared to monomeric PCP, and is likely to be a concentration-driven effect. We conclude that the trimerization is not necessary for PCP to function as a light-harvesting complex, although it was observed in X-ray crystallography. The chlorophyll a-chlorophyll c2-peridinin-protein (apcPC), another major light harvesting component in peridinin-containing dinoflagellates, was purified in this study. Both SDS-PAGE and electrospray ionization mass spectrometry (ESI-MS) analysis quantified the denatured subunit polypeptide molecular weight (MW) to be 18 kDa. The size-exclusion chromatography (SEC), blue native gel electrophoresis (BN-PAGE) and native ESI-MS estimated that the native acpPC complex has a molecular weight of 66 kDa. Based the pigment stoichiometry (Chl a/Chl c2/peridinin/diadinoxanthin 4/6/6/1), we propose that the functional oligomeric state of acpPC is a trimer