The different functions of BglF, the E. coli β-glucoside permease and sensor of the bgl system, have different structural requirements

The Escherichia coli BglF protein (EII(bgl)) is an Enzyme II (EII) of the phosphoenolpyruvate-dependent phosphotransferase system (PTS) which catalyses transport and phosphorylation of β-glucosides. In addition to its transport function, BglF serves as a β-glucoside sensor which reversibly phosphorylates BglG, the transcription regulator of the bgl operon. Like many other PTS sugar permeases, the BglF protein is composed of three discrete functional and structural domains: IIA(bgl) and IIB(bgl), which are hydrophilic, and IIC(bgl), which is hydrophobic. The domains of BglF are covalently linked to one another in the order BCA. The IIA(bgl) domain contains the first phosphorylation site, which accepts a phosphoryl group from the general PTS protein HPr and delivers it to the second phosphorylation site, located in the IIB(bgl) domain. This second site can deliver the phosphoryl group either to a β-glucoside or to BglG. To elucidate the mechanism by which such different substrates can be phosphorylated by the same active site, we decided to try to separate the different phosphorylation activities catalyzed by BglF. To this end we rearranged the BglF domains and constructed IICBA(bgl) (scrambled-BglF). Scrambled-BglF behaved like wild-type BglF in its ability to be phosphorylated and to phosphorylate BglG in vitro and in vivo. However, it could not catalyze phosphorylation of β-glucosides in vitro nor their phosphotransfer in vivo, and it could not catalyze BglG dephosphorylation in vitro or in vivo. Therefore, the two reactions induced by β-glucosides, sugar phosphorylation and BglG dephosphorylation, seem to require a specific domain organization: IIB(bgl) should precede IIC(bgl). The order of the B and C domains is irrelevant for BglG phosphorylation, which occurs in the absence of β-glucosides. Because the domain order affects the way that the domains are able to interact, our results suggest that catalysis of the sugar- induced functions depends on specific interactions between IIB(bgl) and IIC(bgl). In light of the previous assumption that domain order in EIIs is immaterial for their function, the finding that the order of the domains is important for the function of BglF as a sugar phosphotransferase raises two possibilities: (a) BglF differs from other EIIs in this regard; (b) BglF represents a subgroup of EIIs in which the requirement for a specific domain order correlates with the ability to transport a set of structurally related sugars.