Regulated high-level expression of the mannitol permease of the phosphoenolpyruvate-dependent sugar phosphotransferase system in Escherichia coli

Transcending membrane barriers: advances in membrane engineering to enhance the production capacity of microbial cell factories

Microbial cell factories serve as pivotal platforms for the production of high-value natural products, which tend to accumulate on the cell membrane due to their hydrophobic properties. However, the limited space of the cell membrane presents a bottleneck for the accumulation of these products. To enhance the production of intracellular natural products and alleviate the burden on the cell membrane caused by product accumulation, researchers have implemented various membrane engineering strategies. These strategies involve modifying the membrane components and structures of microbial cell factories to achieve efficient accumulation of target products. This review summarizes recent advances in the application of membrane engineering technologies in microbial cell factories, providing case studies involving Escherichia coli and yeast. Through these strategies, researchers have not only improved the tolerance of cells but also optimized intracellular storage space, significantly enhancing the production efficiency of natural products. This article aims to provide scientific evidence and references for further enhancing the efficiency of similar cell factories.

Inducible intracellular membranes: molecular aspects and emerging applications

Membrane remodeling and phospholipid biosynthesis are normally tightly regulated to maintain the shape and function of cells. Indeed, different physiological mechanisms ensure a precise coordination between de novo phospholipid biosynthesis and modulation of membrane morphology. Interestingly, the overproduction of certain membrane proteins hijack these regulation networks, leading to the formation of impressive intracellular membrane structures in both prokaryotic and eukaryotic cells. The proteins triggering an abnormal accumulation of membrane structures inside the cells (or membrane proliferation) share two major common features: (1) they promote the formation of highly curved membrane domains and (2) they lead to an enrichment in anionic, cone-shaped phospholipids (cardiolipin or phosphatidic acid) in the newly formed membranes. Taking into account the available examples of membrane proliferation upon protein overproduction, together with the latest biochemical, biophysical and structural data, we explore the relationship between protein synthesis and membrane biogenesis. We propose a mechanism for the formation of these non-physiological intracellular membranes that shares similarities with natural inner membrane structures found in α-proteobacteria, mitochondria and some viruses-infected cells, pointing towards a conserved feature through evolution. We hope that the information discussed in this review will give a better grasp of the biophysical mechanisms behind physiological and induced intracellular membrane proliferation, and inspire new applications, either for academia (high-yield membrane protein production and nanovesicle production) or industry (biofuel production and vaccine preparation).

Ectopic Neo-Formed Intracellular Membranes in Escherichia coli: A Response to Membrane Protein-Induced Stress Involving Membrane Curvature and Domains

Bacterial cytoplasmic membrane stress induced by the overexpression of membrane proteins at high levels can lead to formation of ectopic intracellular membranes. In this review, we report the various observations of such membranes in Escherichia coli, compare their morphological and biochemical characterizations, and we analyze the underlying molecular processes leading to their formation. Actually, these membranes display either vesicular or tubular structures, are separated or connected to the cytoplasmic membrane, present mono- or polydispersed sizes and shapes, and possess ordered or disordered arrangements. Moreover, their composition differs from that of the cytoplasmic membrane, with high amounts of the overexpressed membrane protein and altered lipid-to-protein ratio and cardiolipin content. These data reveal the importance of membrane domains, based on local specific lipid⁻protein and protein⁻protein interactions, with both being crucial for local membrane curvature generation, and they highlight the strong influence of protein structure. Indeed, whether the cylindrically or spherically curvature-active proteins are actively curvogenic or passively curvophilic, the underlying molecular scenarios are different and can be correlated with the morphological features of the neo-formed internal membranes. Delineating these molecular mechanisms is highly desirable for a better understanding of protein⁻lipid interactions within membrane domains, and for optimization of high-level membrane protein production in E. coli.

