Bilin--apoprotein linkages in rhodophytan phycobiliproteins: the role of cysteine.

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The photosynthetically active algal biliproteins phycocyanin and allophycocyanin contain the hnear tetrapyrrole, phycocyanobilin, which is covalently attached to apoprotein [l] . The structure of phycocyanobilin has been established [2,3] but the nature of the covalent linkages to apoprotein has remained controversial [l] . Attempts to elucidate bilin-apoprotein linkages have included treatment of phycobiliproteins with agents which specifically cleave ester, ether and peptide bonds [4], sequencing [5] and amino acid analysis [6,7] of chromopeptides obtained by proteolysis of phycobiliproteins, and analysis of the pattern of imides derived from the chromophore of phycobiliproteins during chemical degradation in chromic acid [8] . We have shown that phycocyanin and allophycocyanin from the unicellular rhodophyte, Cyanidium caldarium, are comprised of dissimilar polypeptide subunits, termed (Y and 0, that the OL : fl chromophore composition of the phycocyanin subunits is 1 : 2, and that the Q : @ chromophore composition of the allophycocyanin subunits is 1 : 1 [9,10] . Clearly the complexity of the subunit and chromophore composition of these phycobiliproteins could allow for more than one type of covalent attachment between phycocyanobilin and amino acids in the subunit polypeptides. 0 Carra and 0 hEocha [ 1 ] concluded that the best evidence seems to indicate that one type of bilin-apoprotein linkage is a thioether bond between a cysteinyl residue and the ethylidine group of ring I of phycocyanobilin and that a second type of linkage is an ester bond between a seryl residue and the propionic Fig.1. Diagram illustrating proposed linkages between phycocyanobilin and apoprotein [ 1 ] .