Escherichia coli P II protein: purification, crystallization and oligomeric structure

Abstract

The Escherichia coli signal transduction protein P II, product of the glnB gene, was overproduced and purified. The predicted molecular weight of the protein based on the correct nucleotide sequence is 12,427 and is very close to the value 12,435 obtained by matrix-assisted laser desorption mass spectrometry. Hexagonal crystals of the unuridylylated form of P II with dimensions 0.2 × 0.2 × 0.3 mm were grown and analysed by X-ray diffraction. The crystals belong to space group P6 3 with a= b=61.6Å, c= 56.3 Å and V m of 2.5 for one subunit in the asymmetric unit. A low-resolution electron density map showed electron density concentrated around a three-fold axis, suggesting the molecule to be a trimer. A sedimentation equilibrium experiment of the meniscus depletion type was used to estimate a molecular weight of 35,000 ± 1,000 for P II in solution. This result is consistent with the native protein being a homotrimer.