PRIMARY STRUCTURE OF PHYCOCYANIN FROM THE UNICELLULAR RHODOPHYTE CYANIDIUM-CALDARIUM .1. COMPLETE AMINO-ACID-SEQUENCE OF THE ALPHA-SUBUNIT

The complete amino acid sequence of the .alpha. subunit of phycocyanin from the unicellular rhodophyte C. caldarium was determined by automated sequential degradation of cyanogen bromide peptides, tryptic peptides derived from protein chemically modified with 1,2-cyclohexanedione or citraconic anhydride and a peptide obtained after cleavage of the protein at the single tryptophan residue. The .alpha. subunit contains 162 amino acids and methionine and serine are the NH2- and carboxyl-terminal amino acids, respectively. The calculated MW of the protein, based on the amino acid sequence, is 18,303, in good agreement with the value of 17,500 .+-. 500, obtained by electrophoresis on calibrated sodium dodecyl sulfate-polyacrylamide gels. One phycocyanobilin chromophore is attached to the .alpha. subunit at residue 84 by a cysteinyl thioether linkage. A 2nd cysteine (residue 98) is present but is not linked to phycocyanobilin. The amino acid sequence of the .alpha. subunit of phycocyanin from C. caldarium is the 1st complete amino acid sequence of a phycobiliprotein from a eukaryotic alga. Extensive homology occurs between the .alpha. subunit of phycocyanin from C. caldarium and from 2 prokaryotic cyanobacteria, and the significance of this is discussed.