Expression and characterization of a structural and functional domain of the mannitol-specific transport protein involved in the coupling of mannitol transport and phosphorylation in the phosphoenolpyruvate-dependent phosphotransferase system of Escherichia coli

Isolation of Escherichia coli mannitol permease, EIImtl, trapped in amphipol A8-35 and fluorescein-labeled A8-35

Amphipols (APols) are short amphipathic polymers that keep integral membrane proteins water-soluble while stabilizing them as compared to detergent solutions. In the present work, we have carried out functional and structural studies of a membrane transporter that had not been characterized in APol-trapped form yet, namely EII(mtl), a dimeric mannitol permease from the inner membrane of Escherichia coli. A tryptophan-less and dozens of single-tryptophan (Trp) mutants of this transporter are available, making it possible to study the environment of specific locations in the protein. With few exceptions, the single-Trp mutants show a high mannitol-phosphorylation activity when in membranes, but, as variance with wild-type EII(mtl), some of them lose most of their activity upon solubilization by neutral (PEG- or maltoside-based) detergents. Here, we present a protocol to isolate these detergent-sensitive mutants in active form using APol A8-35. Trapping with A8-35 keeps EII(mtl) soluble and functional in the absence of detergent. The specific phosphorylation activity of an APol-trapped Trp-less EII(mtl) mutant was found to be ~3× higher than the activity of the same protein in dodecylmaltoside. The preparations are suitable both for functional and for fluorescence spectroscopy studies. A fluorescein-labeled version of A8-35 has been synthesized and characterized. Exploratory studies were conducted to examine the environment of specific Trp locations in the transmembrane domain of EII(mtl) using Trp fluorescence quenching by water-soluble quenchers and by the fluorescein-labeled APol. This approach has the potential to provide information on the transmembrane topology of MPs.

Mapping of the dimer interface of the Escherichia coli mannitol permease by cysteine cross-linking

A cysteine cross-linking approach was used to identify residues at the dimer interface of the Escherichia coli mannitol permease. This transport protein comprises two cytoplasmic domains and one membrane-embedded C domain per monomer, of which the latter provides the dimer contacts. A series of single-cysteine His-tagged C domains present in the native membrane were subjected to Cu(II)-(1,10-phenanthroline)(3)-catalyzed disulfide formation or cysteine cross-linking with dimaleimides of different length. The engineered cysteines were at the borders of the predicted membrane-spanning alpha-helices. Two residues were found to be located in close proximity of each other and capable of forming a disulfide, while four other locations formed cross-links with the longer dimaleimides. Solubilization of the membranes did only influence the cross-linking behavior at one position (Cys(73)). Mannitol binding only effected the cross-linking of a cysteine at the border of the third transmembrane helix (Cys(134)), indicating that substrate binding does not lead to large rearrangements in the helix packing or to dissociation of the dimer. Upon mannitol binding, the Cys(134) becomes more exposed but the residue is no longer capable of forming a stable disulfide in the dimeric IIC domain. In combination with the recently obtained projection structure of the IIC domain in two-dimensional crystals, a first proposal is made for alpha-helix packing in the mannitol permease.

BglF, the Escherichia coli beta-glucoside permease and sensor of the bgl system: domain requirements of the different catalytic activities

The Escherichia coli BglF protein, an enzyme II of the phosphoenolpyruvate-dependent carbohydrate phosphotransferase system, has several enzymatic activities. In the absence of beta-glucosides, it phosphorylates BglG, a positive regulator of bgl operon transcription, thus inactivating BglG. In the presence of beta-glucosides, it activates BglG by dephosphorylating it and, at the same time, transports beta-glucosides into the cell and phosphorylates them. BglF is composed of two hydrophilic domains, IIAbgl and IIBbgl, and a membrane-bound domain, IICbgl, which are covalently linked in the order IIBCAbgl. Cys-24 in the IIBbgl domain is essential for all the phosphorylation and dephosphorylation activities of BglF. We have investigated the domain requirement of the different functions carried out by BglF. To this end, we cloned the individual BglF domains, as well as the domain pairs IIBCbgl and IICAbgl, and tested which domains and which combinations are required for the catalysis of the different functions, both in vitro and in vivo. We show here that the IIB and IIC domains, linked to each other (IIBCbgl), are required for the sugar-driven reactions, i. e., sugar phosphotransfer and BglG activation by dephosphorylation. In contrast, phosphorylated IIBbgl alone can catalyze BglG inactivation by phosphorylation. Thus, the sugar-induced and noninduced functions have different structural requirements. Our results suggest that catalysis of the sugar-induced functions depends on specific interactions between IIBbgl and IICbgl which occur upon the interaction of BglF with the sugar.

The thermal stability and domain interactions of the mannitol permease of Escherichia coli. A differential scanning calorimetry study

The thermal stability and domain interactions in the mannitol transporter from Escherichia coli, enzyme IImtl, have been studied by differential scanning calorimetry. To this end, the wild type enzyme, IICBAmtl, as well as IICBmtl and IICmtl, were reconstituted into a dimyristoylphosphatidylcholine lipid bilayer. The changes in the gel to liquid crystalline transition of the lipid indicated that the protein was inserted into the membrane, disturbing a total of approximately 40 lipid molecules/protein molecule. The thermal unfolding profile of EIImtl exhibited three separate transitions, two of which were overlapping, that could be assigned to structural domains in the protein. Treatment with trypsin, resulting in the degradation of the water-soluble part of the enzyme while leaving the binding and translocation capability of the enzyme intact, resulted in a decrease of the Tm and enthalpy of unfolding of the membrane-embedded C domain. This effect was much more apparent in the presence of the substrate but only partly so in the presence of the substrate analog perseitol. These results are consistent with a recently proposed model (Meijberg, W., Schuurman-Wolters, G. K., and Robillard, G. T. (1998) J. Biol. Chem. 273, 7949-7946), in which the B domain takes part in the conformational changes during the substrate binding process.

