Crystallization and molecular packing analysis of barstar crystals.

Barstar, the natural inhibitor of barnase crystallizes in many different crystal forms under almost identical conditions. Although barstar is a monomeric protein, it crystallizes with four molecules in the asymmetric unit in two crystal forms, rhombohedral (space group R3; a = b = 118.0 angstrum; c = 75.5 angstrum) and tetragonal (space group P4; a = b = 105.1 angstrum; c = 36.0 angstrum), which exist simultaneously under identical crystallization conditions. The relation between the four molecules in the asymmetric unit of the crystals belonging to space group P4 can be interpreted in terms of a small distortion in the crystallographic symmetry of the higher symmetry space group P422.