Patent Publication Number: US-9416388-B2

Title: Method for producing disulfide bond containing proteins in a prokaryotic cytoplasm

Description:
PRIORITY 
     This application is a national phase application of PCT/F12012/050105 filed on Feb. 3, 2012 and claiming priority of FI20115115, which was filed on Feb. 4, 2011, and both of which are incorporated herein by reference. 
     SEQUENCE LISTING 
     This application contains sequence listing which is provided on written format as well as on a computer readable format, both of which are identical. 
     FIELD OF THE INVENTION 
     This invention relates to a method, a host cell and a vector system for producing protein(s) of interest containing one or more disulfide bonds in their native state in a prokaryotic host. 
     Many proteins and enzymes of biotechnological importance contain structure stabilizing disulfide bonds, with an estimated one third of all human proteins folding in the endoplasmic reticulum (ER) and acquiring disulfide bonds there. This includes most proteins which get secreted or end up on the outer membrane. Since any two cysteine residues in a protein have the potential to form a disulfide bond, the correct formation of native disulfide bonds is not trivial. Hence, it is unsurprising that native disulfide bond formation is often the rate-limiting step in the folding of proteins in vitro and in vivo. 
     Currently proteins that contain disulfide bonds are difficult for the biotech industry to produce on a large scale. The most common route is to produce these proteins in the cytoplasm of  E. coli . There are no mechanisms for disulfide bond formation in the cytoplasm. This, combined with the efficient pathways for disulfide bond reduction in the cytoplasm, means that recombinant proteins which fold in the cytoplasm of bacteria, such as  E. coli , lack structural and/or functional disulfides and usually form insoluble, inactive inclusion bodies. Inclusion body refolding is a widely studied and patented field. However, it is costly, complex and generally inefficient. 
     Alternative routes for producing disulfide bonded proteins in prokaryotes also have drawbacks.
         I. Disulfide bond formation in the periplasm. The process of native disulfide bond formation in the periplasm of bacteria is a multi-factorial process, which shows variations between species. Disulfide bond formation (oxidation of a dithiol to a disulfide) is catalysed by the transmembrane proteins DsbB and/or vitamin-K oxidoreductases (VKOR). These have a transmembrane topology which has their active site(s) located on the periplasmic side of the membrane. DsbB and VKOR family members use an intermediary protein, for example DsbA, rather than interacting directly with substrates. However, some examples exist where the two components (transmembrane protein and intermediary protein) are fused, for example VKOR from  Synechococcus  sp. Native disulfide bond formation often requires isomerization of disulfide bonds and this is catalysed by a disulfide isomerase, for example DsbC. Both DsbA and DsbC are targeted to the periplasm by an N-terminal signal sequence. In the periplasm DsbC requires the action of the transmembrane protein DsbD which has its active site located on the periplasmic side of the membrane.   While native  E. coli  disulfide bond containing proteins fold efficiently in the periplasm, the yields of heterologously expressed proteins are often very low, in part due to the small size of the periplasm. In addition, the outer membrane of  E. coli  is freely diffusible to most small molecules which means that the biophysical environment of the periplasm is dependent on the external media.   EP 0 992 588 A1 describes bacterial expression plasmids encoding DsbA, DsbB, DsbC and DsbD in addition to the heterologous protein to be produced in the bacterial cell. The expression plasmid enables expression of active heterologous protein in both periplasmic and spheroplast fraction, but not in the cytoplasm.   II. Disulfide bond formation in the cytoplasm of modified  E. coli. E. coli  has two pathways to ensure that its cytoplasm is reducing: i) using thioredoxins/thioredoxin reductases and ii) using glutathione/glutaredoxin/glutathione reductase. When both pathways are knocked out, for example in the commercial Rosetta-Gami™ (Novagen) or SHuffle® (New England Biolabs) strains, the cytoplasm is less reducing and disulfide bonds form in proteins. However, there is no active oxidase in the system and so disulfide bond formation is still slow and inefficient and dependent on external factors. In addition, these strains grow significantly more slowly than wild type strains. While some disulfide bond containing proteins can be formed in the cytoplasm of Rosetta-gami, or equivalent strains, the yields of many proteins are often below that required for commercial production.       

     Since the prior art routes for producing disulfide bonded proteins in prokaryotes have various drawbacks, there is a need for improved systems for the production of desired disulfide bonded proteins in prokaryotic hosts, in particular in bacterial hosts. 
     SUMMARY 
     One object of the present invention is to provide a method for producing protein(s) of interest containing one or more disulfide bonds in their native state in a prokaryotic host. 
     Another object of the present invention is to provide a prokaryotic host cell for producing protein(s) of interest containing one or more disulfide bonds in their native state. 
     Still another object of the invention is to provide a vector system which can be used in introducing into and/or expressing desired genes in said prokaryotic hosts. 
     To achieve these objects the invention is characterized by the features that are enlisted in the independent claims. Other claims represent the preferred embodiments of the invention. 
     There exist at least two pathways to ensure that disulfide bond formation in proteins in the cytoplasm is minimal: (i) the glutathione/glutathione reductase pathway and (ii) the thioredoxin/thioredoxin pathway. Due to these cytoplasmic pathways, and the lack of existence of pathways to catalyse cytoplasmic disulfide bond formation, non-regulatory disulfide bond formation in bacteria is restricted to the periplasmic space. 
     It has now been surprisingly found that the use of expression of inverted DsbB (iDsbB) or inverted VKOR (iVKOR) in prokaryotes, generates disulfide bonds in folding proteins in the cytoplasm. Equally surprisingly after engineering inversion of one DsbB, some naturally occurring inverted VKOR family members were identified. 
     DsbB and VKOR catalyse the reaction:
 
Dithiol+Quinone→Disulfide+Quinol
 
where the quinone is either ubiquinone (aerobic conditions) or menaquinone (anaerobic conditions) for DsbB and is thought to be a form of vitamin K for VKOR. Since DsbB is specific for the oxidation of DsbA, cytoplasmic expression of DsbA is required along with iDsbB to allow for disulfide bond formation in cytoplasmically expressed proteins. Typically VKOR family members also require the use of an intermediary protein. For example the active site of VKOR or iVKOR as disclosed herein also requires expression of DsbA or a corresponding protein capable of providing electrons to the active site(s) of VKOR or iVKOR in order to function efficiently. Some VKOR family members are formed from a fusion of the VKOR domain and the intermediary protein.
 
     By inverting the topology of DsbB (iDsbB) such that the cysteines of the active site(s) are now cytoplasmically localised, it is possible for iDsbB to accept electrons directly from a cytoplasmically expressed version of DsbA (cDsbA). This would allow reconstruction of the natural pathway for disulfide bond formation pathway not in the periplasm but in the cytoplasm of prokaryotes. Inversion is needed also if a VKOR species is used not having the cysteines of the active site(s) cytoplasmically localised. In a similar manner VKOR or iVKOR requires expression of DsbA or a corresponding protein capable of providing electrons to the active site(s) of VKOR or iVKOR. 
     In one aspect the present invention provides a method for producing disulfide bond containing proteins in prokaryotic cells, which comprises the steps of
         expressing in the cytoplasm of a prokaryotic cell   protein(s) of interest that naturally contain disulfide bonds,   naturally occurring or inverted transmembrane enzyme, wherein the cysteines of the active site(s) are naturally or after genetic engineering located towards the prokaryotic cytoplasm and said enzyme is capable of catalysing the process of native disulfide bond formation, said enzyme being selected from the group of VKOR, inverted VKOR (iVKOR) and inverted DsbB (iDsbB), and   cytoplasmic DsbA or a corresponding protein being capable of providing electrons to the active site(s) of VKOR, iVKOR or iDsbB   whereby natively folded disulfide bond containing protein(s) of interest are formed in the prokaryotic cytoplasm.       

     As disclosed herein the orientation of the transmembrane enzyme VKOR, iVKOR or iDsbB depends on the charge distribution of the said enzyme in the membrane. 
     In an embodiment of the invention the removal or addition of one or more lysine and/or arginine residues changes the charge distribution of the enzyme VKOR, iVKOR or iDsbB across the membrane. 
     The change of orientation of the transmembrane enzymes is typically done using genetic engineering methods. In an embodiment genetic engineering of said DsbB or VKOR comprises the following steps of
         removing one or more of the cytoplasmic lysine and/or arginine residues of DsbB or VKOR, and/or   fusing DsbB or VKOR to a transmembrane helix or transmembrane helices of any transmembrane protein,
 
whereby the charge distribution of the said enzyme in the membrane is changed and the said active site(s) of said DsbB or VKOR protein is shifted towards the prokaryotic cytoplasm.
       

     In an embodiment one or more cytoplasmic lysine and/or arginine residues are removed by mutagenesis or deletion of the N- and/or C-terminus of the DsbB or VKOR protein. 
     In an embodiment the prokaryote is a bacterium, preferably a Gram negative bacterium, such as  E. coli.    
     In another aspect the present invention provides a prokaryotic host cell genetically engineered to express
         transmembrane enzyme selected from the group of VKOR, inverted VKOR (iVKOR) and inverted DsbB (iDsbB) catalysing the process of native disulfide bond formation and having the cysteines of the active site naturally or after genetic engineering located towards the prokaryotic cytoplasm, and   cytoplasmic DsbA or a corresponding protein being capable of providing electrons to the active site(s) of VKOR, iVKOR or iDsbB,   said host cell being capable of forming natively folded disulfide bond containing protein(s) of interest in the cytoplasm.       

     In an embodiment the method or host cell further comprises expressing in the cytoplasm of a prokaryotic host cell a thiol-disulfide isomerase, such as DsbC or PDI. 
     In an embodiment a nucleic acid sequence encoding transmembrane enzyme VKOR or inverted VKOR (iVKOR) or inverted DsbB (iDsbB) is chromosomally integrated or is in a vector. 
     In an embodiment a nucleic acid sequence encoding DsbA or a corresponding protein being capable of providing electrons to the active site(s) of VKOR, iVKOR or iDsbB, or a nucleic acid sequence encoding a thiol-disulfide isomerase, such as DsbC or PDI, is chromosomally integrated or is in a vector. 
     In another aspect the present invention provides a method for producing a prokaryotic host cell for producing disulfide bond containing protein(s) of interest. 
     In still another aspect the present invention provides a vector system, which comprises
         a vector encoding VKOR, iVKOR and/or iDsbB and cytoplasmic DsbA or a corresponding protein being capable of providing electrons to the active site(s) of VKOR, iVKOR or iDsbB, and optionally a site for a nucleic acid sequence encoding protein(s) of interest, or   a first vector encoding VKOR, iVKOR and/or iDsbB, and   a second vector encoding cytoplasmic DsbA or a corresponding protein being capable of providing electrons to the active site(s) of VKOR, iVKOR or iDsbB,   said first or said second vector optionally having a site for a nucleic acid sequence encoding protein(s) of interest.       

