An analysis of the fucoxanthin-chlorophyll proteins and the genes encoding them in the unicellular marine raphidophyte, Heterosigma carterae: characterization and evolution

• Main Author: Durnford, Dion Glenn
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/7251

The light-harvesting complexes (LHC) of the unicellular marine chromophyte, Heterosigma carterae, were fractionated by sucrose-density gradient centrifugation, following digitonin solubilization, and by non-denaturing SDS-PAGE. The sucrose gradient allowed for the isolation of a major light-harvesting complex fraction, containing approximately 53% of the total chlorophyll, the majority of the chlorophyll c and a single polypeptide of 19.5 kDa. Up to 12 different polypeptides immunologically related to both the fucoxanthin-Chi a/c complexes (FCPs) and to the chlorophyll a + b-binding proteins (CABs) were detected in thylakoids and in the lower photosystem I (PS I) enriched fractions. Using a modification of the non-denaturing gel system of Allen and Staehelin (1991 Anal. Biochem. 194, 2 14-222) allowed the resolution of a number of large pigment-protein complexes which included several PS I and PS II fractions along with a predominant LHC fraction, an improvement over previously published methods. A Fcp eDNA from Heterosigina carterae has been cloned and sequenced. It encodes a 210 amino acid polypeptide that has similarity to other FCPs and to the CABs of terrestrial plants and green algae. Comparison of the FCP sequence to the recently determined 3- dimensional structure of the pea LHC II complex indicates that many of the key amino acids thought to participate in the binding of chlorophyll and in the formation of complex-stabilizing ionic interactions between hydrophobic regions of the protein are well conserved. In addition, the Fcp genes are part of a large multigene family with greater than 20 related members in Heterosigina. Phylogenetic analyses of the LHC protein sequences shows that the FCPs form a natural group separate from the iPCPs of the dinoflagellates. Though there are obvious similarities between the FCPs and the CABs, the relationships are very distant. Analyses of polypeptides in the red algae Aglaotharnn ion neglectuin and Porphyridium cruentum, in collaboration with Greg Wolfe and Beth Gantt, were the first to demonstrate that polypeptides immunologically related to the CABs and the FCPs are present within the Rhodophyceae. In addition, CAB/FCP-related LHCs have not been detected in a cyanobacterium (Nostoc) and a prochiorophyte (Prochlorothrix). This suggests the CAB/FCP related LHCs arose only once after the establishment of the chioroplast and provides some evidence that suggests chioroplasts evolved from a symbiotic cyanobacterium-like organism only once (monphyletic). The organization of the antennae in Heterosigma carterae is equally as complex as that in the terrestrial plants. This is indicated by the detection of at least 12 LHC-related polypeptides and the presence of a large multigene family encoding the FCPs. In addition, the immunological relatedness and the sequence conservation of the FCPs with the CABs indicates that the structure of the LHCs has been conserved throughout evolution and that these different antennae complexes share a common ancestor.