Isolation and characterization of three membranebound chlorophyll-protein complexes from four dinoflagellate species

Abstract

Abstract Employing discontinuous sucrose density gradient centrifugation of n-dodecyl β-d-maltoside-solubilized thylakoid membranes, three chlorophyll (Chl)-protein complexes containing Chl a, Chl c2 and peridinin in different proportions, were isolated from the dinoflagellates Symbiodinium microadriaticum, S. kawagutii, S. pilosum and Heterocapsa pygmaea. In S. microadriaticum, the first complex, containing 13% of the total cellular Chl a, and minor quantities of Chl c2 and peridinin, is associated with polypeptides with apparent molecular mass (Mr) of 8-9 kDa, and demonstrated inefficient energy transfer from the accessory pigments to Chl a. The second complex contains Chl a, Chl c2 and peridinin in a molar ratio of 1:1:2, associated with two apoproteins of Mr19-20 kDa, and comprises 45%, 75% and 70%, respectively, of the cellular Chl a, Chl c2 and peridinin. The efficient energy transfer from Chl c2 and peridinin to Chl a in this complex is supportive of a light-harvesting function. This Chl a-c2- peridin-protein complex represents the major light-harvesting complex in dinoflagellates. The third complex obtained contains 12% of the cellular Chl a, and appears to be the core of photosystem I, associated with a light-harvesting complex. This complex is spectroscopically similar to analogous preparations from different taxonomic groups, but demonstrates a unique apoprotein composition. Antibodies against the water-soluble peridinin-Chl a-protein (sPCP) light-harvesting complexes failed to cross-react with any of the thylakoid-associated complexes, as did antibodies against Chl a-c-fucoxanthin apoprotein (from diatoms). Antibodies against the P700 apoprotein of plants did not cross-react with the photosystem I complex. Similar results were observed in the other dinoflagellates.