Sequence–function relationships in MalG, an inner membrane protein from the maltose transport system in Escherichia coli

The maIG gene encodes a hydrophobic cytoplasmic membrane protein which is required for the energy-dependent transport of maltose and maltodextrins in Escherichia coli. The MalG protein, together with MalF and MalK proteins, forms a multimeric complex in the membrane consisting of two MalK subunits for each MalF and MalG subunit. Fifteen mutations have been isolated in malG by random linker insertion mutagenesis. Two regions essential for maltose transport have been identified. In particular, a hydro philic region containing the peptidic motif EAA—G———I-LP, highly conserved among inner membrane proteins from binding protein-dependent transport systems, is essential for maltose transport. The results also show that several regions of MalG are not essential for function. A region (residues 30–50) encompassing the first predicted transmembrane segment and the first periplasmic loop in MalG may be modified extensively with little effect on maltose transport and no effect on the stability and the localization of the protein. A region located at the middle of the protein (residues 153–157) is not essential for the function of the protein. A region, essential for maltodextrin utilization but not for maltose transport, has been identified near the C-terminus of the protein.     

Overproduction, solubilization, and reconstitution of the maltose transport system from Escherichia coli

Maltose is transported across the cytoplasmic membrane of Escherichia coli by a binding protein-dependent transport system. We observed a 10-fold increase in the level of transport activity in assays with membrane vesicles when the three membrane-associated components of the transport system (the MalF, MalG, and MalK proteins) were overproduced. In addition, we have successfully reconstituted maltose transport activity in proteoliposome vesicles from solubilized proteins using a detergent dilution procedure. The addition of ATP as an energy source was sufficient to obtain transport, and this activity was dependent on the presence of maltose binding protein and was not seen in proteoliposomes prepared from a strain with a deletion of the maltose genes. We determined that hydrolysis of ATP was directly coupled to maltose uptake. In the majority of these experiments, an average of 1.4 mol of ATP was hydrolyzed for each mole of maltose accumulated. However, in the remaining experiments, ATP hydrolysis was observed to be much higher and averaged 17 mol of ATP hydrolyzed per mol of maltose transported. Possible explanations for a variable stoichiometry are discussed. These results provide strong evidence that it is the hydrolysis of ATP by a component of the transport complex that provides the energy required for active maltose transport.     

Membrane topology of the Escherichia coli ExbD protein.

The ExbD protein is involved in the energy-coupled transport of ferric siderophores, vitamin B12, and B-group colicins across the outer membrane of Escherichia coli. In order to study ExbD membrane topology, ExbD-beta-lactamase fusion proteins were constructed. Cells expressing beta-lactamase fusions to residues 53, 57, 70, 76, 78, 80, 92, 121, and 134 of ExbD displayed high levels of ampicillin resistance, whereas fusions to residues 9 and 19 conferred no ampicillin resistance. It is concluded that the only hydrophobic segment of ExbD, encompassing residues 23 to 43, forms a transmembrane domain and that residues 1 to 22 are located in the cytoplasm and residues 44 to 141 are located in the periplasm.     

Membrane topology of penicillin-binding protein 3 of Escherichia coli

The beta-lactamase fusion vector, pJBS633, has been used to analyse the organization of penicillin-binding protein 3 (PBP3) in the cytoplasmic membrane of Escherichia coli. The fusion junctions in 84 in-frame fusions of the coding region of mature TEM beta-lactamase to random positions within the PBP3 gene were determined. Fusions of beta-lactamase to 61 different positions in PBP3 were obtained. Fusions to positions within the first 31 residues of PBP3 resulted in enzymatically active fusion proteins which could not protect single cells of E. coli from killing by ampicillin, indicating that the beta-lactamase moieties of these fusion proteins were not translocated to the periplasm. However, all fusions that contained greater than or equal to 36 residues of PBP3 provided single cells of E. coli with substantial levels of resistance to ampicillin, indicating that the beta-lactamase moieties of these fusion proteins were translocated to the periplasm. PBP3 therefore appeared to have a simple membrane topology with residues 36 to the carboxy-terminus exposed on the periplasmic side of the cytoplasmic membrane. This topology was confirmed by showing that PBP3 was protected from proteolytic digestion at the cytoplasmic side of the inner membrane but was completely digested by proteolytic attack from the periplasmic side. PBP3 was only inserted in the cytoplasmic membrane at its amino terminus since replacement of its putative lipoprotein signal peptide with a normal signal peptide resulted in a water-soluble, periplasmic form of the enzyme. The periplasmic form of PBP3 retained its penicillin-binding activity and appeared to be truly water-soluble since it fractionated, in the absence of detergents, with the expected molecular weight on Sephadex G-100 and was not retarded by hydrophobic interaction chromatography on Phenyl-Superose.     

