STRUCTURAL AND FUNCTIONAL CHARACTERIZATION OF C-PHYCOCYANIN FROM THE CYANOPHYTE AGMENELLUM QUADRUPLICATUM

Native C-phycocyanin from the blue-green alga Agmenellum quadruplicatum and its separated alpha and beta subunits are characterized. The holoprotein has a molecular weight of 34,500, absorption maxima at 622 nm in phosphate buffer and 665 nm in 8.0 M acid urea, a flourescence emission maximum at 650 nm, and a molar absorbance coefficient in urea of 94,400. Both the absorbance and fluorescence maxima are shifted in the denatured state, in acid urea. The holoprotein consists of an alpha and a beta subunit with molecular weights of 16,000 and 18,500 respectively. The absorption maximum of the alpha chain is at 622 nm and the beta chain is at 608 nm. These absorption maxima are also shifted in 8 M acid urea. The spectra in 8 M urea incidate that the alpha chain possesses one and the beta chain two phycocyanobilin chromophores. Amino acid compositions of the holoprotein and separated chains are given, along with tryptic and chymotryptic peptide maps for the alpha chain. The partial amino acid sequence of the alpha subunit of the Agmenellum quadruplicatum C-phycocyanin and the amino-terminal sequence of its beta subunit are presented. These are compared with the amino acid sequences of other cyanophytan phycocyanin alpha and beta subunits. The evolutionary, structural and functional implications are discussed.