Engineering membrane morphology and manipulating synthesis for increased lycopene accumulation in Escherichia coli cell factories

The goal of this work was to improve the lycopene storage capacity of the E. coli membrane by engineering both morphological and biosynthetic aspects. First, Almgs, a protein from Acholeplasma laidlawii that is involved in membrane bending is overexpressed to expand the storage space for lycopene, which resulted in a 12% increase of specific lycopene production. Second, several genes related to the membrane-synthesis pathway in E. coli, including plsb, plsc, and dgka, were also overexpressed, which led to a further 13% increase. In addition, membrane separation and component analysis confirmed that the increased amount of lycopene was mainly accumulated within the cell membranes. Finally, by integrating both aforementioned modification strategies, a synergistic effect could be observed which caused a 1.32-fold increase of specific lycopene production, from the 27.5 mg/g of the parent to 36.4 mg/g DCW in the engineered strain. This work demonstrates that membrane engineering is a feasible strategy for increasing the production and accumulation of lycopene in E. coli.

Cardiolipin plays an essential role in the formation of intracellular membranes in Escherichia coli

Mitochondria, chloroplasts and photosynthetic bacteria are characterized by the presence of complex and intricate membrane systems. In contrast, non-photosynthetic bacteria lack membrane structures within their cytoplasm. However, large scale over-production of some membrane proteins, such as the fumarate reductase, the mannitol permease MtlA, the glycerol acyl transferase PlsB, the chemotaxis receptor Tsr or the ATP synthase subunit b, can induce the proliferation of intra cellular membranes (ICMs) in the cytoplasm of Escherichia coli. These ICMs are particularly rich in cardiolipin (CL). Here, we have studied the effect of CL in the generation of these membranous structures. We have deleted the three genes (clsA, clsB and clsC) responsible of CL biosynthesis in E. coli and analysed the effect of these mutations by fluorescent and electron microscopy and by lipid mass spectrometry. We have found that CL is essential in the formation of non-lamellar structures in the cytoplasm of E. coli cells. These results could help to understand the structuration of membranes in E. coli and other membrane organelles, such as mitochondria and ER.

Massive formation of intracellular membrane vesicles in Escherichia coli by a monotopic membrane-bound lipid glycosyltransferase

The morphology and curvature of biological bilayers are determined by the packing shapes and interactions of their participant molecules. Bacteria, except photosynthetic groups, usually lack intracellular membrane organelles. Strong overexpression in Escherichia coli of a foreign monotopic glycosyltransferase (named monoglycosyldiacylglycerol synthase), synthesizing a nonbilayer-prone glucolipid, induced massive formation of membrane vesicles in the cytoplasm. Vesicle assemblies were visualized in cytoplasmic zones by fluorescence microscopy. These have a very low buoyant density, substantially different from inner membranes, with a lipid content of > or = 60% (w/w). Cryo-transmission electron microscopy revealed cells to be filled with membrane vesicles of various sizes and shapes, which when released were mostly spherical (diameter approximately 100 nm). The protein repertoire was similar in vesicle and inner membranes and dominated by the glycosyltransferase. Membrane polar lipid composition was similar too, including the foreign glucolipid. A related glycosyltransferase and an inactive monoglycosyldiacylglycerol synthase mutant also yielded membrane vesicles, but without glucolipid synthesis, strongly indicating that vesiculation is induced by the protein itself. The high capacity for membrane vesicle formation seems inherent in the glycosyltransferase structure, and it depends on the following: (i) lateral expansion of the inner monolayer by interface binding of many molecules; (ii) membrane expansion through stimulation of phospholipid synthesis, by electrostatic binding and sequestration of anionic lipids; (iii) bilayer bending by the packing shape of excess nonbilayer-prone phospholipid or glucolipid; and (iv) potentially also the shape or penetration profile of the glycosyltransferase binding surface. These features seem to apply to several other proteins able to achieve an analogous membrane expansion.