The nucleotide sequence of the gene for malF protein, an inner membrane component of the maltose transport system of Escherichia coli. Repeated DNA sequences are found in the malE-malF intercistronic region

The malF gene product is an inner membrane component of the maltose transport system in Escherichia coli. Some gene fusions between malF and lacZ (encoding the normally cytoplasmic enzyme beta-galactosidase) produce hybrid proteins which are membrane-bound while other fusions produce hybrid proteins which are cytoplasmic (Silhavy, T. J., Casadaban, M. J., Shuman, H. A., and Beckwith, J. R. (1976) Proc. Natl. Acad. Sci. U. S. A. 73, 3423-3427). To further analyze the localization properties of the different classes of fusion proteins and of the intact MalF protein, we have obtained the DNA sequence of 5 malF-lacZ fusions and the wild type malF gene. From the predicted amino acid sequence, MalF protein contains 514 amino acids and has a molecular weight of 56,947. Analysis of the hydropathic character of MalF using the Kyte-Doolittle assignments (Kyte, J., and Doolittle, R. F. (1982) J. Mol. Biol. 157, 105-132), indicates that the protein may have 2 or 3 amino-terminal membrane-spanning segments and 4 or 5 carboxy-terminal membrane-spanning segments separated by a region of 181 hydrophilic residues. Localization properties of the different fusion proteins correspond with degree of hydrophobicity. By sequencing upstream from malF, the malE-malF intercistronic region was found to be 153 base pairs in length and to contain inverted repeats, homologous to intercistronic repeats of many other operons. Further analysis of this region may help in understanding the observed step-down in synthesis of the MalF protein.     

Purification and characterization of the membrane-associated components of the maltose transport system from Escherichia coli

Maltose is transported across the cytoplasmic membrane of Escherichia coli by a binding protein-dependent transport system. The three membrane-associated components of the transport system, the MalK, MalF, and MalG proteins, have been solubilized from the membrane and maltose transport activity has been reconstituted in proteoliposome vesicles (Davidson, A. L., and Nikaido, H. (1990) J. Biol. Chem. 265, 4254-4260). A modification of the reconstitution technique is presented which permits reconstitution from the detergent dodecyl maltoside. Utilizing reconstitution of maltose transport as an assay, we have purified these proteins in the presence of n-dodecyl-beta-D-maltoside. The purified proteins catalyze both maltose transport activity and ATP hydrolysis. In all experiments, the MalF, MalG, and MalK proteins behaved as a multiprotein complex; all three proteins were immunoprecipitated using antibody prepared against MalF, and they copurified, eluting from a gel filtration column between markers of Mr 160,000 and 200,000. Each complex contains two MalK, one MalF, and one MalG proteins, providing two putative sites for ATP hydrolysis. Chemical cross-linking detected specific interactions between MalF and MalG and between MalF and MalK.     

Membrane topology of the N-terminus of the Escherichia coli FtsK division protein

The Escherichia coli FtsK protein targets the septum, is essential for cell division and may play a role in DNA partitioning. Computer modelling suggests that the first 180 amino acids of the protein are embedded in the cytoplasmic membrane by up to six transmembrane domains. We demonstrate, using gene fusions, that the N-terminus contains four transmembrane helices that link two periplasmic domains. The first periplasmic domain contains an HEXXH amino acid sequence characteristic of zinc metalloproteases. We show by mutation analysis that the conserved glutamic acid of the HEXXH sequence is essential for FtsK function during septation.     

Use of phoA fusions to study the topology of the Escherichia coli inner membrane protein leader peptidase.

