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

Abstract

The complete amino acid sequence of the a subunit of phycocyanin from the unicellular rhodophyte Cyanidium caldarium has been determined by automated sequential degradation of cyanogen bromide peptides, tryptic peptides derived from protein chemically modified with 1,2-~yclohexanedione or citraconic anhydride, and a peptide obtained after cleavage of the protein at the single tryptophan residue.The a subunit contains 162 amino acids and methionine and serine are the NH2and carboxyl-terminal amino acids, respectively.The calculated molecular weight of the protein, based on the amino acid sequence, is 18,303, in good agreement with the value of 17,500 f 500, obtained by electrophoresis on calibrated sodium dodecyl sulfate-polyacrylamide gels.One phycocyanobilin chromophore is attached to the a subunit at residue 84 by a cysteinyl thioether linkage.A second cysteine (residue 98) is present but is not linked to phycocyanobilin.The amino acid sequence of the a subunit of phycocyanin from C. caldarium is the first complete amino acid sequence of a phycobiliprotein from a eukaryotic alga.Extensive homology occurs between the a subunit of phycocyanin from C. caldarium and from two prokaryotic cyanobacteria, and the significance of this is discussed.