High yield cell-free production of integral membrane proteins without refolding or detergents

Integral membrane proteins act as critical cellular components and are important drug targets. However, difficulties in producing membrane proteins have hampered investigations of structure and function. In vivo production systems are often limited by cell toxicity, and previous in vitro approaches have required unnatural folding pathways using detergents or lipid solutions. To overcome these limitations, we present an improved cell-free expression system which produces high yields of integral membrane proteins without the use of detergents or refolding steps. Our cell-free reaction activates an Escherichia coli-derived cell extract for transcription and translation. Purified E. coli inner membrane vesicles supply membrane-bound components and the lipid environment required for insertion and folding. Using this system, we demonstrated successful synthesis of two complex integral membrane transporters, the tetracycline pump (TetA) and mannitol permease (MtlA), in yields of 570+/-50 microg/mL and 130+/-30 microg/mL of vesicle-associated protein, respectively. These yields are up to 400 times typical in vivo concentrations. Insertion and folding of these proteins are verified by sucrose flotation, protease digestion, and activity assays. Whereas TetA incorporates efficiently into vesicle membranes with over two-thirds of the synthesized protein being inserted, MtlA yields appear to be limited by insufficient concentrations of a membrane-associated chaperone.

Subcellular location of phage infection protein (Pip) in Lactococcus lactis

The amino acid sequence of the phage infection protein (Pip) of Lactococcus lactis predicts a multiple-membrane-spanning region, suggesting that Pip may be anchored to the plasma membrane. However, a near-consensus sortase recognition site and a cell wall anchoring motif may also be present near the carboxy terminus. If functional, this recognition site could lead to covalent linkage of Pip to the cell wall. Pip was detected in both plasma membranes and envelopes (plasma membrane plus peptidoglycan) isolated from the wild-type Pip strain LM2301. Pip was firmly attached to membrane and envelope preparations and was solubilized only by treatment with detergent. Three mutant Pip proteins were separately made in which the multiple-membrane-spanning region was deleted (Pip-Deltammsr), the sortase recognition site was converted to the consensus (Pip-H841G), or the sortase recognition site was deleted (Pip-Delta6). All three mutant Pip proteins co-purified with membranes and could not be solubilized except with detergent. When membranes containing Pip-Deltammsr were sonicated and re-isolated by sucrose density gradient centrifugation, Pip-Deltammsr remained associated with the membranes. Strains that expressed Pip-H841G or Pip-Delta6 formed plaques with near unit efficiency, whereas the strain that expressed Pip-Deltammsr did not form plaques of phage c2. Both membranes and cell-free culture supernatant from the strain expressing Pip-Deltammsr inactivated phage c2. These results suggest that Pip is an integral membrane protein that is not anchored to the cell wall and that the multiple-membrane-spanning region is required for productive phage infection but not phage inactivation.

The First Cytoplasmic Loop of the Mannitol Permease from Escherichia coli is Accessible for Sulfhydryl Reagents from the Periplasmic Side of the Membrane

The mannitol permease (EII(Mtl)) from Escherichia coli couples mannitol transport to phosphorylation of the substrate. Renewed topology prediction of the membrane-embedded C domain suggested that EII(Mtl) contains more membrane-embedded segments than the six proposed previously on the basis of a PhoA fusion study. Cysteine accessibility was used to confirm this notion. Since cysteine 384 in the cytoplasmic B domain is crucial for the phosphorylation activity of EII(Mtl), all cysteine mutants contained this activity-linked cysteine residue in addition to those introduced for probing the membrane topology of the protein. To distinguish between the activity-linked cysteine and the probed cysteine, either trypsin was used to specifically digest the two cytoplasmic domains (A and B), thereby removing Cys384, or Cys384 was protected by phosphorylation from alkylation by N-ethylmaleimide (NEM). Our data show that upon phosphorylation EII(Mtl) undergoes major conformational changes, whereby residues in the putative first cytoplasmic loop become accessible to NEM. Other residues in this loop were accessible to NEM in intact cells and inside-out membrane vesicles, but cysteine residues at these positions only reacted with the membrane-impermeable sulfhydryl reagent from the periplasmic side of the protein. These and other results suggest that the predicted loop between TM2 and TM3 may fold back into the membrane and form part of the translocation path.