A topology of the Escherichia coli leader peptidase has been previously proposed on the basis of proteolytic studies. Here, a collection of alkaline phosphatase fusions to leader peptidase is described. Fusions to the periplasmic domain of this protein exhibit high alkaline phosphatase activity, while fusions to the cytoplasmic domain exhibit low activity. Elements within the cytoplasmic domain are necessary to stably anchor alkaline phosphatase in the cytoplasm. The amino-terminal hydrophobic segment of leader peptidase acts as a weak export signal for alkaline phosphatase. However, when this segment is preceded by four lysines, it acts as a highly efficient export signal. The coherence of in vitro studies with alkaline phosphatase fusion analysis of the topology of leader peptidase further indicates the utility of this genetic approach to membrane protein structure and insertion.     

Identification of the malK gene product. A peripheral membrane component of the Escherichia coli maltose transport system

The malK gene product of Escherichia coli has been identified through the use of a previously described technique that employs gene fusions (Shuman, H. A., Silhavy, T. J., and Beckwith, J. R. (1980) J. Biol. Chem. 255, 168-174). This protein, along with the four other products of the malB locus, comprise the complete maltose transport system. The malK protein has a molecular weight of approximately 40,000 and is located in the cell envelope. In mutant strains which lack another component of the transport system, the malG protein, the malK protein is located in the cytoplasm. This alteration in location suggests that the malK protein is associated with the inner surface of the cytoplasmic membrane via an interaction with the malG protein.     

Versatility of inner membrane protein biogenesis in Escherichia coli

To further our understanding of inner membrane protein (IMP) biogenesis in Escherichia coli, we have accomplished the widest in vivo IMP assembly screen so far. The biogenesis of a set of model IMPs covering most IMP structures possible has been studied in a variety of signal recognition particle (SRP), Sec and YidC mutant strains. We show that the assembly of the complete set of model IMPs is assisted (i.e. requires the aid of proteinaceous factors), and that the requirements for assembly of the model IMPs into the inner membrane differ significantly from each other. This indicates that IMP assembly is much more versatile than previously thought.     

Antibodies against synthetic peptides and the topology of LamB, an outer membrane protein from Escherichia coli K12

LamB, an outer membrane protein from Escherichia coli K12, is involved in the transport of maltose and maltodextrins across the outer membrane and constitutes a receptor for a number of bacteriophages. A recent folding model proposes that LamB spans the outer membrane through a number of transmembranous segments separated by regions exposed either to the cell exterior or to the periplasm. This model is essentially based on predictions of structure and genetic arguments relying on the hypothesis that the mutations studied did not alter the folding of the protein. In order to obtain direct evidence with the unaltered protein, we elicited polyclonal antibodies against synthetic peptides corresponding to several LamB sequences. We chose four regions. Three of them [aa 147-161 (peptide 2), aa 371-385 (peptide 3), and aa 399-413 (peptide 4)] are predicted to face the outside of the cell, and the fourth (aa 19-33 (peptide 1)] is predicted to be periplasmic. By immunoblotting against extracts of various mutants, these antibodies were shown to be specific for LamB and targeted to the selected regions. In some cases, the recognition sites for antibodies were narrowed down to parts of a region. In vivo, on intact cells, anti-peptides 2, 3, and 4 reacted with LamB in an ELISA; this confirmed that regions of peptide 2 and 3 are located, at least in part, at the cell exterior and provided the first proof for a similar, situation of the region of peptide 4. Under the same conditions, anti-peptide 1 did not react with LamB.(ABSTRACT TRUNCATED AT 250 WORDS)     

Uncoupling substrate transport from ATP hydrolysis in the Escherichia coli maltose transporter

