Rod structure of a phycoerythrin II-containing phycobilisome. II. Complete sequence and bilin attachment site of a phycoerythrin gamma subunit.

The major phycoerythrins of marine unicellular cyanobacteria Synechococcus spp. and those of red algae are isolated as stable complexes with the composition (alpha beta)6 gamma. The gamma subunits carry bilins and in this respect are unique among phycobilisome rod linker polypeptides. There has been no complete amino acid sequence data on any gamma subunit. Synechococcus sp. WH8020 phycoerythrin II (PE II) gamma subunit was isolated from purified PE II hexamers. Partial amino acid sequence determination showed that the gamma subunit was encoded by the mpeC gene, an open reading frame 275 base pairs 3' of mpeA and mpeB, which encode the alpha and beta subunits of PE II, respectively (Wilbanks, S. M., and Glazer, A. N. (1993) J. Biol. Chem. 268, 1226-1235). A single phycourobilin is attached through a thioether bond to gamma-Cys-49. Derivatization with 4-vinylpyridine showed that the only other cysteinyl residue, gamma-Cys-64, is unsubstituted. MpeC encodes a polypeptide of 293 residues with a predicted molecular weight of 32,100 and a pI of 8.9. These properties are like those of non-bilin-bearing linker polypeptides associated with C-phycoerythrin and hence the gamma subunit is designated gamma (LR32). Alignment of the sequence of the PE II-gamma with those of the latter polypeptides shows that PE II-gamma has a 49-residue extension at the N terminus, that encompasses the phycourobilin attachment site, and is shorter by a similar number of residues at the C terminus. These differences in linker polypeptide structure offer a possible explanation for the observed much higher stability of PE II hexamers relative to those of C-phycoerythrins.