DnaK and DnaJ facilitated the folding process and reduced inclusion body formation of magnesium transporter CorA overexpressed in Escherichia coli

Overexpression of CorA, the major magnesium transporter from bacterial inner membrane, in Escherichia coli resulted in the synthesis of 60mg of protein per liter of culture, most of which however was in the form of inclusion bodies. The levels of inclusion body formation were reduced by lowering the cell culture temperature. To dissect CorA inclusion body formation and the folding process involved, we co-expressed the protein with various chaperones and other folding modulators. Expression of DnaK/DnaJ (Hsp70) prevented inclusion bodies from forming and resulted in the integration of more CorA into the membrane. GroEL/GroES (Hsp60/Hsp10) were less effective at reducing CorA inclusion body formation. Co-expression with either Ffh/4.5S-RNA, the signal recognition particle, or SecA, the ATPase that drives protein insertion into the membrane, had little effect on CorA folding. These results indicate: (1) that CorA inclusion bodies form immediately after synthesis at 37 degrees C, (2) that CorA solubility in the cytosol can be increased by co-expressing a chaperone system, (3) membrane targeting is probably not a rate-limiting factor, and (4) that membrane insertion becomes a limitation only when large amounts of soluble CorA are present in the cytosol. These co-expression systems can be used for producing other membrane proteins in large quantities.

Efficient biosynthetic incorporation of tryptophan and indole analogs in an integral membrane protein

Biosynthetic incorporation of tryptophan (Trp) analogs such as 7-azatryptophan, 5-hydroxytryptophan, and fluorotryptophan into a protein can facilitate its structural analysis by spectroscopic techniques such as fluorescence, phosphorescence, nuclear magnetic resonance, and Fourier transform infrared. Until now, the approach has dealt primarily with soluble proteins. In this article, we demonstrate that four different Trp analogs can be very efficiently incorporated into a membrane protein as demonstrated for the mannitol transporter of Escherichia coli (EII(mtl)). EII(mtl) overexpression was under control of the lambdaP(R) promoter, and the E. coli Trp auxotroph M5219 was used as host. This strain constitutively expresses the heat labile repressor protein of the lambdaP(R) promoter. Together with the presence of the repressor gene on the EII(mtl) plasmid, this resulted in a tightly controlled promoter system, a prerequisite for high Trp analog incorporation. A new method for determining the analog incorporation efficiency is presented that is suitable for membrane proteins. The procedure involves fitting of the phosphorescence spectrum as a linear combination of the Trp and Trp analog contributions, taking into account the influence of the protein environment on the Trp analog spectrum. The data show that the analog content of EII(mtl) samples is very high (>95%). In addition, we report here that biosynthetic incorporation of Trp analogs can also be effected with less expensive indole analogs, which in vivo are converted to L-Trp analogs.

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.

Thermodynamic evidence for conformational coupling between the B and C domains of the mannitol transporter of escherichia coli, enzyme IImtl

The transport across the cytoplasmic membrane and concomitant phosphorylation of mannitol in Escherichia coli is catalyzed by the mannitol-specific transport protein from the phosphoenolpyruvate-dependent phosphotransferase system, enzyme IImtl. Interactions between the cytoplasmic B and the membrane embedded C domain play an important role in the catalytic cycle of this enzyme, but the nature of this interaction is largely unknown. We have studied the thermodynamics of binding of (i) mannitol to enzyme IImtl, (ii) the substrate analog perseitol to enzyme IImtl, (iii) perseitol to phosphorylated enzyme IImtl, and (iv) mannitol to enzyme IImtl treated with trypsin to eliminate the cytoplasmic domains. Analysis of the heat capacity increment of these reactions showed that approximately 50-60 residues are involved in the binding of mannitol and perseitol, but far less in the phosphorylated state or after removal of the B domain. A model is proposed in which binding of mannitol leads to the formation of a contact interface between the two domains, either by folding of unstructured parts or by docking of preexisting surfaces, thus positioning the incoming mannitol close to the phosphorylation site on the B domain to facilitate the transfer of the phosphoryl group.

