Molecular architecture of a light-harvesting antenna. Core substructure in Synechococcus 6301 phycobilisomes: two new allophycocyanin and allophycocyanin B complexes.

Two new allophycocyanin-containing complexes were found among the products of partial dissociation of the phycobilisomes of Synechococcus 6301 strain AN112. These complexes were purified to homogeneity and characterized with respect to composition, stability, and spectroscopic properties. The structures of the complexes were established to be (alpha AP beta AP)3 . 10.5K and (alpha 1APB alpha 2AP beta 3AP) . 10.5 K, where alpha AP and beta AP are subunits of allophycocyanin, and alpha APB is the subunit of allophycocyanin B (see Lundell, D. J., and Glazer, A. N. (1981) J. Biol. Chem. 256, 12600-12606), and 10.5K is an uncolored polypeptide of 10.5-kilodaltons. These complexes are derived from the core substructure of the phycobilisome. Electron microscopic studies of the morphology of the core of strain AN112 phycobilisomes (Yamanaka, G., Glazer, A. N., and Williams, R. C. (1980) J. Biol. Chem. 255, 11004-11010) as well as structural studies of an 18 S subassembly derived from the phycobilisomes by partial dissociation (Yamanaka, G., Lundell, D. J., and Glazer, A. N. (1982) J. Biol. Chem. 257, 4077-4086) indicated that the core assembly consisted of two cylindrical elements each made up of the same four distinct "trimeric" biliprotein-containing complexes. Two such core components, (alpha AP beta AP)3 and alpha 2AP beta 2AP. 18.3K . 75K (where 18.3K and 75K are polypeptides of 18.3- and 75-kilodaltons), were shown to be contained within the 18 S subassembly (Lundell, D. J., and Glazer, A. N. (1983) J. Biol. Chem. 258, 894-901). The isolation of the two allophycocyanin-containing complexes described here completes the characterization of the four types of components in the Synechococcus 6301 phycobilisome core. Two lines of evidence indicate that each of the four complexes is present twice in the core: comparison of the compositions (and yields) of the complexes with that of the intact AN112 phycobilisome, and near-coincidence of the molar absorption spectrum of the phycobilisome with that generated by summing the spectra of the constituent complexes taken in appropriate molar proportions.