Crystallization and 2.2 Å E resolution structure of R-phycoerythrin from 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 necessary 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 radiation at beamline BM14 of the European Synchrotron Radiation Facility (ESRF) at Grenoble, France and was used for structure determination. Data were collected at 100 K to a completeness of 98.6%. The crystal was trigonal, space group R3, with unit-cell parameters a = b = 187.3, c = 59.1 Angstrom, alpha = 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). Molecular 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 used for the crystallographic refinement. The twin fraction was 0.48, indicating a nearly perfect hemihedrally twinned crystal. The final R-work and R-free factors are 0.16 and 0.25, respectively. All the residues and chromophores of the alpha- and beta -chains are well defined in the electron-density maps. Some residues belonging to the gamma -linker are also recognizable.