The first step in sugar transport: crystal structure of the amino terminal domain of enzyme I of the E. coli PEP: sugar phosphotransferase system and a model of the phosphotransfer complex with HPr

Abstract Background: The bacterial phosphoenolpyruvate (PEP): sugar phosphotransferase system (PTS) transports exogenous hexose sugars through the membrane and tightly couples transport with phosphoryl transfer from PEP to the sugar via several phosphoprotein intermediates. The phosphate group is first transferred to enzyme I, second to the histidine-containing phosphocarrier protein HPr, and then to one of a number of sugar-specific enzymes II. The structures of several HPrs and enzymes IIA are known. Here we report the structure of the N-terminal half of enzyme I from Escherichia coli (EIN). Results The crystal structure of EIN (MW ∼30 kDa) has been determined and refined at 2.5 a resolution. It has two distinct structural subdomains; one contains four α helices arranged as two hairpins in a claw-like conformation. The other consists of a β sandwich containing a three-stranded antiparallel β sheet and a four-stranded parallel β sheet, together with three short α helices. Plausible models of complexes between EIN and HPr can be made without assuming major structural changes in either protein. Conclusion The α / β subdomain of EIN is topologically similar to the phospho-histidine domain of the enzyme pyruvate phosphate dikinase, which is phosphorylated by PEP on a histidyl residue but does not interact with HPr. It is therefore likely that features of this subdomain are important in the autophosphorylation of enzyme I. The helical subdomain of EIN is not found in pyruvate phosphate dikinase; this subdomain is therefore more likely to be involved in phosphoryl transfer to HPr.