Biochemical characterization of a delta12 acyl-lipid desaturase after overexpression of the enzyme in Escherichia coli

The Delta12 acyl-lipid desaturase of Synechocystis sp. PCC 6803 was overexpressed in Escherichia coli as an active enzyme. The overexpressed protein was associated with cell membranes; it represented about 10% of the total cellular protein and 25% of the total membrane protein. The enzyme in the membrane fraction exhibited strong fatty-acid desaturase activity. The desaturase in salt-washed membranes was stabilized by the presence of sorbitol. Storage of salt-washed membranes in 2 M sorbitol at 4 degrees C and at pH 7-8 for six days resulted in the loss of less than 10% of the desaturase activity. The desaturase activity had a positive temperature coefficient, a result that suggests that the increase in the desaturation of fatty acids at low temperature might not be caused by the activation of desaturases at low temperature but, rather, by the increased synthesis of desaturases de novo.

Overproduction of a foreign membrane protein in Escherichia coli stimulates and depends on phospholipid synthesis

When the Pseudomonas oleovorans alk system, consisting of the alkBFGHJKL and alkST genes, is expressed in Escherichia coli W3110, significant changes in phospholipid metabolism of the host are observed. A major role seems to be played by the cytoplasmic membrane protein alkane hydroxylase (AlkB), which is synthesized as up to 10-15% of the total protein in this strain [Nieboer, M., Kingma, J. & Witholt, B. (1993) The alkane oxidation system of Pseudomonas oleovorans: induction of the alk genes in Escherichia coli W3110[pGEc47] affects membrane biogenesis and results in overexpression of alkane hydroxylase in a distinct cytoplasmic membrane subfraction, Mol. Microbiol. 8, 1039-1051]. In the present paper, we have studied the link between synthesis of the membrane protein and the synthesis of phospholipids and fatty acids by examining the kinetics of these processes. Using [35S]methionine labeling, it was shown that induction of AlkB was maximal within 30-60 min after addition of inducer, when up to 15% of all newly synthesized protein is AlkB. Phospholipid synthesis was followed by measuring the incorporation of 14C-labeled acetate and 33P-labeled phosphoric acid into phospholipids. Despite a negative effect of the inducer on the growth rate of W3110[pGEc47], net phospholipid synthesis was significantly enhanced as a result of the expression of alkB. Synthesis of all three major phospholipids were stimulated to comparable extents by the induction of alkB. Induction did not increase 33P incorporation into lipids in the control recombinant alk+ strain which lacked alkB. Simultaneous with AlkB synthesis, the conversion of unsaturated 9-hexadecenoic acid (C16:1) into 9,10-methylene hexadecanoic acid (C17:ocyc) was reduced in the alk+ recombinant. Overall, these data show that the production of a foreign membrane protein in E. coli can engender a response of the phospholipid-synthesizing system of the host. In the absence of such a response, induction of the alk system would be much more toxic to the cells. Apparently, the increased phospholipid synthesis plays an important role in enabling the AlkB overproducing strain to grow.

Overexpression of integral membrane proteins for structural studies

Determination of the structure of integral membrane proteins is a challenging task that is essential to understand how fundamental biological processes (such as photosynthesis, respiration and solute translocation) function at the atomic level. Crystallisation of membrane proteins in 3D has led to the determination of four atomic resolution structures [photosynthetic reaction centres (Allenet al. 1987; Changet al. 1991; Deisenhofer & Michel, 1989; Ermleret al. 1994); porins (Cowanet al. 1992; Schirmeret al. 1995; Weisset al. 1991); prostaglandin H2synthase (Picotet al. 1994); light harvesting complex (McDermottet al. 1995)], and crystals of membrane proteins formed in the plane of the lipid bilayer (2D crystals) have produced two more structures [bacteriorhodopsin (Hendersonet al. 1990); light harvesting complex (Kühlbrandtet al. 1994)].

