Engineering stable cytoplasmic intrabodies with designed specificity

Many attempts have been made to develop antibody fragments that can be expressed in the cytoplasm ("intrabodies") in a stable and functional form. The recombinant antibody fragment scFv(F8) is characterised by peculiarly high in vitro stability and functional folding in both prokaryotic and eukaryotic cytoplasm. To dissect the relative contribution of different scFv(F8) regions to cytoplasmic stability and specificity we designed and constructed five chimeric molecules (scFv-P1 to P5) in which several groups of residues important for antigen binding in the poorly stable anti-hen egg lysozyme (HEL) scFv(D1.3) were progressively grafted onto the scFv(F8) scaffold. All five chimeric scFvs were expressed in a soluble form in the periplasm and cytoplasm of Escherichia coli. All the periplasmic oxidised forms and the scFv(P3) extracted from the cytoplasm in reducing conditions had HEL binding affinities essentially identical (K(d)=15nM) to that of the cognate scFv(D1.3) fragment (K(d)=16nM). The successful grafting of the antigen binding properties of D1.3 onto the scFv(F8) opens the road to the exploitation of this molecule as a scaffold for the reshaping of intrabodies with desired specificities to be targeted to the cytoplasm.     

Efficient isolation of soluble intracellular single-chain antibodies using the twin-arginine translocation machinery

One of the most commonly used recombinant antibody formats is the single-chain variable fragment (scFv) that consists of the antibody variable heavy chain connected to the variable light chain by a flexible linker. Since disulfide bonds are often necessary for scFv folding, it can be challenging to express scFvs in the reducing environment of the cytosol. Thus, we sought to develop a method for antigen-independent selection of scFvs that are stable in the reducing cytosol of bacteria. To this end, we applied a recently developed genetic selection for protein folding and solubility based on the quality control feature of the Escherichia coli twin-arginine translocation (Tat) pathway. This selection employs a tripartite sandwich fusion of a protein-of-interest with an N-terminal Tat-specific signal peptide and C-terminal TEM1 β-lactamase, thereby coupling antibiotic resistance with Tat pathway export. Here, we adapted this assay to develop intrabody selection after Tat export (ISELATE), a high-throughput selection strategy for the identification of solubility-enhanced scFv sequences. Using ISELATE for three rounds of laboratory evolution, it was possible to evolve a soluble scFv from an insoluble parental sequence. We show also that ISELATE enables focusing of an scFv library in soluble sequence space before functional screening and thus can be used to increase the likelihood of finding functional intrabodies. Finally, the technique was used to screen a large repertoire of naïve scFvs for clones that conferred significant levels of soluble accumulation. Our results reveal that the Tat quality control mechanism can be harnessed for molecular evolution of scFvs that are soluble in the reducing cytoplasm of E. coli.     

In vitro folding and thermodynamic stability of an antibody fragment selected in vivo for high expression levels in Escherichia coli cytoplasm.

We recently isolated a mutant of a human anti-beta-galactosidase single chain antibody fragment (scFv) able to fold at high levels in Escherichia coli cytoplasm. When targeted to the periplasm, this mutant and the wild-type scFv are both expressed at comparable levels in a soluble, active and oxidized form. If a reducing agent is added to the growth medium, only the mutant scFv is still able to fold, showing that in vivo aggregation is a direct consequence of the lack of disulphide bond formation and not of the cellular localization. In vitro denaturation/renaturation experiments show that the mutant protein is more stable than the wild-type scFv. Furthermore, refolding kinetics under reducing conditions show that the mutant folds faster than the wild-type protein. Aggregation does not proceed from the native or unfolded conformation of the protein, but from a species only present during the unfolding/refolding transition. In conclusion, the in vivo properties of the mutant scFv can be explained by, first, an increase in the stability of the protein in order to tolerate the removal of the two disulphide bonds and, second, a modification of its folding properties that reduces the kinetic competition between folding and aggregation of a reduced folding intermediate.     

