X-ray structure of the signal transduction protein from Escherichia coli at 1.9 Å

The structure of the bacterial signal transduction protein P_II has been refined to an R factor of 13.2% using 3σ data between 10 and 1.9 Å. The crystals exhibited twinning by merohedry and X-ray intensities were corrected using the method of Fisher & Sweet [Fisher & Sweet (1980). Acta Cryst. A36, 755–760] prior to refinement. Our earlier 2.7 Å structure [Cheah, Carr, Suffolk, Vasudevan, Dixon & Ollis (1994). Structure, 2, 981–990] served as a starting model. P_II is a trimeric molecule, each subunit has a mass of 12.4 kDa and contains 112 amino-acid residues. The refined model includes all 1065 protein atoms per subunit plus 312 water molecules. The high-resolution refinement confirms the correctness of our 2.7 Å model, although it leads to a redefinition of the extent of various secondary-structural elements. The monomeric structure of P_II exhibits an interlocking double βαβ fold. This is a stable fold found in a number of proteins with diverse functions. The association of the protein into a trimer leads to a new structure which we describe in detail. The effects of crystal packing forces are discussed and potential interaction sites with other proteins and effector molecules are identified.