Crystallization and 2.2 angstrom E resolution structure of R-phycoerythrinfrom Gracilaria chilensis: a case of perfect hemihedral twinning

R-phycoerythrin, a light-harvesting component from the red algae Gracilaria chilensis, was crystallized by vapour diffusion using ammonium sulfate as precipitant agent. Red crystals grew after one week at 293 K and diffracted to 2.70 Angstrom resolution. Three serial macroseeding assays were necessa ry to grow a second larger crystal to dimensions of 0.68 x 0.16 x 0.16 mm. This crystal diffracted to 2.24 Angstrom resolution using synchrotron radia tion at beamline BM14 of the European Synchrotron Radiation Facility (ESRF) at Grenoble, France and was used for structure determination. Data were co llected at 100 K to a completeness of 98.6%. The crystal was trigonal, spac e group R3, with unit-cell parameters a = b = 187.3, c = 59.1 Angstrom, alp ha = beta = 90, gamma = 120 degrees. Data treatment using the CCP4 suite of programs indicated that the crystal was twinned ([I-2]/[I](2) = 1.41). Mol ecular replacement was performed with AMoRe using the R-phycoerythrin from Polysiphonia urceolata [Chang et al. (1996), J. Mol. Biol. 249, 424-440] as a search model. In order to overcome the twinning problem, SHELX97 was use d for the crystallographic refinement. The twin fraction was 0.48, indicati ng a nearly perfect hemihedrally twinned crystal. The final R-work and R-fr ee factors are 0.16 and 0.25, respectively. All the residues and chromophor es of the alpha- and beta -chains are well defined in the electron-density maps. Some residues belonging to the gamma -linker are also recognizable.