Abstract
The bacterial phosphotransferase system (PTS) consists of two energy-coupling soluble proteins (enzyme I and HPr) and a large number of inner membrane transporters (enzymes II) that mediate concomitant phosphorylation and translocation of sugars and hexitols. The transporters consist of three functional units (IIA, IIB, IIC), which occur either as protein subunits or domains of a multidomain polypeptide. The membrane-spanning IIC domain contains the substrate binding site; IIA and IIB are phosphorylation domains that transfer phosphate from HPr to the transported sugar. The transporter complexes of the PTS are good examples for variation of design by modular assembly of domains and subunits. The domain order is IIC-IIB in the membrane subunit of the Escherichia coli glucose transporter (IICBGlc) and IIB-IIC in Salmonella typhimurium sucrose transporter (IIBCScr). The phosphorylation domain of IICBGlc was translocated from the carboxyl-terminal to the amino-terminal end of the IIC domain, and the activity of the circularly permuted form was optimized by variation of the length and the composition of the interdomain linker. IIBapCGlc with an alanine-proline-rich interdomain linker has 70% of the control specific activity after purification and reconstitution into proteoliposomes. These results indicate that the amino-terminal end of IICBGlc must be on the cytoplasmic side of the inner membrane, that membrane insertion of the IIC domain is insensitive to the modification of its amino-terminal end, and that a domain swap as it could occur by a single DNA translocation event can rapidly lead to a functional protein. However, IIB could not be substituted for by glucokinase. Fusion proteins between the IIC domain and glucokinase do not transport and phosphorylate glucose in an ATP-dependent mechanism, although the IIC moiety displays transport activity upon complementation with soluble subclonal IIB, and the glucokinase moiety retains ATP-dependent nonvectorial kinase activity. This indicates that IIC and IIB are two cooperative units and not only sequentially acting upon a common substrate, and that translocation of glucose must be conformationally coupled to the phosphorylation/dephosphorylation cycle of IIB.
Highlights
The EMBL/SwissProt data base lists 24 homologous proteins from 12 different Gram-positive and -negative bacteria in the glucose-sucrose family.2 Nine have the domain order CB, and 15 have the order BC (Fig. 1)
It has been speculated that the different types of phosphotransferase system (PTS) transporters are circularly permuted forms of a consensus structure envisaged to consist of two transmembrane portions and of two large loops, forming cytoplasmic subdomains [11]
We show that the C and B domains of of the glucose transporter of E. coli can be circularly permuted with only minor effects on the activity and without detectable effects on membrane insertion
Summary
Bacterial Strains and Growth Conditions—E. coli K-12 ZSC112L⌬HIC (ptsG manZ glk ptsH ptsI crr) [23] and E. coli K-12 ZSC112L⌬G (ptsG::cat manZ glk) [24] were used as hosts for in vivo complementation assays on McConkey plates and for protein expression. Plasmid Construction—Plasmids pJFBxC and pJFBapC (Fig. 2) encode under the control of the Ptac promoter the circularly permuted IIBCGlc variants with a hexahistidine tag at the carboxyl termini They were constructed from plasmids pJBH and pJCH [24] encoding the IIB and IIC domains, respectively. PJFCK, pMSKxC, and pMSKapC encode fusion proteins between the IIC domain and glucokinase (K) with hexahistidine tags at the carboxyl termini. They were constructed from plasmids pJCH, pJFBxC, pJFBapC, and pMSGlk. pMSGlk was constructed as follows. A cell-free cytoplasmic extract from E. coli ZSC112L⌬G(pTSHIC9) overexpressing enzymes I, HPr, and IIAGlc or the proteins purified from this extract were used to complement phosphotransferase activity [33]. Protein concentrations were determined by a modified Lowry assay [35] with bovine serum albumin as the standard
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