Lactose-specific enzyme II of the phosphoenolpyruvate-dependent phosphotransferase system of Staphylococcus aureus. Purification of the histidine-tagged transmembrane component IICBLac and its hydrophilic IIB domain by metal-affinity chromatography, and functional characterization

The lactose-specific integral-membrane-protein enzyme II (IICBLac) of the bacterial phosphoenolpyruvate-dependent phosphotransferase system of Staphylococcus aureus catalyses the uptake and phosphorylation of lactose. It consists of an N-terminal membrane-spanning IIC domain and a C-terminal hydrophilic IIB domain. IICBLac was fused with a C-terminal tag of six histidine residues using recombinant DNA technology. The resulting protein, IICBLac-His, was produced in Escherichia coli and purified under nondenaturing conditions to homogenity. The purification procedure consists of a NaOH extraction step followed by solubilisation with Triton X-100, and metal-affinity chromatography using Ni(2+)-nitrilotriacetic acid resin. The purified recombinant His-tagged protein possessed substrate specificity identical to that of the wild-type protein. To investigate the hydrophilic IIB domain, the DNA sequence coding for IIB and the His tag were fused in-frame to a DNA sequence specific for an initiation signal. The overproduced recombinant IIBLac-His was obtained by metal-affinity chromatography in pure form. Bacterial phosphotransferase-system-dependent phosphorylation of IIB-His was demonstrated in a photometric assay and by urea/polyacrylamide gel electrophoresis. The phosphorylation activity of the mutant protein [C476S]-IICBLac, containing the mutagenized phosphorylation site, was restored in the presence of IIBLac-His in a phosphorylation assay.

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)

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

The mannitol-specific transport protein in Escherichia coli, EIImtl, consists of three structural and functional domains: a hydrophilic EIII-like domain (the A domain); a hydrophobic transmembrane domain (the C domain); and a second hydrophilic domain (the B domain) which connects the A and C domains together. The A domain contains the first phosphorylation site, His554, while the B domain contains the second phosphorylation site, Cys384. The phosphoryl group which is needed for the active transport of mannitol is sequentially transferred from P-enolpyruvate via the two phosphorylation sites to mannitol bound to the substrate binding site. In this paper, the expression, purification, and initial characterization of the B domain, IIBmtl, are described. Oligonucleotide-directed mutagenesis was used to produce an amber stop codon (TAG) and HindIII restriction site in a flexible loop between the B and A domains in the subcloned gene fragment coding for IIBAmtl (van Weeghel et al., 1991c). The gene fragment coding for IIBmtl was then subcloned behind strong promoters, located in two different expression/mutagenesis vectors, which directed the expression of the 15.3-kDa polypeptide in Escherichia coli. The domain was purified from E. coli crude cell extracts by using Q-Sepharose Fast Flow, S-Sepharose Fast Flow, and hydroxylapatite column steps. This purification procedure resulted in 1 mg of pure IIBmtl/g of cell, wet weight. The purified B domain was analyzed in vitro for its catalytic activity with membranes containing the phosphorylation site mutant form of EIImtl, C384S, and with the transmembrane domain, IICmtl. The B domain, together with purified IIA, was able to restore the P-enolpyruvate-dependent phosphorylation activity of the membrane-bound C domain. Steady-state mannitol phosphorylation kinetics at saturating EI, HPr, and IIAmtl yielded an apparent Km of P-IIBmtl for IICmtl of 200 microM and an apparent Vmax of 71 nmol of mtl-P min-1 mg of membrane protein)-1. This Vmax value is comparable to that of wild-type EIImtl measured under the same experimental conditions.

Phosphoenolpyruvate:carbohydrate phosphotransferase systems of bacteria

Numerous gram-negative and gram-positive bacteria take up carbohydrates through the phosphoenolpyruvate (PEP):carbohydrate phosphotransferase system (PTS). This system transports and phosphorylates carbohydrates at the expense of PEP and is the subject of this review. The PTS consists of two general proteins, enzyme I and HPr, and a number of carbohydrate-specific enzymes, the enzymes II. PTS proteins are phosphoproteins in which the phospho group is attached to either a histidine residue or, in a number of cases, a cysteine residue. After phosphorylation of enzyme I by PEP, the phospho group is transferred to HPr. The enzymes II are required for the transport of the carbohydrates across the membrane and the transfer of the phospho group from phospho-HPr to the carbohydrates. Biochemical, structural, and molecular genetic studies have shown that the various enzymes II have the same basic structure. Each enzyme II consists of domains for specific functions, e.g., binding of the carbohydrate or phosphorylation. Each enzyme II complex can consist of one to four different polypeptides. The enzymes II can be placed into at least four classes on the basis of sequence similarity. The genetics of the PTS is complex, and the expression of PTS proteins is intricately regulated because of the central roles of these proteins in nutrient acquisition. In addition to classical induction-repression mechanisms involving repressor and activator proteins, other types of regulation, such as antitermination, have been observed in some PTSs. Apart from their role in carbohydrate transport, PTS proteins are involved in chemotaxis toward PTS carbohydrates. Furthermore, the IIAGlc protein, part of the glucose-specific PTS, is a central regulatory protein which in its nonphosphorylated form can bind to and inhibit several non-PTS uptake systems and thus prevent entry of inducers. In its phosphorylated form, P-IIAGlc is involved in the activation of adenylate cyclase and thus in the regulation of gene expression. By sensing the presence of PTS carbohydrates in the medium and adjusting the phosphorylation state of IIAGlc, cells can adapt quickly to changing conditions in the environment. In gram-positive bacteria, it has been demonstrated that HPr can be phosphorylated by ATP on a serine residue and this modification may perform a regulatory function.

