THE FIRST STEP IN SUGAR-TRANSPORT - CRYSTAL-STRUCTURE OF THE AMINO-TERMINAL DOMAIN OF ENZYME-I OF THE ESCHERICHIA-COLI PEP - SUGAR PHOSPHOTRANSFERASE SYSTEM AND A MODEL OF THE PHOSPHOTRANSFER COMPLEX WITH HPR

Background: The bacterial phosphoenolpyruvate (PEP): sugar phosphotran sferase system (PTS) transports exogenous hexose sugars through the me mbrane 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-contai ning phosphocarrier protein HPr, and then to one of a number of sugar- specific enzymes II, The structures of several HPrs and enzymes IIA ar e 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 similar to 30 kDa) has been determined and refined at 2.5 Angstrom resolution, It has two distinct structural subdomains; one contains f our ex helices arranged as two hairpins in a claw-like conformation, T he other consists of a beta sandwich containing a three-stranded antip arallel beta sheet and a four-stranded parallel beta sheet, together w ith three short alpha helices. Plausible models of complexes between E IN and HPr can be made without assuming major structural changes in ei ther protein, Conclusions: The alpha/beta subdomain of EIN is topologi cally similar to the phospho-histidine domain of the enzyme pyruvate p hosphate dikinase, which is phosphorylated by PEP on a histidyl residu e 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 dik inase; this subdomain is therefore more likely to be involved in phosp horyl transfer to HPr.