Preliminary crystallographic investigations of two phycobiliproteins

Single crystal x-ray diffraction investigations are in progress on two phycobiliproteins. C-phycocyanin from Anabaena variabilis crystallizes in space group P63 with a = b = 154 A and c = 40 A. The crystallographic asymmetric unit is (alphabeta)2 with a total molecular mass of 7.0-10(4) daltons. B-phycoerythrin from Porphyridium cruentum crystallizes in space group R3 with a = b = 189 A and c = 60 A. This molecule has the unusual molecular stoichiometry (alphabeta)6gamma and the crystallographic asymmetric unit is (alphabeta)2gamma1/3. This requires that the gamma chain undergo a perfect 3-fold disordering about the crystallographic 3 axis, i.e. the gamma chain must occupy three symmetry-equivalent positions, each with an occupancy of one-third.