Crystallization and preliminary X-ray analysis of tryptophan synthase alpha-subunits from Escherichia coli

Tryptophan synthase alpha-subunit (alphaTS) catalyzes the cleavage of indole-3-glycerolphosphate to glyceraldehyde-3-phosphate and indole, which is channelled to the active site of the associated beta-subunit (betaTS), where it reacts with serine to yield the amino acid tryptophan in tryptophan biosynthesis. The alphaTS from Escherichia coli is a 268 amino-acid protein with no disulfide bonds or prosthetic groups. Although the crystallization of the subunits from E. coli has been attempted over many years, there have been no reports of an X-ray structure. To explore the molecular origin of the conformational stabilization mechanism of alphaTS, the alpha-subunit protein was overexpressed in E. coli and crystallized using the hanging-drop vapour-diffusion method at 298 K. A native data set to 2.8 A resolution was obtained from a flash-cooled crystal upon exposure to Cu Kalpha X-rays. The crystal belongs to the monoclinic space group C2, with unit-cell parameters a = 162.27, b = 44.48, c = 71.52 A, beta = 106.56 degrees. The asymmetric unit contains two molecules of alphaTS, giving a crystal volume per protein mass (V(M)) of 2.16 A(3) Da(-1) and a solvent content of 43.18%.