     In an embodiment the vector system further comprises that one of the vectors encodes cytoplasmic thiol-disulfide isomerase, such as DsbC or PDI.
         The advantage of the present invention is that the protein of interest is formed in soluble form. Typically no denaturation and renaturation steps of the protein are needed. Furthermore, the protein is produced directly in biologically active form. Compared to prokaryotic hosts used in the prior art, such as  E. coli  the prokaryotic hosts as described herein are capable of producing even 100× times higher level of desired proteins in an active form.       

    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1 . Schematic of the topology of wild type DsbB (Panel A) and iDsbB (Panel B). The black circles represent the active site cysteine residues. The additional transmembrane region from MalF with the PW linker is shown in shaded circles. 
         FIG. 2 . The activity of inverted DsbB requires expression of cDsbA and active site Cys46. Activity measurements of exogenous cPhoA in the  E. coli  strain BL21(DE3) pLysS. Data represents the mean relative activity (%) to the maximal exogenous cPhoA activity for these data sets (ΔA 410 =19.5 mAU/min)±sd (n=2). 
       Panel A: Expression of cPhoA+/−cDsbA from T7 promoter of pET23 with or without pre-expression of H 0 DsbB 1/9b  from an arabinose promoter in pLysSBAD. For these experiments induction of H 0 DsbB 1/9b  was with 0.5% arabinose. Neither cDsbA nor inverted DsbB are able by themselves to increase the yield of active cPhoA.
 
Panel B: Expression of cPhoA+cDsbA from an arabinose promoter in pLysS-BAD (induction with 0.5% arabinose) with pre-induction of wild type DsbB, H 0 DsbB 1/9b  or the active site mutant H 0 DsbB 1/9b  C46A from the T7 promoter of pET23 using 10 μM IPTG. Neither pre-expression of wild-type DsbB nor the active site mutant of H 0 DsbB 1/9b  results in the gain of significant cPhoA activity while pre-expression of H 0 DsbB 1/9b  does.
 