Effects of variable domain orientation on anti-HER2 single-chain variable fragment antibody expressed in the Escherichia coli cytoplasm

Single-chain variable fragment (scFv) antibodies have great potential for a range of applications including as diagnostic and therapeutic agents. However, production of scFvs is challenging because proper folding and activity depend on the formation of two intrachain disulfide bonds that do not readily form in the cytoplasm of living cells. Functional expression in bacteria therefore involves targeting to the more oxidizing periplasm, but yields in this compartment can be limiting due to secretion bottlenecks and the relatively small volume compared to the cytoplasm. In the present study, we evaluated an anti-HER2 scFv, which is specific for human epidermal growth receptor 2 (HER2) overexpressed in breast cancer, for functional expression in the cytoplasm of Escherichia coli strains BL21(DE3) and SHuffle T7 Express, the latter of which is genetically engineered for cytoplasmic disulfide bond formation. Specifically, we observed much greater solubility and binding activity with SHuffle T7 Express cells, which likely resulted from the more oxidative cytoplasm in this strain background. We also found that SHuffle T7 Express cells were capable of supporting high-level soluble production of anti-HER2 scFvs with intact disulfide bonds independent of variable domain orientation, providing further evidence that SHuffle T7 Express is a promising host for laboratory and preparative expression of functional scFv antibodies.     

Structural basis of neutralization of the major toxic component from the scorpion Centruroides noxius Hoffmann by a human-derived single-chain antibody fragment

It has previously been reported that several single-chain antibody fragments of human origin (scFv) neutralize the effects of two different scorpion venoms through interactions with the primary toxins of Centruroides noxius Hoffmann (Cn2) and Centruroides suffusus suffusus (Css2). Here we present the crystal structure of the complex formed between one scFv (9004G) and the Cn2 toxin, determined in two crystal forms at 2.5 and 1.9 Å resolution. A 15-residue span of the toxin is recognized by the antibody through a cleft formed by residues from five of the complementarity-determining regions of the scFv. Analysis of the interface of the complex reveals three features. First, the epitope of toxin Cn2 overlaps with essential residues for the binding of β-toxins to its Na⁺ channel receptor site. Second, the putative recognition of Css2 involves mainly residues that are present in both Cn2 and Css2 toxins. Finally, the effect on the increase of affinity of previously reported key residues during the maturation process of different scFvs can be inferred from the structure. Taken together, these results provide the structural basis that explain the mechanism of the 9004G neutralizing activity and give insight into the process of directed evolution that gave rise to this family of neutralizing scFvs.     

X-ray and biochemical analysis of N370S mutant human acid β-glucosidase

Gaucher disease is caused by mutations in the enzyme acid β-glucosidase (GCase), the most common of which is the substitution of serine for asparagine at residue 370 (N370S). To characterize the nature of this mutation, we expressed human N370S GCase in insect cells and compared the x-ray structure and biochemical properties of the purified protein with that of the recombinant human GCase (imiglucerase, Cerezyme®). The x-ray structure of N370S mutant acid β-glucosidase at acidic and neutral pH values indicates that the overall folding of the N370S mutant is identical to that of recombinant GCase. Subtle differences were observed in the conformation of a flexible loop at the active site and in the hydrogen bonding ability of aromatic residues on this loop with residue 370 and the catalytic residues Glu-235 and Glu-340. Circular dichroism spectroscopy showed a pH-dependent change in the environment of tryptophan residues in imiglucerase that is absent in N370S GCase. The mutant protein was catalytically deficient with reduced V_max and increased K_m values for the substrate p-nitrophenyl-β-d-glucopyranoside and reduced sensitivity to competitive inhibitors. N370S GCase was more stable to thermal denaturation and had an increased lysosomal half-life compared with imiglucerase following uptake into macrophages. The competitive inhibitor N-(n-nonyl)deoxynojirimycin increased lysosomal levels of both N370S and imiglucerase 2–3-fold by reducing lysosomal degradation. Overall, these data indicate that the N370S mutation results in a normally folded but less flexible protein with reduced catalytic activity compared with imiglucerase.     

Generic Approach for the Generation of Stable Humanized Single-chain Fv Fragments from Rabbit Monoclonal Antibodies

Despite their favorable pharmacokinetic properties, single-chain Fv antibody fragments (scFvs) are not commonly used as therapeutics, mainly due to generally low stabilities and poor production yields. In this work, we describe the identification and optimization of a human scFv scaffold, termed FW1.4, which is suitable for humanization and stabilization of a broad variety of rabbit antibody variable domains. A motif consisting of five structurally relevant framework residues that are highly conserved in rabbit variable domains was introduced into FW1.4 to generate a generically applicable scFv scaffold, termed FW1.4gen. Grafting of complementarity determining regions (CDRs) from 15 different rabbit monoclonal antibodies onto FW1.4 and their derivatives resulted in humanized scFvs with binding affinities in the range from 4.7 × 10⁻⁹ to 1.5 × 10⁻¹¹ m. Interestingly, minimalistic grafting of CDRs onto FW1.4gen, without any substitutions in the framework regions, resulted in affinities ranging from 5.7 × 10⁻¹⁰ to <1.8 × 10⁻¹² m. When compared with progenitor rabbit scFvs, affinities of most humanized scFvs were similar. Moreover, in contrast to progenitor scFvs, which were difficult to produce, biophysical properties of the humanized scFvs were significantly improved, as exemplified by generally good production yields in a generic refolding process and by apparent melting temperatures between 53 and 86 °C. Thus, minimalistic grafting of rabbit CDRs on the FW1.4gen scaffold presents a simple and reproducible approach to humanize and stabilize rabbit variable domains.     

