Bilin attachment sites in the alpha and beta subunits of B-phycoerythrin. Amino acid sequence studies.

The amino acid sequence about the sites of attachment of all of the bilin prosthetic groups of the alpha and beta subunits of Porphyridium cruentum B-phycoerythrin has been determined. The sequences of five unique tryptic peptides, each carrying one phycoerythrobilin, are as follows: alpha-1 Ile-Asx-Lys-Cys*-Tyr-Arg alpha-2 Asx-Arg-Leu-Cys*-Val-Pro-Arg beta-1 Met-Ala-Ala-Cys*-Leu-Arg beta-2 Met-Ser-Phe-Ala-Ala-Gly-Asp-Cys*-Thr-Ser-Leu-Ala-Ser-Glu-Val-Ala-Gln-Tyr - Phe-Asp-Arg beta-3 Leu-Asp-Ala-Val-Asn-Ser-Ile-Val-Ser-Asn-Ala-Ser-Cys*-Met-Val-Ser-Asp-Ala - Val-Ser-Gly-Met-Ile-Cys*-Glu-Asn-Pro-Gly-Leu-Ile-Ser-Pro-Gly-Gly-Asn-Cys -Tyr- Thr-Asn-Arg where the designations alpha and beta refer to the subunit from which the peptide was derived. Cysteinyl residues involved in bilin attachment are indicated with an asterisk. The bilins in peptides alpha-1, alpha-2, beta-1, and beta-2 are attached to the peptide through a single thioether linkage to a cysteinyl residue. In contrast, the phycoerythrobilin on peptide beta-3 is attached through two thioether linkages to cysteinyl residues 10 residues apart. This appears to be the first report of a prosthetic group covalently bound to a polypeptide through two linkages separated by such a considerable distance in the linear sequence. The visible absorption spectrum of peptide beta-3 is red-shifted by about 10 nm relative to the spectra of the other four bilin peptides. Comparison of the sequences from B-phycoerythrin to sequences of several other biliproteins has revealed the presence of a number of invariant tyrosyl and arginyl residues near bilin attachment sites.