       Note: The strain, plasmids and expression conditions used here are different than in  FIGS. 4, 6, 8 and 9 , to ensure that the results can be generally applied, hence the difference in maximal exogenous cPhoA activity. 
         FIG. 3 . Original and engineered membrane topology of DsbB. (A) The active site cysteines (black circles) of wild-type DsbB are periplasmic. Electrons (black arrows) flow from folding proteins in the periplasm to DsbA, then to DsbB, then to quinones. (B) Reducing the number of lysine and arginine residues (light grey circles) that are naturally on the cytoplasmic side of DsbB induces partial topology inversion so that it can catalyse disulfide bond formation in the periplasm and the cytoplasm providing DsbA is expressed in both compartments. (C) Addition of an N-terminal TMH (transmembrane helix) fusion to the DsbB mutants allows oxidative folding to occur only in the cytoplasm. 
         FIG. 4 . The activity of DsbB can be inverted across the membrane. DsbB mutants were expressed under a pBAD promoter in the ΔdsbB strain FSH8. The periplasmic activity of DsbB mutants was determined by measuring the activity of pPhoA (white bars) after growth in low-phosphate minimal media while the cytoplasmic activity was determined by expressing cPhoA and cDsbA under a lac promoter in LB media and measuring cPhoA activity (black bars). High pPhoA and cPhoA activities indicate dual topology of the DsbB 2/5  and DsbB 1/5  mutants. N-terminal fusion of the first TMH, H 0 (N in ) of MalF (M1-G38), to these mutants results in inverted topology, while C-terminal fusions of the last TMH, H5(C in ) of MalF (G478-D515) do not. Data represents mean relative activity (%)±sd (n=4). The maximal endogenous pPhoA activity was ΔA 410 =27.0 mAU/min, the maximal exogenous cPhoA activity was ΔA 410 =75.6 mAU/min. The cPhoA and pPhoA activities from the negative control, the vector without DsbB, have been subtracted. 
         FIG. 5 . H 0 DsbB 1/9b  expression does not restore ΔdsbB motility. Motility halos of  E. coli  strain JW5182 harboring a plasmid encoding A) wt DsbB (DsbB 9/5 ) and B) H 0 DsbB 1/9b  were examined after 48 hours growth at 30° C. 
         FIG. 6 . Efficient oxidative folding in the cytoplasm catalyzed by H 0 DsbB 1/9a  requires an efficient electron transport chain. cPhoA activity was determined in the  E. coli  strain AN384 (ΔubiA ΔmenA) with 0.04 mM or 2 mM quinone precursor hydroxybenzoic acid (HBA) or in the ΔdsbB strain FSH8. Data represents mean relative activity (%) to the maximal exogenous cPhoA activity (ΔA 410 =75.6 mAU/min)±sd (n=4). The cPhoA activity from the negative control, the vector without DsbB, has been subtracted. 
         FIG. 7 . Sequence alignment of apVKOR1 and apVKOR2. Grey bars represent the four putative transmembrane regions shared by both proteins. 
         FIG. 8 .  A. pernix  has two copies of VKOR that are have different topologies. When expressed in the ΔdsbB  E. coli  strain FSH8 under the pBAD promoter, apVKOR1 and apVKOR2 can catalyse periplasmic and cytoplasmic disulfide bond formation, respectively as determined by the activities of pPhoA (white bars) and cPhoA (black bars). Data represents mean relative activity (%) to the maximal endogenous pPhoA (ΔA 410 =27.0 mAU/min) or exogenous cPhoA (ΔA 410 =75.6 mAU/min) activity±sd (n=4). The cPhoA and pPhoA activities from the negative control, the vector without DsbB or VKOR, have been subtracted. 
         FIG. 9 . apVKOR2 dependence on cDsbA. Cells harboring vectors encoding cPhoA or cPhoA+ cDsbA were cotransformed with empty vector or with apVKOR2. After induction cPhoA activity was measured. Data represents mean relative activity (%) to the maximal exogenous cPhoA (ΔA 410 =75.6 mAU/min) activity±sd (n=4). The cPhoA activity from the negative control, the vector without VKOR, has been subtracted. 
         FIG. 10 . Inverted DsbB and naturally inverted VKOR can catalyse disulfide bond formation in eukaryotic proteins. Activity measurements of exogenous MBP-vtPA in the  E. coli  strain origami. Data represents the mean relative activity (%) to the maximal exogenous vtPA activity for these data sets (ΔA 405 =7.9 mAU/min). All samples, except the negative control (Neg), have a polycistronic vector expressing MBP-vtPA+DsbA+DsbC under IPTG induction. apVKOR1, apVKOR2 and H 0 DsbB 1/9b  (iDsbB, inverted DsbB) are pre-expressed under the pBAD promoter from a modified version of pLysS. apVKOR1 is a VKOR i.e. not inverted, apVKOR2 is a naturally occurring inverted VKOR. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Definitions 
     By “transmembrane proteins” are meant proteins in which the polypeptide chain is exposed on both sides of the membrane. 
     “Dsb” means disulfide bond forming enzymes. Dsb&#39;s are enzymes which catalyse thiol-disulfide exchange reactions in the periplasm of prokaryotes. DsbA, DsbC and DsbG belong to the thioredoxin superfamily and are soluble proteins. DsbB and DsbD are transmembrane proteins. 
     “DsbA” is a soluble periplasmic enzyme, a direct donor of disulfides (acceptor of electrons) when disulfide bonds are being formed in the bacterial periplasm. 
     “DsbB” is a protein that oxidizes the periplasmic protein DsbA, which in turn oxidizes cysteines in other periplasmic proteins in order to make disulfide bonds. DsbB is a transmembrane protein. There are four catalytic cysteines in DsbB, which are necessary for it to function properly in vivo. DsbB spans the membrane four times and both N-terminus and C-terminus are located in the cytoplasm. Both periplasmic loops of DsbB have two essential catalytic cysteines. DsbB may contain additional non-catalytic cysteines. 
     “VKOR” is vitamin-K dependent oxidoreductase. VKOR family members are found in bacteria, archaea and eukaryotes. VKOR is a transmembrane protein. VKOR family members include proteins in which VKOR is genetically fused to one or more other proteins. As such VKOR family members exhibit variable numbers of transmembrane regions, but the core VKOR domain usually spans the membrane four times. The two non-cytoplasmic loops in the core VKOR domain are located in the periplasm in bacteria and in the endoplasmic reticulum lumen in eukaryotes and each contain two active site cysteines. In some fusion forms the number of catalytic cysteines may be higher. VKOR may contain additional non-catalytic cysteines. 
     “Inverting” means altering the membrane topology of a transmembrane protein such that some or all parts of the protein has a reversed topology. More specifically it means that the charge distribution of a transmembrane protein, here in particular DsbB or VKOR, is changed across the membrane and the said active site(s) of said enzyme(s) is/are shifted towards the cytoplasm, in Gram-negative bacteria from periplasmic space towards the cytoplasm. 
     “iDsbB” means an inverted form of protein DsbB with the active site(s) towards the cytoplasm. 
     “iVKOR” means an inverted form of VKOR with the active site(s) towards the cytoplasm. 
     “Active site(s)” means the one or more site(s) on the surface of the enzyme molecule that has just the right shape and functional groups to bind to at least one of the reacting molecules and to catalyse the reaction. 
     The active site(s) of DsbB or VKOR includes cysteines. As disclosed herein for example the active site(s) of VKOR, iVKOR or iDsbB comprise(s) at least four cysteines located towards the prokaryotic cytoplasm. 
     “Charge distribution” means the distribution of amino acids with charged side chains in a transmembrane protein across the membrane. In particular, the distribution of amino acids with positively charged side chains (lysine, arginine and to a lesser extent histidine). 
     When inverting a transmembrane enzyme DsbB or VKOR by genetic engineering methods, a fusion step of said transmembrane enzyme to a transmembrane helix (TMH) or transmembrane helices (TMHs) of a transmembrane protein may be needed. Any transmembrane protein or transmembrane helix can be used in the fusion construct. Examples of transmembrane proteins are “MalF” and “Leader peptidase Lep”. The use of MalF has been exemplified in the examples. 
     “MalF” means an  E. coli  membrane protein which is part of the Maltose operon and acts as a maltose transporter subunit. The first transmembrane segment has been previously used to stabilize and reserve the native topology of a fragment of the  E. coli  protein DsbD. It has also been previously used to generate protein fusions in order to tether proteins to the periplasmic space of the inner membrane of  E. coli  and to study protein transport across the membrane. 
     “Leader peptidase Lep” means an  E. coli  membrane protein, which catalyses the cleavage of signal sequences in the periplasm of  E. coli.    
     “Genetical engineering methods” mean the process by which the genetic information of an organism is changed in a stable manner. This can be made experimentally by the use of for example molecular biological techniques, chemicals or radiation. The term mutagenesis is here used as a synonym for genetical engineering. 
     “Cytoplasmically targeted” means something that is located in the cytoplasm. 
     Thioredoxin superfamily members are for example thioredoxins, protein disulfide isomerases (PDI&#39;s) and disulfide bond forming enzymes (Dsb&#39;s). 
     Herein, by “disulfide bond containing proteins” are meant in particular proteins produced recombinantly in a prokaryotic host. The proteins contain one or more disulfide bonds in their native state which are required to attain their native conformation. Many such proteins when expressed in a system in which disulfide bond formation is limited form insoluble inclusion bodies within the host. 
     Herein, by “protein of interest” is meant proteins that contain (comprise) one or more disulfide bonds in their native state which are required to attain their native conformation. Many such proteins when expressed in a system in which disulfide bond formation is limited form insoluble inclusion bodies within the host. A protein of interest is here typically a eukaryotic protein, usually a mammalian protein, in particular a human protein. The method of the present invention has been exemplified by producing  E. coli  alkaline phosphatase and a fragment of human tissue plasminogen activator (vtPA). 
     The protein may have in its native state multiple disulfide bonds. The method of the present invention is particularly suitable for producing a protein having in its native state two or more disulfide bonds. 
     Compared to prokaryotic hosts used in the prior art, such as  E. coli , the prokaryotic hosts as described herein are capable of producing proteins of interest at least 4× times, typically at least 10× times, preferably at least 15× times, more preferably at least 50× times, still more preferably at least 50× times higher level of desired proteins in an active form. 
     The term “a prokaryote” has here its ordinary meaning comprising bacteria and archaea. 
     By “a host” is meant here in particular a prokaryotic host. More specifically the host can be a bacterial host, in particular a gram negative host, such as  Escherichia coli  ( E. coli ). In one preferred embodiment the host is  E. coli.    
     According to this disclosure it is possible to produce a natively folded disulfide bond containing protein in a prokaryotic host. According to a preferred embodiment of the invention the protein may be recovered and optionally purified from the cultured host cells. The protein may be lyophilized or formulated with a carrier or diluents, if needed. 
     The advantage of the present invention is that the protein of interest is produced directly in a biologically active form. Typically no denaturation and renaturation steps of the protein are needed. Commercially significant proteins which may be produced by using the present invention comprise for example insulin, blood coagulation factors, cytokines, chemokines, interferons, growth hormones and single chain antibodies and many others. 
     In the disclosure the “biological activity” of a protein is deduced by well known methods in the art appropriate for the individual proteins being assayed. The biological activity or function of a protein reflects characteristics of the protein that result from the structure and conformational flexibility of the protein. These in turn are often dependent on the formation of native disulfide bonds. Hence biological activity, for example the ability of an enzyme to catalyze a specific enzymatic activity, is a measure of the attainment of the formation of native disulfide bonds within a protein. 
     The present invention provides a method for the production of proteins that require disulfide bond formation to reach their native biologically active conformation. The methods presented are particularly suitable for the expression of biologically active proteins that require the formation of multiple disulfide bonds. By multiple disulfide bonds is here meant two or more than two disulfide bonds. 
     “A vector” means a genetic element which is used as a vehicle to transfer typically foreign genetic material into the genome of a host cell. 
     According to the present disclosure the vector or vectors are constructed to be capable of expressing VKOR, iVKOR or iDsbB and cytoplasmic DsbA or a corresponding protein being capable of providing electrons to the active site(s) of VKOR, iVKOR or iDsbB, and optionally a protein of interest in the cytoplasm of the prokaryotic host cell. 
     The system may comprise that VKOR, iVKOR or iDsbB are encoded by one vector and cytoplasmic DsbA or a corresponding protein being capable of providing electrons to the active site(s) of VKOR, iVKOR or iDsbB by another vector. The first or the second vector may optionally have a site for a nucleic acid sequence encoding (a) protein of interest(s). 
     Any of VKOR, iVKOR or iDsbB or cytoplasmic DsbA or a corresponding protein being capable of providing electrons to the active site(s) of VKOR, iVKOR or iDsbB may be chromosomally integrated or in a vector. Also the protein of interest may be chromosomally integrated or in a vector. 
     The vector system can be introduced to a suitable prokaryotic host cell and the host can be cultured to produce a protein of interest comprising natively folded disulfide bonds. 
     The vector system furthermore comprises regulatory elements for multiplying and expressing the nucleic acid sequences in a prokaryotic host. The vector system may comprise also selection markers. The vector system may comprise also a system for inducing expression. 
     Host cells encoding VKOR, iVKOR or iDsbB and cytoplasmic DsbA and a protein of interest are cultured to produce the protein of interest in a biologically active form. 
     Methods for cloning the genes of interest into appropriate vectors and culturing prokaryotic organisms are well known in the art. 
     The construction of suitable vectors has been exemplified here in the examples which report the use of pET23 and pLysS plasmid derivatives. These have ampicillin and chloramphenicol selection markers, respectively. T7 and arabinose inducible expression systems were used. 
     Any suitable culture media may be used for the cultivation of the prokaryotic organisms. In the examples reported here Luria-Bertani Media (LB media) was used. 
     The protein may be obtained from the cultured cells in a soluble form by routine cell lysis methods. 
     Cell lysis can be performed for example by the addition of lysozyme to the resuspended cell pellet followed by freeze-thawing. 
     The protein of interest can be isolated from the cell lysate in substantially pure form by methods well known in the art and that are appropriate for the individual proteins and final application, for example column chromatography, polyacrylamide electrophoresis, or HPLC analysis. This can include the addition of a fusion tag to the protein of interest to aid purification. 
     Useful purification methods are for example methods where N-terminal hexahistidine (HIS) or N-terminal maltose binding protein (MBP) tags are used to facilitate purification using immobilized metal affinity chromatography or amylose resin, respectively. 
     “A substantially pure protein” means a preparation which is at least 60% by weight (dry weight) the protein of interest. Preferably the preparation is at least 75%, more preferably at least 80%, still more preferably at least 90%, still more preferably at least 95%, most preferably at least 99% by weight of the protein of interest. 
     In some applications the protein product comprising the protein of interest may be used with the cell culture without recovery, isolation and/or without purification. In other applications the protein product comprising the protein of interest may be recovered from the cell culture or cell medium or from host cells with or without purification. Furthermore, in some applications the protein product or purified protein may be diluted or concentrated, or lyophilized. 
     According to a preferred embodiment of the invention the vector or vectors is a plasmid or plasmids. 
     Disulfide bond formation in the periplasm is catalysed by enzymes belonging to the Dsb and VKOR families. DsbA, the first family discovered, catalyses the co- and post-translational formation of disulfide bonds in secreted and membrane proteins. In order for DsbA to oxidise a protein dithiol to a disulfide, the active site of DsbA must be regenerated by the transmembrane protein DsbB. DsbB uses quinones as a cofactor. The flow of electrons from substrate proteins via DsbA and DsbB terminates with molecular oxygen under aerobic growth or with fumarate (or nitrate) under an aerobic growth. The key enzyme player in this process which links the respiratory chain within the lipid bilayer of the inner membrane to disulfide bond formation is DsbB. 
     The active site of DsbA, or a corresponding protein being capable of providing electrons to the active site(s) of iDsbB, VKOR or iVKOR after oxidizing a dithiol to a disulfide in a substrate protein must have its own active site disulfide regenerated by the membrane embedded protein iDsbB, VKOR or iVKOR. 
     By “a corresponding protein being capable of providing electrons to the active site(s) of VKOR, iVKOR or iDsbB” as cytoplasmic DsbA is here meant a protein which is able to take electrons from a substrate protein during the process of disulfide bond formation in said substrate protein and transfer them to the active site of VKOR, iVKOR or iDSbB. All known proteins with this ability are in the thioredoxin superfamily. 
     DsbB has four transmembrane segments that form a helical bundle embedded in the membrane ( FIG. 1 ). The four helices are connected by two periplasmic loops, each of which contains two catalytic cysteines, C43/C46 and C106/C132 (numbering for the longer variant of full length  E. coli  DsbB), and by a short cytoplasmic loop. The first active site is close to the periplasmic leaflet of membrane and is oxidised directly by quinones. 
     There is limited evidence for a function of the cytoplasmic regions of DsbB or for cytoplasmic factors in the function of DsbB. Therefore, in the present invention the idea to invert the topology of DsbB was based on the observations that there is abundance of positively charged residues (K and R) on the cytoplasmic side of the membrane (the so-called “positive inside” rule). The majority of the cytoplasmic lysine and arginine residues were therefore removed, either by mutagenesis and/or by deletion of the N- and C-terminus of the protein. In addition, the truncated N-terminus of DsbB was fused to the first transmembrane helix of the maltose transporter MalF ( FIG. 1 ). Alternatively, the truncated C-terminus of DsbB was fused to the last transmembrane helix of the maltose transporter Mal F. 
     While MalF was used as an example, DsbB or VKOR or mutants thereof can be fused to a part of any suitable transmembrane protein (natural or artificial) to mediate inversion. It is important that the final construct has inverted DsbB or VKOR topology based on the membrane distribution of positively charged residues (lysine, arginine and to a lesser extent histidine). The membrane proteins are orientated in such a way that positively charged amino acids are found predominantly in the cytoplasm. In the method of the present disclosure this was achieved by the deletion of the positively charged N- and C-terminus of DsbB as well as removing the cytoplasmic lysine and arginine residues by internal mutations. A transmembrane region of MalF was also added as a fusion. 
     After successfully inverting the topology of  E. coli  DsbB naturally occurring topologically inverted DsbB and VKOR family members were searched for. Using global membrane topology prediction algorithms (SCAMPI) examined 1042 DsbB and 243 prokaryotic and archaeal VKOR family members from Pfam were searched. While no naturally occurring DsbBs with inverted topology were identified, 5 species were found that contain VKOR homologues that are predicted to have cytoplasmic localization of their catalytic sites. All 5 species were hyperthermophiles. It is important that these natural proteins with putative inverted topology were identified using the same membrane distribution of positively charged residues as was used to engineer inversion of DsbB, hence there is no intrinsic difference between naturally inverted proteins and those engineered to have inversion of topology. Subsequently one of these naturally occurring inverted VKOR family members from  Aeropyrum pernix  was shown to have cytoplasmic disulfide bond forming activity. 
     As is shown below DsbBs and VKORs can be obtained from various sources from prokaryotic as well as from eukaryotic sources or artificial DsbBs or VKORs may be used. The sequences of these transmembrane enzymes may be modified or they may vary naturally in broad range. When inverting these transmembrane enzymes it is essential to ensure an appropriate charge distribution, which means the distribution of amino acids with charged side chains in the transmembrane protein across the membrane. As can be seen below DsbBs may originate from various prokaryotes, typically from bacterial genera, such as  Aeromonas, Azospirillum, Rhodoferax, Burkholderia, Pseudoalteromonas . VKORs may originate from various prokaryotes, typically from archaeal or bacterial genera, such as  Aeropyrum, Bifidobacterium , or from eukaryotes, such as human ( Homo sapiens ), or zebrafish ( Danio rerio ). Naturally inverted examples of VKOR are from archaeal genera, such as  Aeropyrum  and  Pyrobaculum . VKOR examples with fusion partner are from bacterial genera, such as  Synechococcus , and  Anaeromyxobacter . Also DsbA may originate from various prokaryotes, typically from bacterial genera, such as  Escherichia , typically  E. coli . DsbCs may originate from bacterial genera, such as  Escherichia , typically  E. coli.    
     Examples of DsbBs, VKORs, DsbAs, DsbCs and PDIs: 
     DsbB Examples (Taken from Various Positions in Full Family Alignment) 
     
