3-DIMENSIONAL STRUCTURE OF THE ESCHERICHIA-COLI PHOSPHOCARRIER PROTEIN-IIIGLC

The crystal structure of a proteolytically modified form of the Escherichia coli phosphocarrier and signal transducing protein III(glc) has been determined by multiple isomorphous and molecular replacement. The model has been refined to an R-factor of 0.166 for data between 6- and 2.1-angstrom resolution with an rms deviation of 0.020 angstrom from ideal bond lengths and 3.2-degrees from ideal bond angles. The molecule is a beta-sheet sandwich, with six antiparallel strands on either side. Several short distorted helices line the periphery of the active site, which is a shallow extremely hydrophobic depression almost-equal-to 18 angstrom in diameter near the center of one face. The side chains of the active site histidine residues 75 and 90 face each other at the center of the depression, with the N3 positions exposed to solvent, separated by 3.3 angstrom in an excellent position to form adducts with phosphate. Chloroplatinate forms a divalent adduct with both histidyl side chains, suggesting that the phosphodonor reaction might proceed through a similar transition state. The hydrophobic patch forms the primary crystal contact, suggesting a mode of association of III(glc) with other components of the phosphoenolpyruvate-dependent phosphotransferase system.