Molecular architecture of a light-harvesting antenna. Quaternary interactions in the Synechococcus 6301 phycobilisome core as revealed by partial tryptic digestion and circular dichroism studies.

The core of the phycobilisomes of Synechococcus 6301 (Anacystis nidulans) strain AN112 consists of two cylindrical elements each made up of the same four distinct subcomplexes: A (alpha AP beta AP)3; B (alpha AP beta AP)2 . 18.3K . 75K; C (alpha 1APB alpha 2AP beta 3AP) . 10.5K; and D (alpha AP beta AP)3 . 10.5K, where alpha AP and beta AP are the subunits of allophycocyanin, alpha APB is the subunit of allophycocyanin B, and 18.3K, 75K, and 10.5K are polypeptides of 18,300, 75,000, and 10,500 Da, respectively. An 18 S subassembly containing subcomplexes A and B has previously been characterized (Yamanaka, G., Lundell, D. J., and Glazer, A. N. (1982) J. Biol. Chem. 257, 4077-4086; Lundell, D. J., and Glazer, A. N. (1983) J. Biol. Chem. 258, 894-901, 902-908). A ternary core subassembly, containing complexes A, B, and C, was isolated from a limited tryptic digest of AN112 phycobilisomes and characterized with respect to composition and spectroscopic properties. Isolation of this ternary subassembly also establishes that subcomplex D must occupy a terminal position in each of the two core cylinders. Spectroscopic studies of the individual complexes, A-D, of the subassemblies AB and ABC, and of intact AN112 phycobilisomes showed core assembly-dependent changes in the circular dichroism spectra indicative of changes in the environment and/or conformation of the bilin chromophores within the individual subcomplexes. Two terminal energy acceptors are present in the phycobilisome core, alpha APB and 75K. No indication of interaction between the chromophores on these polypeptides was detected by circular dichroism spectroscopy. This result indicates that the bilins on alpha APB and 75K act as independent energy acceptors rather than as exciton pairs.