Background: The histidine-containing phosphocarrier protein (HPr) functions in the bacterial phosphoenolpyruvate:sugar phosphotransferase system (PTS). His15 on HPr accepts a phosphoryl group from Enzyme I and transfers it to the Enzyme IIA domain of a sugar-specific PTS permease. In addition, HPrs from Gram-positive bacteria undergo phosphorylation on a serine residue, Ser46, which inhibits phosphorylation at His15 and sugar transport. The questions to be addressed at the molecular level are: what is the mechanism of each of the phosphoryl transfers and what conformational transitions are associated with each event?
Results: Thus, the crystal structures of the mutants Ser83-->Cys HPr (fully active protein) and Ser46-->Asp HPr (impaired protein which mimics Ser46 approximately P HPr) from Bacillus subtilis have been determined at 2 A resolution. They have been crystallized from high-salt and low-salt solutions respectively, and in two different space groups. Analysis of the two crystal forms reveals some significant differences but these do not alter the overall fold of the protein. In each structure, the side chain of His15 caps the following helix. Two alternative side-chain conformations of Arg17 are observed; it either forms an ion pair with a sulfate ion, presumably resembling the phosphorylated state of the protein (high-salt crystal) or with Glu84 (low-salt crystal). The main-chain conformation in the region of residue 46 is the same in the two crystal forms, with both Ser46 and Asp46 capping the following helix.
Conclusions: The analysis suggests that phosphorylation of either His15 or Ser46 is not associated with main-chain conformational transitions. Rather, the protein is poised to accept the respective phosphoryl group with minor adjustments to side chains. The inhibitory effect of phosphorylation on Ser46 is attributed to the altered surface electrostatics, which impairs protein-protein interaction.