       
         
           
               
               
            
               
                   E. coli  DsbB (P0A6M2, long variant AAA23711.1)  
                   
               
               
                 (SEQ ID NO: 1) 
                   
               
               
                 MIMLRFLNQCSQGRGAWLLMAFTALALELTALWFQHVMLLKP   C   VL   C   IYERCALFGVLGAALIGAIAPKTPLR 
                   
               
               
                   
               
               
                 YVAMVIWLYSAFRGVQLTYEHTMLQLYPSPFAT   C   DFMVRFPEWLPLDKWVPQVFVASGD   C   AERQWDFLGLEM 
               
               
                   
               
               
                 PQWLLGIFIAYLIVAVLVVISQPFKAKKRDLFGR 
               
               
                   
               
               
                   E. coli  DsbB (P0A6M2, short variant AAC23711.1)  
               
               
                 (SEQ ID NO: 2) 
                   
               
               
                 MLRFLNQCSQGRGAWLLMAFTALALELTALWFQHVMLLKP   C   VL   C   IYERCALFGVLGAALIGAIAPKTPLRYV 
                   
               
               
                   
               
               
                 AMVIWLYSAFRGVQLTYEHTMLQLYPSPFAT   C   DFMVRFPEWLPLDKWVPQVFVASGD   C   AERQWDFLGLEMPQ 
               
               
                   
               
               
                 WLLGIFIAYLIVAVLVVISQPFKAKKRDLFGR 
               
               
                   
               
               
                   Aeromonas Salmonicida  DsbB (A4SN81)  
               
               
                 (SEQ ID NO: 3) 
                   
               
               
                 MIEFLRRIAAHRLAWGLLAASALFLELSALFFQYVLGLHP   C   VM   C   VYERLAILGVLSAGLLGMVAPEKWYLRW 
                   
               
               
                   
               
               
                 SALLLWGYSAFRGLQLALKHVDYQMNPSPFNV   C   SPFADFPSWAPLDQWLPWLFFPDGD   C   SEISWQFLSFSMP 
               
               
                   
               
               
                 QWLVAIFAAYLLVFVVVTIGNLVKGRCCS 
               
               
                   
               
               
                   Azospirillum  sp. DsbB (D3NXM8)  
               
               
                 (SEQ ID NO: 4) 
                   
               
               
                 MGVADGWPNGSARANLSAMMIQTLLSRVFDDPRIAAPLLALASAGVLLSALFFQFVLGYQP   C   VL   C   IMQRWPY 
                   
               
               
                   
               
               
                 VAVMALGLVTWLFRRWRGVGDALLVVSGLALLAGAGIAAYHVGVEQHWWAGTSS   C   GGSAPANSLEALRAQVL 
               
               
                   
               
               
                 AAPVTR   C   DEVAWSLFGISMAGYNVVISLALAAYAFIAARIAYTRTPVSRTAL 
               
               
                   
               
               
                   Rhodoferax ferrireducens  DsbB (Q21WM5)  
               
               
                 (SEQ ID NO: 5) 
                   
               
               
                 MSFQVVTGWLDNSPRRIFAFVSLASIGMLAFGQYLQHVVGLEP   C   PM   C   IVQRYALVLVAIIAGLTGASGRKGL 
                   
               
               
                   
               
               
                 HLGGAVLMLGSSGFGAYVAARQSWLQWYPPEVVS   C   GRDFYGMITTFPLQRAIPMIFKGSGD   C   SKVDWTFLGG 
               
               
                   
               
               
                 SIANWTFVVFGLIVLLSLALIWRRVSRRVS 
               
               
                   
               
               
                   Burkholderia cenocepacia  DsbB (Q1BY52)  
               
               
                 (SEQ ID NO: 6) 
                   
               
               
                 MNDYTLALRRERRLLMLLGWVCIALLAGALYLQYVKNEDP   C   PL   C   IIQRYFFCAIGIFAFLAAGIRNWRGVWV 
                   
               
               
                   
               
               
                 LELLIAIAAAGGVGTAARHLSIQMNPGFS   C   GFDTLQPIVDSLPPAQWFPGMFKVAGL   C   ETVYPPIFGILLPG 
               
               
                   
               
               
                 WALIGFAVILVAVVASLWRHRRKLAS 
               
               
                   
               
               
                   Pseudoalteromonas atlantica  DsbB (Q15S31)  
               
               
                 (SEQ ID NO: 7) 
                   
               
               
                 MTFISNLADTRLAWGLLFLSALVLVAYALFSQHAMGLQP   C   IM   C   IYQRTAIFGIMFACVPVLAANNMLTRLFA 
                   
               
               
                   
               
               
                 FTVWGISAIWGGLIAWEHYDIQNAANPFFAT   C   EIVPNFPSWLPLHEWLPNLFAATGD   C   GNIDWVFMDMSMPQ 
               
               
                   
               
               
                 WMMVVFAIYSSIWFVVLASRLIGNRAI 
               
            
           
         
       
     
     VKOR Examples 
     
       
         
           
               
               
            
               
                   Aeropyrum pernix  (Q9Y922) 
                   
               
               
                 (SEQ ID NO: 8) 
                   
               
               
                 MSRLSHALLAVTLVGYAASIAGYFEYVSGSGV   C   EIGDVGFAVVN   C   SSVYDIPEAVVFGVVHLSVLAPVYFTL 
                   
               
               
                   
               
               
                 LSLVALAYWIRRSRIFLIASSFLSLVGVVTVPYLVYLELFVAGAV   C   LW   C   TVMHISILLAFALAIVGLRIGGHT 
               
               
                   
               
               
                   Bifidobacterium longum  (Q8G7Z6) 
               
               
                 (SEQ ID NO: 9) 
                   
               
               
                 MTRNTATDTSSSTTNTTPLGLAEARPLIGWRHSATWTYLIMLIASAVALGASLILSAETLQLARHPESALG   C     
                   
               
               
                   
               
               
                 DLNSVVS   C   SAVAQSWQAEIAKFGGLSYPNAFFGIAAESVFITIAVIGLARVKVPRWFATCTWLGGLAALAYS 
               
               
                   
               
               
                 YWLSTQSLFVIHAL   C   PW   C   LTLMFSTTIQFMALSHATVAVQGLPSRKAVAADDSDGEAEVAAVPAGLNKYYRL 
               
               
                   
               
               
                 NIDLMVDILWIVAIVVLIIVTEGAALFAA 
               
               
                   
               
               
                   Homo sapiens  VKORC1 (Q9BQB6) 
               
               
                 (SEQ ID NO: 10) 
                   
               
               
                 MGSTWGSPGWVRLALCLTGLVLSLYALHVKAARARDRDYRAL   C   DVGTAIS   C   SRVFSSRWGRGFGLVEHVLGQ 
                   
               
               
                   
               
               
                 DSILNQSNSIFGCIFYTLQLLLGCLRTRWASVLMLLSSLVSLAGSVYLAWILFFVLYDF   C   IV   C   ITTYAINVS 
               
               
                   
               
               
                 LMWLSFRKVQEPQGKAKRH 
               
               
                   
               
               
                   Danio rerio  vkorc111 (Q502B2) 
               
               
                 (SEQ ID NO: 11) 
                   
               
               
                 MAAPVLRVSTPRWERIVRLLVCLSGILLSLYSFHVEREKTRDANYRAM   C   DLSSSIS   C   SKVFTSRWGRGFGLL 
                   
               
               
                   
               
               
                 GSIFGNDSAVNQPNSVYGIFFYVFQLLLGLTASAMAALILMTTSIASVMGSLYLGYILYFVLKDF   C   VI   C   ITT 
               
               
                   
               
               
                 YALNFILFVLNYKRLVYLNEAWKQQLQAKRD 
               
            
           
         
       
     
     VKOR Examples with Fusion of Partner that Plays Role of DsbA 
     
       
         
           
               
               
            
               
                   Synechococcus  sp VKOR (Q0I736) 
                   
               
               
                 (SEQ ID NO: 12) 
                   
               
               
                 MATQRLTSRRRQDQGSKWVRIVMAVLATVGVIDTGSITLKFWGVLGDLT   C   PMGAGG   C   DKVLNSPWGTLFQGD 
                   
               
               
                   
               
               
                 GFSIPLSFSGLIAYLAVLVMAVVPLLPGLSENKADLSRRTWWGLFTVSLVMAVFSLVLVGLMVIKIQAF   C   FF 
               
               
                   
               
               
                     C   VLSAVLSLTLLVLSLAGGGWDDPSQLLFRGFLLALAVLLGGLIWASVLDPARPDAVATGPGAPPPVLTESN 
               
               
                   
               
               
                 PAKISLAEHLTASGAVMYSAYW   C   PH   C   HEQKEMFGQEAAKTLKVVECAPTGQNNEAKLCQSKGIEGFPTWEIN 
               
               
                   
               
               
                 GELDSGVKKLPELARLSGYQGSKDF 
               
               
                   
               
               
                   Anaeromyxobacter  sp. VKOR (A7HF26) 
               
               
                 (SEQ ID NO: 13) 
                   
               
               
