Crystallogenesis studies on yeast aspartyl-tRNA synthetase: use of phase diagram to improve crystal quality.

Aspartyl-tRNA synthetase (AspRS) extracted from yeast is heterogeneous owing to proteolysis of its positively charged N-terminus; its crystals are of poor quality. To overcome this drawback, a rational strategy was developed to grow crystals of sufficient quality for structure determination. The strategy is based on improvement of the protein homogeneity and optimization of crystallization, taking advantage of predictions from crystal-growth theories. An active mutant lacking the first 70 residues was produced and initial crystallization conditions searched. The shape and habit of initial crystals were improved by establishing a phase diagram of protein versus crystallizing-agent concentrations. Growth of large well faceted crystals takes place at low supersaturations near the isochronic supersolubility curve. Further refinement led to reproducible growth of two crystalline forms of bipyramidal (I) or prismatic (II) habit. Both diffract X-rays better than crystals previously obtained with native AspRS. Complete data sets were collected at 3 A resolution for form I (space group P41212) and form II (space group P3221) and molecular-replacement solutions were found in both space groups.