Crystallization and preliminary X-ray investigation of non-specific complexes of a mutant ribonuclease T1 (Y45W) with 2'AMP and 2'UMP.

We have succeeded in crystallizing complexes of a mutant ribonuclease T1 (Y45W) with the non-cognizable ribonucleotides 2'AMP and 2'UMP by macroscopic seeding of microcrystals of the mutant enzyme complexed with 2'GMP, which is the cognizable nucleotide inhibitor. The mutant enzyme has a tryptophan residue instead of Tyr45 of the wild-type enzyme and thus this mutation enhances the binding of ribonucleotides to the enzyme. The space group is P212121 with unit cell dimensions a = 49.40 A, b = 46.71 A, c = 41.02 A for the complex with 2'AMP and a = 48.97, b = 46.58 A, c = 40.97 A for the complex with 2'UMP, both of which are poorly isomorphous to the mother crystals. Diffraction data for the complexes with 2'AMP and 2'UMP were collected on a diffractometer at 1.7 A and 2.4 A resolution, respectively. The present studies show that crystallization of non-specific complexes of other protein-ligand systems with the dissociation constants around 10(-3) M, or even larger, could be feasible by application of the seeding technique. A comparison of the crystal structures of the complexes with that with 2'GMP may serve as a structural basis for the determination of differences between the specific and non-specific interactions of the enzyme.