                 MTRDRKKKPDRRPSAPTPAPPRAALLVASLLLALGGVALSVALARLHARAHAGLSSF   C   AINDVVN   C   DRVALS 
                   
               
               
                   
               
               
                 RFSTFLGLPVALWGALGYGLAAVLAARALAHARRGVTAARGLLFAVAAVAVAASAALAVVSELAIGAW   C   LL   C     
               
               
                   
               
               
                 MASWATAAGLLATAWRACPSGPAAAVAADVAVLRARPARTAALALVALVAVVGARAAYARYAATVPRAPAAS 
               
               
                   
               
               
                 AGARAPGPISPAPVAAGGVVVEFSDYE   C   PF   C   ARAHEQLATLRAARPDLEIVRRHFPLDAACNPALARSIHPS 
               
               
                   
               
               
                 ACALARAAICAEAQGRFAEMDDALFRNQQAREPASRLAARLGLDVAAFEACLASPATEARLARDVEDGMRAG 
               
               
                   
               
               
                 VRATPSYVVGGKVYAGELPPGLLAAPAAPAPPPRAAER 
               
            
           
         
       
     
     Naturally Inverted VKOR Examples: 
     
       
         
           
               
               
            
               
                   Aeropryum pernix  (Q9YB70) 
                   
               
               
                 (SEQ ID NO: 14) 
                   
               
               
                 MVEARLLDALYLAALAVGWLASIGGFIEFRRSLLGGGFV   C   KADAKGWIN   C   RSAYVIPQAFIAGRIHLSELAP 
                   
               
               
                   
               
               
                 IYFTATLATAVLGVLLDIDLLAKLSYLLAAGGAASVPYLVYLEVRVAKAI   C   LW   C   TIMHLSIILAVASATAKI 
               
               
                   
               
               
                 LGG 
               
               
                   
               
               
                 Pyrobaculum aerophilum (Q8ZXF9) 
               
               
                 (SEQ ID NO: 15) 
                   
               
               
                 MALYILTGLLAALGVAVGLLGSRLIALSLLAAAGLLHTLFNKPSAF   C   AKYKIGG   C   EAVLSSPYARPFGIPLE 
                   
               
               
                   
               
               
                 YLGAAWFAGVPIAYYLGIGLVWSVMAFAGVIALVAIEAKLRAF   C   IY   C   TVAHVIGLAAAFLLL 
               
            
           
         
       
     
     DsbA Sequence Example 
     
       
         
           
               
               
            
               
                   E. coli  mature DsbA (DsbA without the signal sequence which will be 
                   
               
               
                 cytoplasmically expressed) 
               
               
                 Residues 20-208 of P0AEG4 with an initiating Met added 
               
               
                 (SEQ ID NO: 16) 
                   
               
               
                 (M)AQYEDGKQYTTLEKPVAGAPQVLEFFSFF   C   PH   C   YQFEEVLHISDNVKKKLPEGVKMTKYHVNFMGGDLG 
                   
               
               
                   
               
               
                 KDLTQAWAVAMALGVEDKVTVPLFEGVQKTQTIRSASDIRDVFINAGIKGEEYDAAWNSFVVKSLVAQQEKA 
               
               
                   
               
               
                 AADVQLRGVPAMFVNGKYQLNPQGMDTSNMDVFVQQYADTVKYLSEKK 
               
            
           
         
       
     
     DsbC Sequence Example 
     
       
         
           
               
               
            
               
                   E. coli  mature DsbC (DsbC without the signal sequence which will be 
                   
               
               
                 cytoplasmically expressed) 
               
               
                 Residues 21-236 of P0AEG6 with an initiating Met added 
               
               
                 (SEQ ID NO: 17) 
                   
               
               
                 (M)DDAAIQQTLAKMGIKSSDIQPAPVAGMKTVLTNSGVLYITDDGKHIIQGPMYDVSGTAPVNVTNKMLLK 
                   
               
               
                   
               
               
                 QLNALEKEMIVYKAPQEKHVITVFTDITCGYCHKLHEQMADYNALGITVRYLAFPRQGLDSDAEKEMKAIWC 
               
               
                   
               
               
                 AKDKNKAFDDVMAGKSVAPASCDVDIADHYALGVQLGVSGTPAVVLSNGTLVPGYQPPKEMKEFLDEHQKMT 
               
               
                   
               
               
                 SGK 
               
            
           
         
       
     
     PDI Sequence Example 
     
       
         
           
               
               
            
               
                 Human mature PDI (PDI without the signal sequence which will be 
                   
               
               
                 cytoplasmically expressed) 
               
               
                 Residues 18-508 of P07237 with an initiating Met added 
               
               
                 (SEQ ID NO: 18) 
                   
               
               
                 (M)DAPEEEDHVLVLRKSNFAEALAAHKYLLVEFYAPWCGHCKALAPEYAKAAGKLKAEGSEIRLAKVDATE 
                   
               
               
                   
               
               
                 ESDLAQQYGVRGYPTIKFFRNGDTASPKEYTAGREADDIVNWLKKRTGPAATTLPDGAAAESLVESSEVAVI 
               
               
                   
               
               
                 GFFKDVESDSAKQFLQAAEAIDDIPFGITSNSDVFSKYQLDKDGVVLFKKFDEGRNNFEGEVTKENLLDFIK 
               
               
                   
               
               
                 HNQLPLVIEFTEQTAPKIFGGEIKTHILLFLPKSVSDYDGKLSNFKTAAESFKGKILFIFIDSDHTDNQRIL 
               
               
                   
               
               
                 EFFGLKKEECPAVRLITLEEEMTKYKPESEELTAERITEFCHRFLEGKIKPHLMSQELPEDWDKQPVKVLVG 
               
               
                   
               
               
                 KNFEDVAFDEKKNVFVEFYAPWCGHCKQLAPIWDKLGETYKDHENIVIAKMDSTANEVEAVKVHSFPTLKFF 
               
               
                   
               
               
                 PASADRTVIDYNGERTLDGFKKFLESGGQDGAGDDDDLEDLEEAEEPDMEEDDDQKAVKDEL 
               
            
           
         
       