A semi-synthetic repertoire of intrinsically stable antibody fragments derived from a single-framework scaffold.

We report the design, construction and use of an antibody bacteriophage display library built on the scaffold of a single-chain variable fragment (scFv) previously proven to be functionally expressed in the reducing environment of both bacterial and plant cytoplasm and endowed with intrinsic high thermodynamic stability. Four amino acid residues of the third hypervariable loop (CDR3) of both VH and VL were combinatorially mutated, generating a repertoire of approximately 5x10(7) independent scFvs, cloned in a phagemid vector. The ability of the antibody phage library to yield specific binders was tested by biopanning against several antigens. Successful selection of fully active scFvs was obtained, confirming the notion that combinatorial mutagenesis of few amino acid residues centrally located in the antigen-binding site is sufficient to provide binding specificities against virtually any target. High yields of both soluble and phage antibodies were obtained in Escherichia coli. Maintenance of the cognate scFv antibody stability in the newly selected scFv fragments was demonstrated by guanidinium chloride denaturation/renaturation studies and by soluble antibody expression in the bacterial cytoplasm. The antibody library described here allows the isolation of new stable binding specificities, potentially exploitable as immunochemical reagents for intracellular applications.     

Redesigning of anti‐c‐Met single chain Fv antibody for the cytoplasmic folding and its structural analysis

Typically, single chain Fv antibodies are unable to fold properly under a reducing cytoplasm because of the reduction of disulfide bonds. The inability to fold limits both the production of the functional scFvs and their targeting against antigens, which are generally executed in a reducing cytoplasm. In this study, the target scFv CDR was grafted with stable human consensus framework sequences, which enabled the generation of a foldable scFv in a reducing cytoplasm of Escherichia coli. Additionally, the structural features affecting the folding efficiency of the engineered scFv were identified by analyzing the predicted structure. An anti‐c‐Met scFv, which was a cytoplasmic non‐foldable protein, was redesigned as the model system. This study confirmed that the engineered anti‐c‐Met scFv was folded into its native form in the cytoplasm of E. coli BL21(DE3) without a significant loss in the specific binding activity against c‐Met antigen. The structures of the wild‐type anti‐c‐Met scFv and the engineered scFv were predicted using homology modeling. A comparative analysis based on the sequence and structure showed that the hydrophobicity of 12 solvent exposed residues decreased, and two newly formed salt bridges might have improved the folding efficiency of the engineered scFv under the reducing condition. Biotechnol. Bioeng. 2010; 106: 367–375. © 2010 Wiley Periodicals, Inc.     

A scFv antibody mutant isolated in a genetic screen for improved export via the twin arginine transporter pathway exhibits faster folding

Proteins destined for export across the cytoplasmic membrane via the post-translational Sec-dependent route have to be maintained in a largely unfolded state within the cytoplasm. In sharp contrast, only proteins that have folded into a native-like state within the cytoplasm are competent for export via the twin arginine translocation (Tat) pathway. Proteins that contain disulfide bonds, such as scFv antibody fragments, can be translocated via Tat only when expressed in Escherichia coli trxB gor mutant strains having an oxidizing cytoplasm. However, export is poor with the majority of the protein accumulating in the cytoplasm and only a fraction exported to the periplasmic space. Using a high throughput fluorescence screen, we isolated a mutant of the anti-digoxin 26-10 scFv from a large library of random mutants that is exported with a higher yield into the periplasm. In vitro refolding experiments revealed that the mutant scFv exhibits a 250% increase in the rate constant of the critical second phase of folding. This result suggests that Tat export competence is related to the protein folding rate and could be exploited for the isolation of faster folding protein mutants.     

