Fucoxanthin-chlorophyll proteins in diatoms: 18 and 19 kDa subunits assemble into different oligomeric states.

Fucoxanthin-chlorophyll proteins were purified from the centric diatom Cyclotella meneghiniana. Two major fractions were observed that differed in their polypeptide composition and oligomeric state. Trimers consist of mainly 18 kDa polypeptides. Higher oligomers are tightly assembled from different trimers, which contain mostly 19 kDa subunits. In both oligomeric states, the excitation energy coupling between fucoxanthin and chlorophyll a was preserved, and chlorophyll c was shown to transfer energy efficiently to chlorophyll a. Circular dichroism spectra showed close interaction between fucoxanthin and chlorophyll a, and different chlorophyll a molecules were demonstrated to interact excitonically. The assembly of trimers of antenna proteins with a distinct subunit composition into higher oligomeric states was not reported so far and differs from the situation found in higher plants. The differences in the supramolecular structure of the fucoxanthin-chlorophyll proteins reflect the dissimilar arrangement of the thylakoid membranes in diatoms, which lack the grana-stroma distinction.