     
     Within the scope of the present invention are also nucleic acid sequences encoding the proteins comprising any of the amino acid sequences of the above gene bank accession numbers or the sequences SEQ ID NO:1 to SEQ ID NO:7 having DsbB activity, or SEQ ID NO: 8 to 15 having VKOR activity or SEQ ID NO: 16 having DsbA activity or SEQ ID NO: 17 having DsbC activity or SEQ ID NO: 18 having PDI activity or nucleic acid sequences encoding a fragment of said sequences or a modified version of said sequences, which sequences still have DsbB or VKOR, or DsbA, or DsbC or PDI activity. Suitable nucleic acid sequences encoding proteins having DsbB or VKOR or DsbA or DsbC or PDI activity are publicly available and can be found in gene banks. 
     A fragment, modified version (or variant) of an enzyme having DsbB or VKOR activity comprises typically an amino acid sequence having at least 25%, preferably at least 40%, more preferably at least 50%, still more preferably at least 60%, still and still more preferably at least 70%, more and more preferably at least 90% identity, most preferably at least 95% or at least 98% identity to any of the above mentioned amino acid sequences SEQ ID NO: 1 to 7 or 8 to 15, respectively. 
     By the term “identity” is here meant the identity between two amino acid sequences compared to each other from the first amino acid encoded by the corresponding gene to the last amino acid. Preferably the identity is measured by comparing the amino acid sequences without the sequences of any signal peptide the protein may have. The identity of the full-length sequences may be measured for example by using sequence analysis software, for example BLAST software available from the National Library of Medicine. 
     The transmembrane enzymes DsbB and VKOR are central to oxidative protein folding in the periplasm of prokaryotes. As described herein the present invention was exemplified by engineering DsbB mutants with inverted membrane topology that catalyze disulfide bond formation in the cytoplasm of  Escherichia coli . In addition, two naturally occurring and oppositely oriented VKOR homologues from the hyperthermophile  Aeropyrum pernix  were identified that promote oxidative protein folding in the periplasm or cytoplasm respectively. 
     PhoA used in this disclosure as an example is a protein with two sequential disulfide bonds. For proteins with an increased number of sequential disulfide bonds or with non-sequential disulfide bonds, the formation of the native disulfide state requires the action of a thiol-disulfide isomerase, for example DsbC or PDI. Hence for these more complicated proteins cytoplasmically targeted DsbC (cDsbC) or cytoplasmically targeted protein disulfide isomerase (cPDI) is required.  E. coli  sequences for DsbC (EC 5.3.4.1) can be found for example as Genbank accession numbers AAA83074 or AAC75931 and sequences for PDI (EC 5.3.4.1) comprise for example human PDI or yeast Pdi1p sequences. Yeast Pdi1p sequences can be found for example as Genbank accession numbers CAA42373 or BAA00723 or CAA38402. Human PDI family members are for example PDI Genbank accession numbers CAA28775 or AAC13652. 
     cDsbA is needed for iDsbB or VKOR or iVKOR to work properly and cytoplasmic DsbC (cDsbC) is needed to sequentially catalyze isomerisation for certain substrate proteins. Both cDsbA and cDsbC as well as cPDI are made cytoplasmic by expressing them without their N-terminal signal sequences. 
     Sequences for DsbA can be found for example as Genbank accession numbers CAA56736 or AAA23715. 
     Tissue plasminogen activator (tPA) is a protease that converts plasminogen to plasmin, the major enzyme involved in the breakdown of blood clots. It is used medically to treat pulmonary embolism, myocardial infarction and stroke. It is a large protein that contains 16 non-sequential and one sequential disulfide bond and in addition has one free thiol group in the native structure. It is available under the names Activase and Retavase. Retavase is a fragment of tPA which only contains the kringle 2 and protease domains. A similar fragment of tPA is known as vtPA and has been by academic researchers to study disulfide bond formation. Both Retavase and vtPA contain only 9 disulfide bonds. 
     iDsbB was constructed by mutagenesis from DsbB. DsbB was truncated from both N and C terminus to exclude the topogenic sequences to give tDB (R14-F168). Then H 0 (N in ) of MalF was fused to the N terminus of tDB. 
     H 0 (N in ) of MalF is the first transmembrane helix and part of the subsequent periplasmic loop of MalF (M1-G38). 
     Two additional residues (K68 and R72) located in the cytoplasmic loop of DsbB were mutated to N. 
     Alkaline phosphatase (pPhoA, where p indicates periplasmic) has two disulfide bonds that are essential for its phosphatase activity Hence, endogenous pPhoA exhibits low activity (ΔA 410 =1.8 mAU/min) when expressed in a ΔdsbB strain, with a circa 15-fold increase in active pPhoA (ΔA 410 =27 mAU/min) upon co-expression of wild-type DsbB (see tables 1 and 2 for strains and plasmids used in the examples). Similarly, expression of alkaline phosphatase devoid of its periplasmic targeting sequence (denoted cPhoA for cytoplasmic PhoA), does not result in the production of active protein in the reducing environment of the cytoplasm, unless an active catalyst of de novo disulfide bond formation is present. The inventors reasoned that pPhoA and cPhoA can be used as reporters for periplasmically or cytoplasmically localized disulfide bond forming activity of DsbB or iDsbB respectively as long as DsbA or a corresponding protein being capable of providing electrons to DsbB or iDsbB was present in the appropriate compartment to mediate electron transfer. 
     In this disclosure the inversion of topology of DsbB was based on the reduction of the number of lysine and arginine residues that normally reside in the cytoplasm ( FIG. 1 ). Wild-type DsbB has 9 (K+R) in the cytoplasm and 5 (K+R) on the same face as the active site in the periplasm (DsbB 9/5 ) as well as one R in the middle of transmembrane helix 3. Deletion of the N and C-terminus to truncate DsbB to give tDB (R14-F168) followed by fusion of H 0 (N in ) of MalF and mutation of K68 and R72 located in the cytoplasmic loop of DsbB to N, resulted in the formation of iDsbB. 
     Expression of cPhoA with or without co-expression of cDsbA from the T7 promoter of pET23 results in the formation of negligible amounts of active cPhoA in  E. coli  strain BL21 (DE3) pLysS ( FIG. 2A ). Similarly when iDsbB is pre-expressed from an arabinose promoter in a modified version of pLysS no active cPhoA is seen unless cDsbA is also expressed. Expression of both iDsbB and cDsbA results in a 100-fold increase in cPhoA activity ( FIG. 2A ). This represents the formation of the two disulfide bonds in PhoA which are essential for activity. iDsbB expression cannot be replaced by DsbB expression and the C46 active site cysteine is required for iDsbB activity ( FIG. 2B ). 
     There are multiple possible ways to invert DsbB, all of which depend on altering the charge distribution of the protein. In addition to inversion, the topology of a transmembrane protein maybe partially inverted or mixed topology, resulting in activity on both sides of the membrane such that both active pPhoA and cPhoA can be produced ( FIG. 3 ). This can be observed by the systematic reduction of the number of lysine and arginine DsbB residues that normally reside in the cytoplasm. 
     Point mutations K68Q (DsbB 8/5 ) or K68Q/R72N (DsbB 7/5 ) removing one or two of the cytoplasmic charges, results in no change in cPhoA or pPhoA activity ( FIG. 4 ). In contrast, deletion of the C-terminus of DsbB (ΔK169-R178) combined with the point mutation R5N (DsbB 3/5a ) or K68Q (DsbB 3/5b ) results in the removal of 6 (K+R) residues from the side opposite to the active site and conferred low but significant cPhoA activity ( FIG. 4 ), corresponding with a partial topology inversion of these mutant DsbBs. Combining R5N/K68Q (DsbB 2/5 ) or making the R72N substitution in this background (DsbB 1/5 ) increased cPhoA activity substantially compared with wild-type DsbB, indicating major topology inversion towards the cytoplasm. The gain of cytoplasmic activity was concomitant with a decrease in periplasmic activity to around 46% of wild-type for DsbB 1/5 . Hence, DsbB 2/5  and DsbB 1/5  can simultaneously catalyse oxidative protein folding in two different cellular compartments indicating mixed or dual topology of these mutants. 
     In order to force DsbB towards a fully inverted topology either the first or the last TMH from  E. coli  maltose transporter MalF was fused to truncated mutated DsbB constructs. A C-terminal fusion of the terminal TMH from MalF, which has 3 (K+R) after the TMH, after F168 of ΔK169-R178 R5N K68Q DsbB (DsbBH 5   2/8 ) and the subsequent R72N mutation (DsbBH 5   1/8 ) resulted in the formation of active proteins, but had a relatively small effect on shifting the topology towards inversion compared with DsbB 2/5  or DsbB 1/5  ( FIG. 4 ). In contrast fusion of the first TMH from MalF, which has 4 (K+R) prior to the TMH, N-terminally to ΔM1-G13 ΔK169-R178 DsbB to give H 0 DsbB 3/9  resulted in a protein that displayed low cPhoA and pPhoA activity. However, a single additional mutation, K68Q in L3 (H 0 DsbB 2/9 ) leads to a dramatic increase in cytoplasmic activity ( FIG. 4 ), suggesting topological instability of H 0 DsbB 3/9 . A further point mutation, R72N giving H 0 DsbB 1/9a  showed no detectable pPhoA activity above the vector negative control, but very high cPhoA activity ( FIG. 4 ). A similarly charged species with K68N instead of K68Q (H 0 DsbB 1/9b ) also showed minimal pPhoA activity, but had increased cPhoA activity, presumably linked to changes in structure. In addition, H 0 DsbB 1/9b  did not restore DsbB-dependent motility ( FIG. 5 ), suggesting a complete relocalization of its disulfide forming activity from the periplasm to the cytoplasm i.e. full inversion of topology from DsbB to iDsbB. As a control for non-specific effects, subcellular fractionation of  E. coli  expressing H 0 DsbB 1/9b , cDsbA and cPhoA showed that cPhoA is retained mostly in the cytoplasm and that the subcellular localization of internal markers such as β-galactosidase, β-lactamase was not affected (Table 3). 
     iDsbB shows here the same dependence on DsbA as an intermediary as the wild-type protein does ( FIG. 2 ). To examine whether the inverted DsbB proteins also use quinone one in  E. coli  strain AN384 (ΔubiA420 ΔmenA401) was expressed and tested for cPhoA activity. Consistent with the results for wild-type DsbB H 0 DsbB 1/9a  failed to catalyse cytoplasmic disulfide bond formation in AN384 ( FIG. 6 ), indicating that quinone dependence is retained with DsbB inversion. Hence inversion occurs with conservation of activity and specificity. 
     Disulfide bond formation in bacteria and archaea is catalysed either by DsbB or VKOR. As per DsbB, VKOR family members have multiple transmembrane helices that form a helix-bundle. That DsbB showed topological plasticity and that cytoplasmic disulfide bonds are readily formed once catalysts for their formation are co-localized, suggested that the occurrence of naturally occurring topologically inverted catalysts of disulfide bond formation is evolutionarily plausible. Within the DsbB and VKOR families was systematically searched for members that are predicted to have their catalytic residues in the cytosol. Using global membrane protein topology prediction algorithms (SCAMPI), we examined 1042 DsbB and 243 VKOR bacterial and archaeal homologues we searched from Pfam. While no naturally occurring DsbB members with inverted topology were found, 5 species containing VKOR homologues that are predicted to have cytoplasmic localization of their catalytic sites were identified, all of which belong to the hyperthermophilic crenarchaeon phylum (Table 5). Each of these had two VKOR homologues in their genome, of similar size and number of TMHs ( FIG. 7 ), which are predicted to possess opposite topologies To test the topology prediction, apVKOR1 and apVKOR2 were expressed from the hyperthermophile  Aeropyrum pernix  in  E. coli  and tested for disulfide bond formation in the cytoplasm and the periplasm. Expression of apVKOR1, with predicted normal VKOR topology and the active sites in the periplasm, induced disulfide bond formation only in pPhoA, whereas apVKOR2, with predicted iVKOR topology, induced disulfide bond formation only in cPhoA ( FIG. 8 ), with iVKOR activity being dependent on co-expression of cDsbA ( FIG. 9 ). This indicates that both proteins can catalyse disulfide bond formation and that they have opposite membrane orientation of their active sites. The molecular mechanism by which the two VKOR family members from  Aeropyrum pernix  evolved to attain two distinctly localized disulfide bond formation activities appears to be a gene duplication event followed by the redistribution of positively charged amino acids across the sequence of one copy by loop deletion and mutational substitution ( FIG. 7 ). This is directly analogous to the in vitro methodology used to create iDsbB. 
     To confirm that the system is not specific for the formation of disulfide bonds in prokaryotic proteins we examined disulfide bond formation in human vtPA. vtPA is a fragment of tissue plasminogen activator which contains 9 disulfide bonds. The activity of the protein is dependent on the formation of native disulfide bonds and requires the presence of an disulfide isomerase as well as catalyst of disulfide bond formation. When we expressed vtPA, tagged to maltose binding protein to increase the solubility of folding intermediates, with cDsbA and cDsbC in the cytoplasm of the  E. coli  strain origami, low activity was obtained. This vtPA activity did not increase when apVKOR1 from  Aeropyrum pernix  was pre-expressed, consistent with its topology having the active site cysteines in the periplasm. However, when apVKOR2 (a naturally inverted VKOR) or iDsbB were pre-expressed a circa 4-5 fold increase in vtPA activity was observed ( FIG. 10 ). 
     The present disclosure presents the first time that the enzymatic activity, including specificity, of a membrane protein can be totally inverted across the membrane. 
     The following non-limiting examples illustrate the invention. 
     EXAMPLES 
     Example 1 
     Materials and Methods 
     Strains: 
     The  E. coli  strains used in this study are shown in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   E. coli  strains 
               
            
           
           
               
               
               
            
               
                 Strain 
                 Relevant genotype 
                 Source 
               
               
                   
               
               
                 BL21 (DE3) 
                 ompT, λ(DE3) 
                 Novagen 
               
               
                 BW25113 
                 Parental strain of Keio collection 
                 Keio collection 
               
               
                 JW5182 
                 BW25113 dsbB::KAN 
                 Keio collection 
               
               
                 FSH8 
                 JW5182 ara+ 
                 This study 
               
               
                 AN384 
                 ubiA420 menA401 
                 (Wallace et al. 1977) 
               
               
                   
               
            
           
         
       
     
     Standard P1 transduction was used to construct dsbB:KAN ara+ with selection on M65 salts with 0.2% arabinose as the sole carbon source. 
     Vectors Constructions: 
     A list of all of the plasmid vectors used in this study is shown in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Plasmid vectors used in this study. Pictorial representations of the 
               
               
                 transmembrane proteins are shown in each case with the active site face upper- 
               
               
                 most and positively charged residues (K + R) indicated by black circles. 
               
