Structural determination of a partially hemihedrally twinned actin crystal

An orthorhombic actin crystal (space group P2₁2₁2₁, unit-cell parameters a = 101.6, b = 103.0, c = 127.0 Å) was converted into a partially hemihedrally twinned tetragonal crystal (space group P4₃, unit-cell parameters a = b = 101.5, c = 104.2 Å) by induced condensation. This condensation (decrease in the c axis) was caused by the flash-freezing of the crystal, with 30% PEG 400 as a cryoprotectant, prior to data collection. Diffraction data for the twinned tetragonal crystal were collected at 100 K to 3.0 Å resolution (99.8% completeness with an R_sym of 8.1%) using synchrotron radiation. The hemihedral twinning of the data was observed by self-rotation function analysis and was determined to have a partial twin fraction of 0.376 from intensity statistics. The structure, with two actin molecules in the crystallographic asymmetric unit, was determined by molecular-replacement methods and refined to an R factor of 0.193. As a consequence of the crystal lattice transformation from the orthorhombic P2₁2₁2₁ to the tetragonal P4₃ space group, actin-actin contacts were rearranged and an inter-actin dimer disulfide bond (Cys374) observed in the orthorhombic crystal form was broken in the tetragonal crystal form.