The Complete Amino-Acid Sequence and the Phylogenetic Origin of Phycocyanin-645 from the Cryptophytan algaChroomonas sp.

The first complete amino-acid sequence of the cryptomonad phycobiliprotein phycocyanin-645 from Chroomonas sp. is presented. The alpha 1-subunit contains 70 amino-acid residues and the alpha 2-subunit 80 residues. In each of the alpha-subunits a green, 697-nm absorbing chromophore is covalently bound to Cys18. Both alpha-subunits contain a high number of charged residues. The phycocyanin-645 beta-subunit consists of 177 amino-acid residues. Two phycocyanobilin chromophores are singly bound to Cys beta 82 and Cys beta 158. A purple cryptoviolin-like chromophore is doubly bound to Cys beta 50 and Cys beta 61. Sequence comparisons revealed that the phycocyanin-645 beta-subunit is closely related to red algal phycoerythrin (73% identical amino-acid residues) and not so close to C-phycocyanin (55% identical amino-acid residues). The phycocyanin-645 alpha-subunits represent a special type of phycobiliprotein and a direct relationship to other phycobiliproteins or any light-harvesting polypeptide-pigment complexes could not be derived by sequence comparisons.