Bilin attachment sites in the alpha and beta subunits of B-phycoerythrin. Structural studies on a doubly...

A bilin-containing fragment of the beta subunit of Porphyridium cruentum B-phycoerythrin produced by cleavage with thermolysin was shown by sequence analysis (Lundell, D.J., Glazer, A.N., DeLange, R.J., and Brown, D.M. (1984) J. Biol. Chem. 259, 5472-5480) to have the following structure. (Formula: see text) Secondary ion mass spectrometry of this bilin-peptide yielded a protonated molecular ion of 1629 mass units corresponding to that predicted from the composition of the fragment, and indicated that the heptapeptide is linked to ring A and the tripeptide to ring D. NMR spectra provided definitive evidence for a thioether linkage at the C-3' carbon of ring A and a second thioether linkage at teh C-18' carbon of ring D of the bilin. This is the first documented report of a bilin linked through two thioether linkages to a polypeptide.