STRUCTURAL COMPARISON OF THE HISTIDINE-CONTAINING PHOSPHOCARRIER PROTEIN HPR

The phosphocarrier protein HPr is a central component of the bacterial phosphoenolpyruvate:sugar phosphotransferase system (PTS) that is res ponsible for carbohydrate uptake in many bacterial species. A number o f three-dimensional structures of HPrs from both Gram-positive and Gra m-negative bacteria have been determined; the overall folding topology of HPr is an open-faced beta-sandwich composed of three alpha-helices and a beta-sheet. A detailed structural comparison of these HPrs has been carried out. Besides the overall main chain folding, many detaile d structural features are well conserved in all HPr structures. The th ree x-ray structures of HPrs from Escherichia coli, Streptococcus faec alis, and Bacillus subtilis show considerable overall similarity with respect to the positions of the C alpha atoms. A significant structura l difference between HPrs from Gram-positive and Gram-negative bacteri a is found in the region of Gly54, owing to the steric effects of Tyr3 7 in HPrs from the Gram-positive species. The region around Gly54 is i nvolved in the binding of HPr to other PTS proteins and the difference s in this region may be responsible for some of the poor functional co mplementation between HPrs from Gram-positive and Gramnegative species . The active center region, residues 12-18, appears to have significan t differences in the comparisons between the overall structures. These differences support the proposal that phosphorylation and dephosphory lation of the active site His15 is accompanied by conformational chang es. However, a local structural comparison of residues 12-18 from the x-ray structures of HPrs from E. coli and B. subtilis, and the two-dim ensional nuclear magnetic resonance structure of B. subtilis HPr sugge sts that there is a conserved active center involving residues His15, Arg17, and Pro18, which shows little conformational change during the phosphorylation cycle. The results of other experimental approaches, i ncluding site-directed mutagenesis and NMR spectroscopy, are in some c ases difficult to rationalize with some of the details of the structur es, but do appear to favour the conclusion that little conformational change occurs.