Core substructure in cyanobacterial phycobilisomes.

The tricylindrical core of Synechocystis 6701 phycobilisomes is made up of four types of allophycocyanin-containing complexes: A, (alpha AP beta AP)3; B, (alpha AP beta AP)3 .10K; C, (alpha APB1 alpha AP2 beta AP3).10K; D, (alpha AP beta AP)2.18.5K.99K; where AP is allophycocyanin, APB is allophycocyanin B, and 10K, 18.5K, and 99K are polypeptides of 10,000, 18,500, and 99,000 daltons, respectively. The 18.5K polypeptide is a hitherto unrecognized biliprotein subunit with a single phycocyanobilin prosthetic group. The tricylindrical core is made up of 12 subcomplexes in the molar ratio of A:B:C:D: of 4:4:2:2. Complexes C and D act as terminal energy acceptors. From these results and previous analysis of the bicylindrical core of Synechococcus 6301 phycobilisomes [14,15] it is proposed that the two cylinders of the Synechocystis 6701 core, proximal to the thylakoid membrane, each have the composition ABCD, and that the distal cylinder has the composition A2B2.