X-RAY STRUCTURE OF THE SIGNAL-TRANSDUCTION PROTEIN P-II FROM ESCHERICHIA-COLI AT 1.9 ANGSTROM

The structure of the bacterial signal transduction protein P-II has be en refined to an R factor of 13.2% using 3 sigma data between 10 and 1 .9 Angstrom. The crystals exhibited twinning by merohedry and X-ray in tensities were corrected using the method of Fisher & Sweet [Fisher & Sweet (1980). Acta Cryst. A36, 755-760] prior to refinement. Our earli er 2.7 Angstrom structure [Cheah, Carr, Suffolk, Vasudevan, Dixon & Ol lis (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 refine ment confirms the correctness of our 2.7 Angstrom model, although it l eads to a redefinition of the extent of various secondary-structural e lements. The monomeric structure of P-II exhibits an interlocking doub le beta alpha beta fold. This is a stable fold found in a number of pr oteins with diverse functions. The association of the protein into a t rimer leads to a new structure which we describe in detail. The effect s of crystal packing forces are discussed and potential interaction si tes with other proteins and effector molecules are identified.