Subparticles of Anabaena phycobilisomes. II. Molecular assembly of allophycocyanin cores in reference to "anchor" protein.

Entire phycobilisomes (PBS) and two derived particles, whole allophycocyanin (APC) cores and the far-red-emitting fragment of APC cores (14.5 S APC), all containing the 115-kDa polypeptide ("anchor protein"), were compared for the readiness with which the 115-kDa protein could be modified chemically, be degraded by chymotrypsin, and react with the anti-115-kDa serum. The 115 kDa in PBS and the whole APC cores were digested slightly by chymotrypsin and did not react with anti-115-kDa IgG. In contrast, the 115 kDa in 14.5 S APC was digested to 42 kDa and showed a positive reaction with anti-115-kDa IgG. Reconstitution of APC cores from 14.5 S APC and APC trimers was inhibited by the anti-115-kDa IgG, and APC particles with partly digested 115-kDa cannot reconstitute APC cores. These results imply that 115 kDa is embedded mostly inside PBS and is involved more in the maintenance of the molecular assembly of APC cores than in the "anchoring" of PBS to thylakoid. The PBS from Anabaena variabilis (M3) have an APC core larger than those of other PBS and show atypical morphology consisting of five APC discs. They have a polypeptide (115 kDa) that is significantly longer than the corresponding polypeptides (around 95 kDa) of other blue-green algae. This can be interpreted by assuming a relationship between the size of APC cores and the length of the polypeptide.