Interaction of the allophycocyanin core complex with photosystem II.

Allophycocyanin core complexes were purified from the thermophilic cyanobacterium Thermosynechococcus elongatus and analysed by negative-stain electron microscopy and single-particle averaging. The purified complex was found to consist of three allophycocyanin cylinders. The single-particle analysis of end-on views of the complex revealed a mirror axis, indicative of two-fold symmetry. This observation allowed the assignment of the allophycocyanin base cylinders and the identification of their potential interaction sites with the thylakoid membrane and with the photosystem II reaction centre in particular. The T. elongatus allophycocyanin core projection map, together with published information on the structure of photosystem II for the same organism, was used to construct a model for the allophycocyanin core-photosystem II dimer supercomplex, from which docking sites between both complexes are suggested. The implications of this association for energy transfer from allophycocyanin to photosystem II are discussed.