Phosphoenolpyruvate-dependent mannitol phosphotransferase system of Escherichia coli: overexpression, purification, and characterization of the enzymatically active C-terminal domain of enzyme IImtl equivalent to enzyme IIImtl

The extreme C-terminus (Ser-490 to Lys-637) of the Escherichia coli EIImtl was subcloned to test structural and mechanistic proposals about the existence of an EIII-like domain in this enzyme. Oligonucleotide-directed mutagenesis was used to produce a unique NcoI restriction site and, at the same time, to change Ser-490 into methionine in a flexible region in front of the proposed EIII-like domain. The 16-kDa C-terminal domain (CI) was overexpressed in Escherichia coli, purified, and analyzed in vitro for catalytic activity in the presence of an EIImtl mutated at its first phosphorylation site, His-554 (EII-H554A). The results presented show that this domain can be expressed as a structurally stable, enzymatically active entity which is able to restore the PEP-dependent phosphorylation activity of the mutant EIImtl-H554A to 25% of wild-type levels. To demonstrate the EIII activity of the CI domain in a more direct way, we also substituted it for EIIImtl in the Staphylococcus carnosus system. The CI domain was active in transferring the phosphoryl group to Staph. carnosus EII; however, it was 6.5 times less active compared to Staph. carnosus EIIImtl itself. EIIImtl from Staph. carnosus, on the other hand, was able to substitute for the isolated C-terminal domain in the E. coli mannitol phosphorylation assay; however, it appeared to be 2 or 3 times less effective.

Details of mannitol transport in Escherichia coli elucidated by site-specific mutagenesis and complementation of phosphorylation site mutants of the phosphoenolpyruvate-dependent mannitol-specific phosphotransferase system

The mannitol transport protein (EIImtl) carries out translocation with concomitant phosphorylation of mannitol from the periplasm to the cytoplasm, at the expense of phosphoenolpyruvate (PEP). The phosphoryl group which is needed for this group translocation is sequentially transferred from PEP via two phosphorylation sites, located exclusively on the C-terminal cytoplasmic domain, to mannitol. Oligonucleotide-directed mutagenesis was used to investigate the precise role of these sites in phosphoryl group transfer, by producing specific amino acid substitutions. The first phosphorylation site, His-554 (P1), was replaced by Ala, which renders the EII-H554A completely inactive in PEP-dependent mannitol phosphorylation, but not in mannitol/mannitol 1-phosphate exchange. The P2 site mutant, EII-C384S, was inactive both in the mannitol phosphorylation reaction and in the exchange reaction, due to replacement of the essential Cys-384 by Ser. Although EII-H554A and EII-C384S were both catalytically inactive in the PEP-dependent phosphorylation, EII-C384S was able to restore up to 55% of the wild-type mannitol phosphorylation activity with the EII-H554A mutant, indicating a direct phosphotransfer between two subunits. These phosphorylation data together with the data obtained from mannitol/mannitol phosphate exchange kinetics, after mixing EII-H554A and EII-C384S, indicated the formation of functionally stable heterodimers, which consist of an EII-H554A and an EII-C384S monomer.