STUDIES ON ENGINEERING CRYSTALLIZABILITY BY MUTATION OF SURFACE RESIDUES OF HUMAN THYMIDYLATE SYNTHASE

A study was made to determine the efficacy of altering a protein's intrinsic solvation properties to improve its crystallization properties. In order to change its solubility properties, twelve mutants of thymidylate synthase (TS) were made altering single amino acids at eleven different positions on the protein surface. The mutations changed either the charge or polarity of the wild-type amino acid. Wild-type TS and each of the mutants were subjected to a matrix of crystallization conditions varying pH, precipitant, and salt. After two weeks, each crystallization attempt was examined and scored for protein solubility and crystallization. Accordingly, the parameters of each condition were adjusted then repeated to drive the protein toward saturation without precipitating nonspecific aggregation. It was found that single amino acid changes on the surface of TS could have a dramatic effect on solubility while not decreasing stability. Furthermore, crystals of some mutant TSs were found to occur in conditions where wildtype TS did not crystallize and some mutant TSs showed enhanced crystallizability. The space groups of resulting crystals found in unique conditions or having unique morphologies were determined. Several of the mutant crystals were of different space groups than wild-type TS.