Expression, purification and characterization of the enzyme II mannitol-specific domain from Staphylococcus carnosus and determination of the active-site cysteine residue

The C-terminal B domain of mannitol-specific enzyme II (enzyme IIB) of the phosphoenolpyruvate-dependent phosphotransferase system for mannitol from Staphylococcus carnosus was subcloned, purified and characterized. In Staphylococcal cells, mannitol-specific enzyme II is composed of a soluble A domain (EIIA) and a transmembrane C domain transporter with a fused enzyme IIB (IIB) domain. We purified large amounts of the IIB domain as an in-frame fusion with six histidine residues. Here, we show that the domain is stable and can be phosphorylated by phosphoenolpyruvate and the phosphotransferase components. It is a dimer over a wide range of pH values and salt conditions. Differences between the published nucleotide sequence data and the mass-spectroscopic data obtained with the purified protein lead to anewed nucleotide sequencing of the gene. Two errors in the original proposed sequence were found, the correction of the second error leading to a frame shift that adds 10 amino acids to the deduced amino acid sequence. The mass of the phosphorylated domain is 20,068 Da, 80 Da more than the mass of the unphosphorylated domain, therefore, no other residues, such as COOH side chains, are directly involved in an additional phosphate linkage concerning the IIB domain. 31P-NMR experiments as well as chemical modification proved that Cys429 is the phosphoamino acid. Titration of the phosphorylated domain during 31P-NMR did not lead to the typical shift for the protonation of the thiophosphate in the resonance spectrum. Thus, the thiophosphate remains in the twofold negatively charged state.

Phosphorylation site mutants of the mannitol transport protein enzyme IImtl of Escherichia coli: studies on the interaction between the mannitol translocating C-domain and the phosphorylation site on the energy-coupling B-domain

Mannitol binding and translocation catalyzed by the C domain of the Escherichia coli mannitol transport protein enzyme IImtl is influenced by domain B. This interaction was studied by monitoring the effects of mutating the B domain phosphorylation site, C384, on the kinetics of mannitol binding to the C domain. The dissociation constants for mannitol to the C384 mutants in inside-out membrane vesicles varied from 45 nM for the wild-type enzyme to 306 nM for the mutants. The rate constants pertinent to the binding equilibrium were also altered by the mutations. The association rate of mannitol to the cytoplasmic binding site in the mutants was accelerated for all mutants. The exchange rate of bound mannitol on the wild-type enzyme was shown to be pH dependent with a pKa of approximately 8 and increasing rates at higher pH. This rate was increased for all the mutants, but the pKas differed for the various mutants. The exchange rate for binding to the isolated IICmtl, however, was not pH dependent and exhibited a low rate. Exchange measured at 4 degrees C showed that, of the two steps, binding and occlusion, involved in binding to wild-type EIImtl in inside-out vesicles, only one could be detected for the C384E and C384L mutants. This suggests that the mutations increased the rate of the occlusion step so that it was no longer separable from the initial binding step or that the mutations eliminated the occlusion step altogether. The change in the mannitol binding kinetics of the C domain indicates that the B and C domains of EIImtl influence each other's conformation.(ABSTRACT TRUNCATED AT 250 WORDS)

Enzyme IIBcellobiose of the phosphoenol-pyruvate-dependent phosphotransferase system of Escherichia coli: backbone assignment and secondary structure determined by three-dimensional NMR spectroscopy

The assignment of backbone resonances and the secondary structure determination of the Cys 10 Ser mutant of enzyme IIBcellobiose of the Escherichia coli cellobiose-specific phosphoenol-pyruvate-dependent phosphotransferase system are presented. The backbone resonances were assigned using 4 triple resonance experiments, the HNCA and HN(CO)CA experiments, correlating backbone 1H, 15N, and 13C alpha resonances, and the HN(CA)CO and HNCO experiments, correlating backbone 1H,15N and 13CO resonances. Heteronuclear 1H-NOE 1H-15N single quantum coherence (15N-NOESY-HSQC) spectroscopy and heteronuclear 1H total correlation 1H-15N single quantum coherence (15N-TOCSY-HSQC) spectroscopy were used to resolve ambiguities arising from overlapping 13C alpha and 13CO frequencies and to check the assignments from the triple resonance experiments. This procedure, together with a 3-dimensional 1H alpha-13C alpha-13CO experiment (COCAH), yielded the assignment for all observed backbone resonances. The secondary structure was determined using information both from the deviation of observed 1H alpha and 13C alpha chemical shifts from their random coil values and 1H-NOE information from the 15N-NOESY-HSQC. These data show that enzyme IIBcellobiose consists of a 4-stranded parallel beta-sheet and 5 alpha-helices. In the wild-type enzyme IIBcellobiose, the catalytic residue appears to be located at the end of a beta-strand.