            
           
           
               
               
               
               
               
            
               
                 Plasmid 
                 Vector 
                 Encodes 
                 Source 
                 Representation 
               
               
                   
               
               
                 pFH313 
                 pMALc2x 
                 Spel site added upstream 
                 New En- 
                   
               
               
                   
                   
                 of rbs 
                 gland Bio- 
                   
               
               
                   
                   
                   
                 labs 
                   
               
               
                 pFH368 
                 pFH313 
                 MH 6 M-cPhoA (R22-K271) 
                 This study 
                   
               
               
                 pFH314 
                 pFH313 
                 MH 6 M -cPhoA (R22- 
                 This study 
                   
               
               
                   
                   
                 K271) and MH 6 M -cDsbA 
                   
                   
               
               
                   
                   
                 (A20-K208) 
                   
                   
               
               
                 pVD80 
                 pET23a 
                 MH 6 M -cPhoA (R22- 
                 (Nguyen et 
                   
               
               
                   
                   
                 K271) 
                 al, 2011) 
                   
               
               
                 pFH258 
                 pET23a 
                 MH 6 M -cPhoA (R22- 
                 This study 
                   
               
               
                   
                   
                 K271) and MH 6 M -cDsbA 
                   
                   
               
               
                   
                   
                 (A20-K208) 
                   
                   
               
               
                 pFH273 
                 pLysSBAD 
                 M-cPhoA (R22-K271) 
                 This study 
                   
               
               
                   
                   
                 and 
                   
                   
               
               
                   
                   
                 MH 6 M -cDsbA (A20- 
                   
                   
               
               
                   
                   
                 K208) 
                   
                   
               
               
                 pFH257 
                 pLysSBAD 
                 Erv1p (Met1-Glu189) 
                 (Nguyen et 
                   
               
               
                   
                   
                   
                 al, 2011) 
                   
               
               
                 pFH277 
                 pLysSBAD 
                 Nothing 
                 This study 
                   
               
               
                   
               
               
                 pFH371 
                 pLysSBAD 
                 apVKOR1 (Met1-Thr145) 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH372 
                 pLysSBAD 
                 apVKOR2 (Met1-Gly147) 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH186 
                 pET23 
                 DsbB 9/5 ; DsbB (M1-R178) 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH315 
                 pLysSBAD 
                 DsbB 9/5 ; DsbB (M1-R178) 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH316 
                 pLysSBAD 
                 DsbB 8/5 ; DsbB (M1-R178) K68Q 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH318 
                 pLysSBAD 
                 DsbB 7/5 ; DsbB (M1-R178) K68Q, R72N 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH323 
                 pLysSBAD 
                 DsbB 4/5 ; DsbB (M1-F168) 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH327 
                 pLysSBAD 
                 DsbB 3/5a ; DsbB (M1- F168) R5N 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH324 
                 pLysSBAD 
                 DsbB 3/5b ; DsbB (M1- F168) K68Q 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH328 
                 pLysSBAD 
                 DsbB 2/5 ; DsbB (M1-F168) R5N, K68Q 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH330 
                 pLysSBAD 
                 DsbB 1/5 ; DsbB (M1-F168) R5N, K68Q, R72N 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH357 
                 pLysSBAD 
                 DsbBH 5   2/8 ; DsbB (M1- F168) R5N, K68Q + GS + MalF (G478-D515) 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH359 
                 pLysSBAD 
                 DsbBH 5   1/8 ; DsbB (M1- F168) R5N, K68Q, R72N + GS + MalF (G478-D515) 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH343 
                 pLysSBAD 
                 H 0 DsbBH 3/9 ; MalF (M1- G38) + PW + DsbB (R14- F168) 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH344 
                 pLysSBAD 
                 H 0 DsbBH 2/9 ; MalF (M1- G38) + PW + DsbB (R14- F168) K68Q 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH347 
                 pLysSBAD 
                 H 0 DsbBH 1/9a ; MalF (M1- G38) + PW + DsbB (R14- F168) K68Q, R72N 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH272 
                 pLysSBAD 
                 H 0 DsbBH 1/9b ; MalF (M1- G38) + PW + DsbB (R14- F168) K68N, R72N 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH265 
                 pET23 
                 MH 6 -H 0 DsbB 1/9b ; MalF (M1-G38) + PW + DsbB (R14-F168) K68N, R72N 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 pFH269 
                 pET23 
                 MH 6 -H 0 DsbB 1/9b ; MalF (M1-G38) + PW + DsbB (R14-F168) C46A, K68N, R72N 
                 This study 
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
         
       
     
     For expression in BL21(DE3), cDsbA (A20-K208), with an N-terminal MHHHHHHM-tag (tag sequence SEQ ID NO:19) was cloned (5′XbaI/3′XhoI) into pVD80 (Nguyen et al. 2011) a pET23a derived plasmid which encodes for cPhoA (R22-K271, with an N-terminal MHHHHHHM-tag) (5′SpeI/3′ XhoI) to give a bicistronic plasmid pFH258 with expression under a T7 promoter. 
     Expression in other  E. coli  strains under a lac promoter/repressor was based on using a modified version of pMALc2x (New England Biolabs). First, a SpeI site was introduced upstream of the start of MBP gene to give pFH313. The bicistronic DNA fragment coding for cPhoA and cDsbA was cloned (5′ XbaI/3′ XhoI) from pFH258 into pFH313 (5′SpeI/3′ SalI). This replaces the original ribosome binding site and MBP gene of pFH313 with cPhoA and cDsbA each preceded by a ribosome binding site carried along from pFH258. 
     For expression of DsbB and VKOR variants we wanted a low copy plasmid that is compatible for cotransformation with pFH258 or pFH314. We used pLysS (Promega) as the backbone and cloned into it pBAD/araC as described (Nguyen et al. 2011). The gene encoding DsbB was amplified by colony PCR from the  E. coli  strain BL21 and cloned into this vector (5′NdeI/3′XhoI) and into pET23. The longer DsbB variant in the database (AAA23711.1), encoding a 178 amino acid protein was chosen as the template. The shorter variant is 176 amino acids in length and lacks the first two amino acids Met-Ile. Lysine and arginine residues in cytoplasmic loops were mutated individually (R5N, R14N, K68N, K68Q and R72N) or in combination along with the active site cysteine (C46A) using the QuikChange site-directed mutagenesis kit (Stratagene) according to the manufacturers&#39; instructions. The first and last TMH of  E. coli  MalF (M1-G38) and (G478-D515) were amplified by PCR and cloned in-frame either 5′BglII/3′NcoI or 5′BamHI/3′XhoI into vectors harboring DsbB variants. This adds the two amino acids PW or GS before the start or after the end of DsbB variant, respectively. apVKOR1 and apVKOR2 were amplified by PCR from the genomic DNA of  Aeropyrum Pernix  (DSM 11879, German Collection of Microorganisms and Cell Cultures) and cloned into the pLysSBAD vector (5′NdeI/3′XhoI). All plasmids were sequenced to ensure no PCR or cloning errors prior to use. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Subcellular fractionation of BL21(DE3)  E. coli  strains 
               
               
                 expressing H 0 DsbB 1/9b . Absorbance of hydrolyzed ONPG (0.4 
               
               
                 mg/ml), Nitrocefin (0.1 mg/ml) and PNPP (1 mg/ml) was measured 
               
               
                 over time at 420, 486 and 410 nm respectively. 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 cPhoA + cDsbA 
                 cPhoA 
               
               
                   
                   
                 co-expression 
                 co-expression 
               
               
                 Marker 
                 Fraction 
                 (mAU/min) 
                 (mAU/min) 
               
               
                   
               
               
                 β-galactosidase 
                 Total 
                 9.43 ± 0.01 
                 6.29 ± 0.32 
               
               
                 β-galactosidase 
                 Cytoplasmic 
                 9.88 ± 1.02 
                 6.11 ± 0.50 
               
               
                 β-galactosidase 
                 Periplasmic 
                 0.00 ± 0.00 
                 0.00 ± 0.00 
               
               
                 β-lactamase 
                 Total 
                 35.1 ± 0.8  
                 34.9 ± 1.4  
               
               
                 β-lactamase 
                 Cytoplasmic 
                 0.92 ± 0.08 
                 1.04 ± 0.02 
               
               
                 β-lactamase 
                 Periplasmic 
                 29.6 ± 1.6  
                 30.6 ± 1.1  
               
               
                 cPhoA 
                 Total 
                 26.2 ± 3.9  
                 1.77 ± 0.03 
               
               
                 cPhoA 
                 Cytoplasmic 
                 20.3 ± 3.8  
                 0.24 ± 0.01 
               
               
                 cPhoA 
                 Periplasmic 
                 0.84 ± 0.22 
                 0.59 ± 0.17 
               
               
                   
               
               
                 Data represents mean ± sd ( n = 2). 
               
            
           
         
       
     
     Motility Assay: 
     A single colony from JW5182 strain carrying DsbB constructs was stabbed into a motility plate (M63 salts, 1 mM MgSO 4 , 50 μg/ml thiamine, 0.4% glycerol, 0.1% N—Z-Case plus (Sigma), 0.3% (w/v) agar, 0.0001% arabinose and appropriate antibiotics. Motility halos were examined after 48 hours growth at 30° C. 
     Alkaline Phosphatase Activity: 
     For endogenous pPhoA cells transformed with pLysSBAD harboring different mutants of DsbB were grown in 5 ml fresh MOPS media (MOPS salts (Sambrook and Russell, 2001), 50 μg/ml thiamine, 1 mM MgSO 4 , 0.5% glycerol, 0.2% N—Z-Case plus (Sigma), 0.0001% arabinose, 35 μg/ml chloramphenicol) without addition of phosphate in order to induce chromosomal expression of PhoA. After 14 hours of growth cells were collected by centrifugation and resuspended in lysis buffer (50 mM Tris pH 8.0, 50 mM NEM, 20 μg/ml DNase and 0.1 mg/ml egg white lysozyme) to give a suspension equivalent to OD 600  of 1 based on the final OD 600  of the culture and then frozen. 
     For exogenous cPhoA activity in the  E. coli  strain FSH8 which is deficient in DsbB, 5 ml pre-cultures, containing suitable antibiotics, were used to seed a 20 ml culture of LB in a 125 ml flask to an OD 600  of 0.05. This was then grown at 37° C., 200 rpm until the OD 600  reached 0.4 when expression from pLysSBAD was induced with 0.5% arabinose and 0.08% glucose. 30 minutes later, expression from the pMaI plasmid was induced with 1 mM IPTG and cells harvested after 3 hours. The rationale behind including 0.08% glucose is that expression of the transmembrane protein DsbB can be toxic causing growth cessation (Badwer et al., 1998). The addition of glucose brings on catabolite repression of the pBAD promoter (Miyada et al., 1984) and thereby reduces expression of DsbB. The combination of arabinose and glucose at specific ratios proved to be very useful to regulate expression of DsbB from the pBAD promoter without causing growth defects (Table 4). Harvested cells were resuspended in lysis buffer as above and frozen. A similar protocol was used for exogenous cPhoA activity in the  E. coli  strain BL21(DE3) except that the DsbB constructs were induced first with either 10 μM IPTG (pET23) or 0.5% arabinose (pLysSBAD), with cPhoA and cDsbA being induced 30 minutes later with 1 mM IPTG (pET23) or 0.5% arabinose (pLysSBAD). 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Modulation of expression from pBAD promoter by catabolite repression 
               
               
                 using glucose.  E. coli  strain FSH8 harbouring plasmids pFH347 
               
               
                 (H 0 DsbB 1/9a ) and pFH314 (cPhoA + cDsbA) was induced with arabinose 
               
               
                 and glucose at OD 600  = 0.4 and 30 mintues later with 1 mM IPTG. 
               
