Understanding the Functional Oligomerization State of Peridinin-Chlorophyll a-Proteins in the Dinoflagellate Symbiodinium

Symbiodinium is the most commonly found group of endosymbiotic dinoflagellates in corals. Peridinin-chlorophyll a-proteins (PCP) are one of the major light harvesting complexes in photosynthetic dinoflagellates. The X-ray crystal structure of MFPCP (main form PCP) of Amphidinium carterae has revealed a trimer [1], while HSPCP (high-salt form PCP) from the same strain and recombinant PCP crystallized as monomers [2, 3]. It is still unclear, whether the trimer of Amphidinium carterae MFPCP is an exception, or trimerization is common among PCPs. Our study utilized native mass spectrometry, gel filtration chromatography, chemical cross-linking and small-angle neutron scattering (SANS) to reveal the functional oligomerization state(s) of PCP in the dinoflagellate Symbiodnium. We found that most PCP in aqueous buffer is monomeric, with the presence of a small amount of trimeric PCP. We conclude that PCP monomer is able to fulfill its function, e.g. light-harvesting, but the oligomerization states in vivo remains to be understood.

[1] E. Hofmann, P.M. Wrench, F.P. Sharples, R.G. Hiller, W. Welte, K. Diederichs, Structural basis of light harvesting by carotenoids: peridinin-chlorophyll-protein from Amphidinium carterae, Science, 272 (1996) 1788-1791.

[2] T. Schulte, F.P. Sharples, R.G. Hiller, E. Hofmann, X-ray structure of the high-salt form of the peridinin-chlorophyll a-protein from the dinoflagellate Amphidinium carterae: modulation of the spectral properties of pigments by the protein environment, Biochemistry, 48 (2009) 4466-4475.

[3] T. Schulte, D.M. Niedzwiedzki, R.R. Birge, R.G. Hiller, T. Polivka, E. Hofmann, H.A. Frank, Identification of a single peridinin sensing Chl-a excitation in reconstituted PCP by crystallography and spectroscopy, Proc Natl Acad Sci U S A, 106 (2009) 20764-20769.