Allophycocyanin B. A common beta subunit in Synechococcus allophycocyanin B (lambda max 670 nm) and allophycocyanin (lambda max 650 nM).

In cyanobacterial phycobilisomes, light energy absorbed by phycocyanin (lambda max 620 nm) is transferred to allophycocyanin (AP; lambda max 650 nm) and allophycocyanin B (AP-B; lambda max 670 nm) and emitted primarily at 680 nm. This emission maximum coincides with that of pure AP-B. Previous studies have shown that Synechococcus 6301 AP and AP-B are both (alpha beta)3 trimers with a number of similar properties. Here, we show that the beta subunits of AP and AP-B have identical molecular weights, isoelectric points, absorption spectra, NH2-terminal sequences, yield almost indistinguishable tryptic peptide maps, and can substitute for each other in hybridization with the alpha subunits. We conclude that the beta subunits are identical polypeptides. By the same criteria, the alpha subunits of these two proteins are clearly unique polypeptides. The alpha subunits carry the information which determines the absorption and fluorescence emission spectra of the proteins, e.g. the hybrid of the alpha subunits of AP-B with the beta subunits of AP has a lambda max of 670 nm and an emission maximum of 680 nm. In mixtures of AP-B and AP, heterologous trimers are formed in a near-statistical manner by subunit exchange. In such trimers, AP-B alpha beta monomers act as terminal energy acceptors.