               
                 cPhoA activity was measured 3 hours after induction. 
               
            
           
           
               
               
               
            
               
                   
                 cPhoA activity 
                 Final 
               
               
                 Inducer 
                 (mAU/min) 
                 OD 600   
               
               
                   
               
               
                 0.5% arabinose/0.08% glucose 
                 62.0 ± 1.7 
                 2.95 ± 0.06 
               
               
                 0.4% arabinose/0.05% glucose 
                 54.4 ± 2.7 
                 2.98 ± 0.01 
               
               
                 0.01% arabinose/0.08% glucose 
                 29.3 ± 1.7 
                 3.75 ± 0.03 
               
               
                 0.01% arabinose 
                 3.30 ± 1.2 
                 0.60 ± 0.01 
               
               
                 0.5% arabinose 
                  2.57 ± 0.22 
                 0.67 ± 0.05 
               
               
                   
               
               
                 Data represents mean ± sd (n = 2). 
               
            
           
         
       
     
     Alkaline phosphatase activity for cPhoA and pPhoA samples was measured in a continuous assay by monitoring ΔA 410  upon the hydrolysis 4-nitrophenyl phosphate (0.1% w/v in 1 M Tris pH 8.0) with a plate reader using 5 μl of cleared lysate and 195 μl of substrate in a 96 well plate at 25° C. 
     Cell Fractionation: 
     For cell fractionation studies none of the ΔdsbB strains could be used as they all lack β-galactosidase. Instead BL21(DE3) cells, which are lacZ + , harboring the plasmid pFH272 encoding H 0 DsbB 119  with either pFH258 (encoding cPhoA and cDsbA) or pVD80 (encoding cPhoA) were grown in LB as above except that cells were collected 2 h post IPTG induction. 1 ml of culture was spun down and the cell pellet resuspended in 0.5 ml of 20% sucrose buffer containing 33 mM Tris pH 8.0, 1 mM EDTA and 50 mM NEM and left at RT for 10 min. The cells were then spun down at 4000 g for 5 min at +4° C., the supernatant discarded and the cells resuspended in ice-cold 5 mM MgSO 4  to give an OD 600  equivalent of 5 and left on ice for 20 minutes. A 100 μl sample was removed and marked as total, and another 100 μl was spun down at 10000 g for 10 min and the periplasmic content removed to a new tube. The pellet was resuspended to the original volume with 5 mM MgSO 4 . 20 μg/ml DNase, 0.1 mg/ml lysozyme and 50 mM Tris pH:8.0 were added to all fractions and then freeze-thawed twice. Subsequently activities of β-galactosidase (Miller, 1992), β-lactamase (O&#39;Callaghan et al. 1972), and alkaline phosphatase (as above) were determined. 
     Bioinformatics Phylogenic Analyses 
     Accession numbers of PF02600 and PF07884 family members were collected from Pfam and used to retrieve full sequences from Uniprot. Sequences that appeared to lack catalytic cysteines were excluded and the remaining sequences were run on SCAMPI to predict transmembrane regions and topology and selected sequences were further analysed by TOPCONS. We further examined all VKOR homologues from thermophiles by BLASTing apVKOR2 within Archaea. Blast results were aligned using ClustalW2. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Species containing VKOR homologues that are predicted 
               
               
                 to have cytoplasmic localization of their catalytic 
               
               
                 sites. Each of the hyperthermophiles are predicted to 
               
               
                 have two VKOR homologues with opposite topologies. 
               
            
           
           
               
               
               
               
            
               
                   
                   
                 Accession 
                 Predicted topology 
               
               
                   
                 Organism 
                 number 
                 of active site 
               
               
                   
                   
               
               
                   
                 
                   Aeropyrum pernix 
                 
                 Q9Y922 
                 Periplasmic 
               
               
                   
                 
                   Aeropyrum pernix 
                 
                 Q9YB70 
                 Cytoplasmic 
               
               
                   
                 
                   Pyrobaculum aerophilum 
                 
                 Q8ZV09 
                 Periplasmic 
               
               
                   
                 
                   Pyrobaculum aerophilum 
                 
                 Q8ZXF9 
                 Cytoplasmic 
               
               
                   
                 
                   Pyrobaculum arsenaticum 
                 
                 A4WKP9 
                 Periplasmic 
               
               
                   
                 
                   Pyrobaculum arsenaticum 
                 
                 A4WI60 
                 Cytoplasmic 
               
               
                   
                 
                   Pyrobaculum calidifontis 
                 
                 A3MSV5 
                 Periplasmic 
               
               
                   
                 
                   Pyrobaculum calidifontis 
                 
                 A3MVJ3 
                 Cytoplasmic 
               
               
                   
                 
                   Pyrobaculum islandicum 
                 
                 A1RVU0 
                 Periplasmic 
               
               
                   
                 
                   Pyrobaculum islandicum 
                 
                 A1RVC9 
                 Cytoplasmic 
               
               
                   
                   
               
            
           
         
       
     
     Example 2 
     A list of all of the plasmid vectors used in this study is shown in Table 6. 
     
       
         
           
               
             
               
                 TABLE 6  
               
             
            
               
                   
               
               
                 Plasmid vectors used in this study 
               
            
           
           
               
               
               
               
            
               
                 Plasmid 
                 Vector 
                 Encodes 
                 Source 
               
               
                   
               
               
                 pFH277 
                 pLysSBAD 
                 Nothing 
                 This study 
               
               
                   
               
               
                 pFH371 
                 pLysSBAD 
                 apVKOR1 (Met1-Thr145) 
                 This study 
               
               
                   
               
               
                 pFH372 
                 pLysSBAD 
                 apVKOR2 (Met1-Gly147) 
                 This study 
               
               
                   
               
               
                 pFH272 
                 pLysSBAD 
                 H 0 DsbB 1/9b ; MalF (M1-G38) + PW + 
                 This study 
               
               
                   
                   
                 DsbB (R14-F168) K68N, R72N 
                   
               
               
                   
               
               
                 pVD171 
                 pET23 
                 Mature  E. coli  MBP (Lys27-Thr392) 
                 (Nguyen et 
               
               
                   
                   
                 plus a linker 
                 al, 2011) 
               
               
                   
                   
                 GSGSGSGSGSIEGRGSGSGSGSGSHM- 
                   
               
               
                   
                   
                 vtPA (Gly211-Pro562) 
                   
               
               
                   
                   
                 (linker SEQ ID NO: 20) 
                   
               
               
                   
               
               
                 pMJS41 
                 pET23 
                 Mature  E. coli  MBP (Lys27-Thr392) 
                 This study 
               
               
                   
                   
                 plus a linker 
                   
               
               
                   
                   
                 GSGSGSGSGSIEGRGSGSGSGSGSHM- 
                   
               
               
                   
                   
                 vtPA (Gly211-Pro562) and cDsbC 
                   
               
               
                   
                   
                 (D21-K236) and cDsbA (A20-K208) 
                   
               
               
                   
                   
                 (linker SEQ ID NO: 21) 
               
               
                   
               
            
           
         
       
     
     The polycistronic vectors pMJS41 was constructed by taking a fragment encoding cDsbA from a pET23 based constructs by XbaI/XhoI digest (which includes the ribosome binding site) and ligating the fragment into SpeI/XhoI cut pVD171. This generates a plasmid that contains a single transcription initiator/terminator and hence makes a single mRNA, but has three ribosome binding sites and makes three proteins by co-expression from three translation initiation sites. 
     For expression in LB media,  E. coli  strains containing expression vectors were streaked out from glycerol stocks stored at −70° C. onto LB agar plates containing suitable antibiotics to allow for selection (100 μg/ml ampicillin for pET23 derivatives, 35 μg/ml chloramphenicol for pLysS derivatives; with 10 μg/ml tetracycline and 15 μg/ml kanamycin for selection of origami strain). The next day one colony from these plates were used to inoculate 5 ml of LB media, containing suitable antibiotics, and grown overnight at 30° C., 200 rpm. This overnight culture was used to seed a 50 ml culture of LB containing suitable antibiotics in a 250 ml conical flask to an optical density of 0.05 at 600 nm (OD 600 ). This culture was grown at 30° C., 200 rpm until the OD 600  reached 0.4 at which point production of protein from pLysSBAD vectors was induced by the addition of 0.5% w/v arabinose followed 30 minutes later by the induction of proteins from pET23 vectors with 0.5 mM IPTG. The cells were then grown for a total of 4 hours post induction at 30° C., 200 rpm and the final OD 600  measured. The cells were collected by centrifugation and resuspended to an OD 600  equivalent of 10 (based on the final OD 600  of the culture) in 20 mM sodium phosphate pH 7.4, 20 μg/ml DNase, 0.1 mg/ml egg white lysozyme and frozen. Such normalization allows for easy correction for differences in the growth rates of the cultures and allows rapid equal total protein loading of samples for activity assay. Cells were lysed by freeze-thawing. The insoluble fraction was removed by centrifugation and the soluble fraction removed quickly to a new container. 
     vtPA activity was measured using a chromagenic substrate, chromozyme t-PA peptide assay (Roche), using a methodology similar that that recommended by the manufacturer but adapted for a continuous assay in 96-well plate format. Since this method showed slight variations in activity with time all of the vtPA activity measurements were repeated using the same batch of substrate and same buffers and that data is presented here. 20 mg of substrate was dissolved in 4 ml of sterilized water to generate a 20× substrate stock solution. 20 μl of soluble fraction from cell lysates were added to 180 μl of substrate diluted in reaction buffer (100 mM tris-HCl, 0.15% tween20; pH 8.5) to give a final concentration that is 1× in a 96 well microtitre plate, thermally equilibrated to 37° C. The absorbance at 405 nm was measured at 3 minute intervals for 30 minutes to determine the rate of formation of the product. All samples were measured in duplicate. 
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