| Summary: | 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.
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