Subunits and chromophores of a type I phycoerythrin from a Chroomonas sp. (Cryptophyceae)

Phycoerythrin from a Chroomonas sp. CS24 has been characterised with respect to relative molecular mass, subunit composition and spectral properties (absorbance and fluoresccence). The native protein has a typical type I absorbance spectrum, except for a shoulder at 644 nm. The proposed structure for the phycoerythrin is α₂β₂ with a relative molecular mass of 63.5 kDa. The β subunit of relative molecular mass 20.5 kDa contains the chromophore phycoerythrobilin. There are four α subunits which differ in charge. Two of these are blue with absorbance and fluorescence maxima in 8 M urea (pH 5.1) that are similar to those of phycocyanin, suggesting the presence of a phycocyanobilin-like chromophore. These two subunits differ in relative molecular mass (11 and 12 kDa). The remaining two α subunits are purple at pH 5.1 in 8 M urea and posses a chromophore similar to the unidentified chromophore in the α subunit of phycoerythrocyanin (PXB). These subunits also differ in relative molecular mass (11 and 12 kDa). Titration of the α subunits with HCl and NaOH results in a reversible change between the phycocyanobilin-like and PXB-like forms. It is proposed that a single novel chromophore is present on the α subunits that mimics phycocyanobilin and PXB according to its environment and is probably identical with cryptoviolin (McColl, R., Guard-Friar, D. and Csatorday, K. (1983) Arch. Microbiol. 135, 194-198).