The use of pseudosymmetry in the rotation function of gamma IVa-crystallin

Bovine lens gamma IVa-crystallin crystallizes in space group C222(1) with cell dimensions a = 35.1, b = 46.2, c = 186.2 A, and contains one molecule in the asymmetric unit. The structure was determined at 3.0 A resolution using cross-rotation functions and R-factor searches with the bovine lens protein gamma II-crystallin as the model structure. The rotation function appears to be very sensitive to the resolution range and type of coefficient employed; the use of normalized structure-factor amplitudes gave the best results. The potential problem of a pseudo solution due to an internal pseudo-twofold axis was put to advantage by aligning this axis parallel to z. The results of the R-factor search were well defined. The molecular replacement solution was improved by rigid-body least-squares refinement, initially of the whole molecule, then for the two domains. The R factor at this stage was 39.4% at 2.3-10.0 A. The gamma IVa structure has an even higher internal symmetry than gamma II, since the two domains are related by a rotation around the pseudo-twofold axis of 178.7 degrees as compared with 176.2 degrees for gamma II.