Biliprotein Assembly in the Disc-Shaped Phycobilisomes of Rhodella violacea. On the Molecular Composition of Energy-Transfering Complexes (Tripartite Units) Forming the Periphery of the Phycobilisome*

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Abstract
Heterogeneous complexes with MW of about 790,000 containing B-phycoerythrin (Bangiales phycoerythrin) and C-phycocyanin (Cyanophycease phycocyanin) in a molar pigment ratio of 2:1 were isolated from purified, dissociated phycobilisomes. EM investigations revealed structures of 3 discs aggregated face to face with an apparent distance of 1.5 nm between each disc. Two discs may represent phycoerythrin and 1 phycocyanin. The complexes are structurally identical with tripartite units of the phycobilisome periphery. Fluorescence data confirmed the integrity of isolated tripartite units. Excitation at 546 nm gives a fluorescence maximum at 644 nm, indicating intermolecular transfer of excitation energy from phycoerythrin to phycocyanin. Comparative subunit analyses and spectral data suggested that no allophycocyanin is present. Cross-linking experiments gave evidence for a polar arrangement of phycocyanin within the complex. This pigment itself is an aggregate of 2 smaller molecules each having a MW of about 140,000. Tripartite units contain all the phycoerythrin and phycocyanin of the phycobilisome. On this basis, a phycobilisome model is proposed which combines the aspects of biliprotein distribution, energy transfer and fine structure.