Rod substructure in cyanobacterial phycobilisomes: phycoerythrin assembly in synechocystis 6701 phycobilisomes.

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Abstract
Synechocystis 6701 phycobilisomes consist of a core of 3 cylindrical elements in an equilateral array from which extend in a fanlike manner 6 rods, each made up of 3-4 stacked disks. Previous studies have shown that the rods consist of 4 disk-shaped complexes of biliproteins with linker polypeptides of 27-, 33.5-, 31.5- and 30.5-kdaltons [kd], listed in order starting with the disk proximal to the core: phycocyanin (.alpha..beta.)6-27 kd, phycocyanin (.alpha..beta.)6-33.5 kd, phycoerythrin (.alpha..beta.)6-31.5 kd, phycoerythrin (.alpha..beta.)6-30.5 kd, where .alpha..beta. is the monomer of the biliprotein. Phycoerythrin complexes of the 31.5 and 30.5-kd polypeptides were isolated in low salt. In 0.05 M K-phosphate-1 mM EDTA at pH 7.0, these complexes had the average composition (.alpha..beta.)2-31.5 and (.alpha..beta.)-30.5 kd polypeptide, respectively. Peptide mapping of purified 31.5- and 30.5-kd polypeptides showed that they differed significantly in primary structure. In 0.65 M Na-K-phosphate at pH 8, these phycoerythrin complexes formed rods of stacked disks of composition (.alpha..beta.)6-31.5 or (.alpha..beta.)6-30.5 kd. For the (.alpha..beta.)-30.5 kd complex, the yield of rod assemblies was variable and the self-association of free phycoerythrin to smaller aggregates was an important competing reaction. Complementation experiments were performed with incomplete phycobilisomes from Synechocystis 6701 mutant strain CM25. These phycobilisomes are totally lacking phycoerythrin and the 31.5- and 30.5-kd polypeptides, but have no other apparent structural defects. In high phosphate at pH 8, the phycoerythrin-31.5-kd complex formed disk assemblies at the end of the rod substructures of CM25 phycobilisomes whereas no interaction with the phycoerythrin-30.5 kd complex was detected. In mixtures of both the phycoerythrin-31.5 and -30.5 kd complexes with CM25 phycobilisomes, both complexes were incorporated at the distal ends of the rod substructures. The efficiency of energy transfer from the added phycoerythrin in complemented phycobilisomes was .apprx. 96%. The ordered assembly of phycoerythrin complexes seen in phycobilisomes is reproduced in the in vitro assembly process.