Complex between bovine ribonuclease A and porcine ribonuclease inhibitor crystallizes in a similar unit cell as free ribonuclease inhibitor.

We obtained three different morphologies of co-crystals of bovine ribonuclease A and porcine ribonuclease inhibitor. X-ray quality crystals were grown in 1.3M ammonium sulfate, 100 mM sodium acetate (pH 5.0) and 20 mM dithiothreitol at 21 degrees C. These crystals have the symmetry of the tetragonal space group I4 with a = 133.3 A and c = 86.7 A and diffract to 2.5 A resolution; they have the same symmetry and only slightly different cell parameters than the crystals of free ribonuclease inhibitor. Polyacrylamide gel electrophoresis and the crystal density indicate that both ribonuclease inhibitor and ribonuclease A are present in the crystals. Although small, crystals are suitable for three-dimensional structural analysis.