The NMR determination of the IIAmtl binding site on HPr of the Escherichia coli phosphoenol pyruvate-dependent phosphotransferase system

The region of the surface of the histidine-containing protein (HPr) which interacts with the A domain of the mannitol-specific Enzyme II (IIAmtl) has been mapped by titrating the A-domain into a solution of 15N-labeled HPr and monitoring the effects on the amide proton and nitrogen chemical shifts via heteronuclear single quantum correlation spectroscopy (HSQC). Fourteen of the eighty-five HPr amino acid residues show large changes in either the 15N or 1H chemical shifts or both as a result of the presence of IIAmtl while a further seventeen residues experience lesser shifts. Most of the residues involved are surface residues accounting for approximately 25% of the surface of HPr. Phosphorylation of HPr with catalytic amounts of Enzyme I (EI), in the absence of IIAmtl resulted in chemical shift changes in a sub-set of the above residues; these were located more in the vicinity of the active site phospho-histidine. Phosphorylation of the HPr/IIAmtl complex resulted in a HSQC spectrum which was indistinguishable from the P-HPr spectrum in the absence of IIAmtl indicating that, as expected, the complex P-HPr/P-IIAmtl does not exist even at the high concentrations necessary for NMR.