Isolation and Characterization of Three Membrane-Bound Chlorophyll-Protein Complexes from Four Dinoflagellate Species

Employing discontinuous sucrose density gradient centrifugation of n-dodecyl $\beta$ -d-maltoside-solubilized thylakoid membranes, three chlorophyll (Chl)-protein complexes containing Chl a, Chl c $\_2$ 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 c $\_2$ and peridinin, is associated with polypeptides with apparent molecular mass (M $\_r$ ) of 8-9 kDa, and demonstrated inefficient energy transfer from the accessory pigments to Chl a. The second complex contains Chl a, Chl c $\_2$ and peridinin in a molar ratio of 1:1:2, associated with two apoproteins of M $\_r$ 19-20 kDa, and comprises 45%, 75% and 70%, respectively, of the cellular Chl a, Chl c $\_2$ and peridinin. The efficient energy transfer from Chl c $\_2$ and peridinin to Chl a in this complex is supportive of a light-harvesting function. This Chl a-c $\_2$ -peridinin-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 P $_{700}$ apoprotein of plants did not cross-react with the photosystem I complex. Similar results were observed in the other dinoflagellates.