| C-Phycocyanin is a major light-harvesting protein in the filamentous cyanobacterium Anabaena variabilis. This phycobiliprotein is composed of two subunits, (alpha) and (beta), each of which has a molecular weight of about 16,000 daltons; the molecular stoichiometry is ((alpha)(beta))(,6). Crystals of the protein suitable for single crystal x-ray diffraction studies were grown by diffusion dialysis. Large crystals could also be reproducibly grown using crystal seeding techniques. The crystals form in space group P6(,3) (a = b = 154 (ANGSTROM), c = 40 (ANGSTROM)) with an asymmetric unit of ((alpha)(beta))(,2). Five three-dimensional data sets were collected to a resolution of 5 (ANGSTROM) by the rotation method. These were the native protein and derivatives of mercuric salicylate, K(,2)PtCl(,4), mercuric salicylate / K(,2)PtCl(,4), and K(,3)UO(,2)F(,5). Heavy atom positions were located by difference-Patterson and difference-Fourier techniques. The significant features of the 5 (ANGSTROM) electron density are as follows: the molecule is shaped like a prolate ellipsoid 110 (ANGSTROM) across, 40 (ANGSTROM) thick, and has symmetry 3. It can readily be divided into three ((alpha)(beta))(,2) equivalents; within each of these domains there are several columns of highly connected density. There is a solvent channel approximately 20 - 30 (ANGSTROM) in diameter that runs through the center of the molecule. |