Members of the ATP-binding cassette superfamily couple the energy from ATP hydrolysis to the active transport of substrates across the membrane. The maltose transporter, a well characterized model system, consists of a periplasmic maltose-binding protein (MBP) and a multisubunit membrane transporter, MalFGK(2). On the basis of the structure of the MBP-MalFGK(2) complex in an outward-facing conformation (Oldham, M. L., Khare, D., Quiocho, F. A., Davidson, A. L., and Chen, J. (2007) Nature 450, 515-521), we identified two mutants in transmembrane domains MalF and MalG that generated futile cycling; although interaction with MBP stimulated the ATPase activity of the transporter, maltose was not transported. Both mutants appeared to disrupt the normal transfer of maltose from MBP to MalFGK(2). In the first case, substitution of aspartate for glycine in the maltose-binding site of MalF likely generated a futile cycle by preventing maltose from binding to MalFGK(2) during the catalytic cycle. In the second case, a four-residue deletion of a periplasmic loop of MalG limited its reach into the maltose-binding pocket of MBP, allowing maltose to remain associated with MBP during the catalytic cycle. Retention of maltose in the MBP binding site in the deletion mutant, as well as insertion of this loop into the binding site in the wild type, was detected by EPR as a change in mobility of a nitroxide spin label positioned near the maltose-binding pocket of MBP.     

YqjD is an inner membrane protein associated with stationary-phase ribosomes in Escherichia coli

Here, we provide evidence that YqjD, a hypothetical protein of Escherichia coli, is an inner membrane and ribosome binding protein. This protein is expressed during the stationary growth phase, and expression is regulated by stress response sigma factor RpoS. YqjD possesses a transmembrane motif in the C-terminal region and associates with 70S and 100S ribosomes at the N-terminal region. Interestingly, E. coli possesses two paralogous proteins of YqjD, ElaB and YgaM, which are expressed and bind to ribosomes in a similar manner to YqjD. Overexpression of YqjD leads to inhibition of cell growth. It has been suggested that YqjD loses ribosomal activity and localizes ribosomes to the membrane during the stationary phase.     

Identification of a cytoplasmic membrane-associated component of the maltose transport system of Escherichia coli

The maltose transport system of Escherichia coli contains at least five components, three of which, i.e. the products of lamB, malE, and malF genes, have so far been identified as constituents of the outer membrane, periplasmic space, and cytoplasmic membrane, respectively. We identified another component, a cytoplasmic membrane protein of an apparent molecular weight of 43,000, as the product of the malK gene on the basis of polyacrylamide gel electrophoretic analysis of various mutants and suppressed strains and by the incorporation of extra tyrosine residue into this proten in malK amber mutants containing the suppressor Su3+ allele. The transport of maltose thus appears to require at least two proteins associated with the cytoplasmic membrane.     

Molecular chaperones and protein translocation across the Escherichia coli inner membrane

Proteins that are able to translocate across biological membranes assume a loosely folded structure. In this review it is suggested that the loosely folded structure, referred to here as the 'pre-folded conformation', is a particular structure that interacts favourably with components of the export apparatus. Two soluble factors, SecB and GroEL, have been implicated in maintenance of the pre-folded conformation and have been termed 'molecular chaperones'. Results suggest that SecB may be a chaperone that is specialized for binding to exported protein precursors, while GroEL may be a general folding modulator that binds to many intracellular proteins.     

Immunological approach of assembly and topology of OmpF, an outer membrane protein of Escherichia coli.

Various monoclonal antibodies (MoF) directed against cell-surface-exposed epitopes of OmpF, one major outer membrane pore protein of Escherichia coli B and K-12, have been used to study the assembly and the topology of the protein. This paper firstly describes the characterization of the OmpF epitopes recognized by the various monoclonal antibodies. A comparison between OmpC, OmpF and PhoE porins with respect to their primary amino acid sequence and their cell-surface exposed regions allows us to propose a rough model including 2 antigenic sites. The second part is focused on the assembly of the OmpF protein in the outer membrane. Various forms, precursor, unassembled monomer, metastable oligomer (pre-trimer) and trimer are detected with immunological probes directed against OmpF during a kinetic analysis of the process. The requirement for a concomitant lipid synthesis during the trimerization has been demonstrated by investigating the presence of a specific native epitope. The role of lipopolysaccharide during the stabilization of the conformation is discussed with regard to the various steps of assembly.     

Energy requirements for protein translocation across the Escherichia coli inner membrane

Both ATP and an electrochemical potential play roles in translocating proteins across the inner membrane of Escherichia coli. Recent discoveries have dissected the overall transmembrane movement into separate subreactions with different energy requirements, identified a translocation ATPase, and reconstituted both energy-requiring steps of the reaction from purified components. A more refined understanding of the energetics of this fundamental process is beginning to provide answers about the basic issues of how proteins move across the hydrophobic membrane barrier.