Further evidence for a phycobilisome model from selective dissociation, fluorescence emission, immunoprecipitation, and electron microscopy.

Phycobilisomes, isolated in 500 mM Sorensen's phosphate buffer pH 6.8 from the red alga, Porphyridium cruetum, were analyzed by selective dissociation at various phosphate concentrations. The results are consistent with a structural model consisting of an allophycocyanin core, surrounding by a hemispherical layer of R-phycocyanin, with phycoerythrin being on the periphery. Such a structure also allows maximum energy transfer. Intact phycobilisomes transfer excitation energy ultimately to a pigment with a fluorescence emission maximum at 675 nm. This pigment is presumed to be allophycocyanin in an aggreagated state. Uncoupling of energy transfer among the pigments, and physical release of the phycobiliproteins from the phycobilisome follow a parallel time-course; phycoerythrin is released first, followed by R-phycocyanin, and then allophycocyanin. In 55 mM phosphate buffer, the times at which 50% of each phycobiliprotein has dissociated are: phycoerythrin 40 min, R-phycocyanin 75 min, and allophycocyanin 140 min. The proposed arrangement of phycobiliproteins within phycobilisomes is also consistent with the results from precipitation reactions with monospecific antisera on intact and dissociated phycobilisomes. Anti-phycoertythrin reacts almost immediately with intact phycobilisomes, but reactivity with anti-R-phycocyanin and anti-allophycocyanin is considerably delayed, suggesting that the antigens are not accessible until a loosening of the phycobilsome structure occurs. Reaction wbilisomes, but is much more rapid in phycobilisomes of Nostoc sp. which contains 6-8 times more allophycocyanin. It is proposed that allophycocyanin is partially exposed on the base of isolated intact phycobilisomes of both algae, but that in P. cruentum there are too few accessible sites to permit a rapid formation of a precipitate with anti-allophyocyanin.