Primary structure of phycocyanin from the unicellular rhodophyte Cyanidium caldarium. II. Complete amino acid sequence of the beta subunit.

The complete amino acid sequence of the beta subunit of phycocyanin from the unicellular rhodophyte Cyanidium caldarium, has been determined by automated sequential degradation of cyanogen bromide, tryptic, and Staphylococcus aureus V8 protease peptides. The beta subunit contains 172 amino acids with methionine and glutamine the NH2- and carboxyl-terminal amino acids, respectively. The calculated molecular weight of the protein, based on the sequence, is 19,572. Two phycocyanobilin chromophores are covalently attached by cysteinyl thioether linkages to residues 82 and 153. A third cystine (residue 109) occurs in the beta subunit, but it is not attached to phycocyanobilin. Comparison of the complete amino acid sequence of the beta subunit of C. caldarium phycocyanin with the sequences of the phycocyanin beta subunits from two cyanobacteria, shows that the sequence homology previously noted at the NH2 terminus of phycobiliproteins from distantly related organisms extends along the entire polypeptide chain. The amino acid sequences of the alpha and beta subunits of C. caldarium phycocyanin are also similar, and by proper alignment of the sequences it can be shown that the beta subunit contains a 12-residue insertion where the second phycocyanobilin chromophore is covalently attached. A matrix comparing the alpha and beta subunits of phycobiliproteins for which the complete sequences are known has been determined, and based on these data, a scheme is proposed for evolution of the family of phycobiliproteins in living cyanobacteria and red algae from a protein precursor which gave rise initially to a beta-type allophycocyanin subunit.