Efficient export of prefolded,disulfide‐bonded recombinant proteins to the periplasm by the Tat pathway in Escherichia coli CyDisCo strains

Numerous high‐value therapeutic proteins are produced in Escherichia coli and exported to the periplasm, as this approach simplifies downstream processing and enables disulfide bond formation. Most recombinant proteins are exported by the Sec pathway, which transports substrates across the plasma membrane in an unfolded state. The Tat system also exports proteins to the periplasm, but transports them in a folded state. This system has attracted interest because of its tendency to transport correctly folded proteins, but this trait renders it unable to export proteins containing disulfide bonds since these are normally acquired only in the periplasm; reduced substrates tend to be recognized as incorrectly folded and rejected. In this study we have used a series of novel strains (termed CyDisCo) which oxidise disulfide bonds in the cytoplasm, and we show that these cells efficiently export a range of disulfide‐containing proteins when a Tat signal peptide is attached. These test proteins include alkaline phosphatase (PhoA), a phytase containing four disulfide bonds (AppA), an antiinterleukin 1β scFv and human growth hormone. No export of PhoA or AppA is observed in wild‐type cells lacking the CyDisCo factors. The PhoA, AppA and scFv proteins were exported in an active form by Tat in the CyDisCo strain, and mass spectrometry showed that the vast majority of the scFv protein was disulfide‐bonded and correctly processed. The evidence indicates that this combination of Tat + CyDisCo offers a novel means of exporting active, correctly folded disulfide bonded proteins to the periplasm. © 2013 American Institute of Chemical Engineers Biotechnol. Prog., 30:281–290, 2014     

The disulfide bonds in antibody variable domains: effects on stability, folding in vitro, and functional expression in Escherichia coli.

The formation of the disulfide bonds in the variable domains VH and VL of the antibody McPC603 was found to be essential for the stability of all antigen binding fragments investigated. Exposure of the Fv fragment to reducing conditions in vitro resulted in irreversible denaturation of both VH and VL. In vitro refolding of the reduced Fv fragment was only possible when the disulfide bonds were allowed to form under oxidizing conditions. The analysis of a series of mutants of the Fv fragment, the Fab fragment and the single-chain Fv fragment, all secreted into the periplasm of Escherichia coli, in which each of the cysteine residues of the variable domains was replaced by a series of other amino acids, showed that functional antigen binding fragments required the presence of both the disulfide bond in VH and the one in VL. These results were also used to devise an alternative expression system based on the production of insoluble fusion proteins consisting of truncated beta-galactosidase and antibody domains, enzymatic cleavage, and refolding and assembly in vitro. This strategy should be useful for providing access to unstable antibody domains and fragments.     

Directed Evolution of a Fluorogen-Activating Single Chain Antibody for Function and Enhanced Brightness in the Cytoplasm

Directed evolution is an exceptionally powerful tool that uses random mutant library generation and screening techniques to engineer or optimize functions of proteins. One class of proteins for which this process is particularly effective is antibodies, where properties such as antigen specificity and affinity can be selected to yield molecules with improved efficacy as molecular labels or in potential therapeutics. Typical antibody structure includes disulfide bonds that are required for stability and proper folding of the domains. However, these bonds are unable to form in the reducing environment of the cytoplasm, stymieing the effectiveness of optimized antibodies in many research applications. We have removed disulfide-forming cysteine residues in a single chain antibody fluorogen-activating protein (FAP), HL4, and employed directed evolution to select a derivative that is capable of activity in the cytoplasm. A subsequent round of directed evolution was targeted at increasing the overall brightness of the fluoromodule (FAP–fluorogen complex). Ultimately, this approach produced a novel FAP that exhibits strong activation of its cognate fluorogen in the reducing environment of the cytoplasm, significantly expanding the range of applications for which fluoromodule technology can be utilized.     

Disulfide bond formation in the Escherichia coli cytoplasm: an in vivo role reversal for the thioredoxins.

Cytoplasmic proteins do not generally contain structural disulfide bonds, although certain cytoplasmic enzymes form such bonds as part of their catalytic cycles. The disulfide bonds in these latter enzymes are reduced in Escherichia coli by two systems; the thioredoxin pathway and the glutathione/glutaredoxin pathway. However, structural disulfide bonds can form in proteins in the cytoplasm when the gene (trxB) for the enzyme thioredoxin reductase is inactivated by mutation. This disulfide bond formation can be detected by assessing the state of the normally periplasmic enzyme alkaline phosphatase (AP) when it is localized to the cytoplasm. Here we show that the formation of disulfide bonds in cytoplasmic AP in the trxB mutant is dependent on the presence of two thioredoxins in the cell, thioredoxins 1 and 2, the products of the genes trxA and trxC, respectively. Our evidence supports a model in which the oxidized forms of these thioredoxins directly catalyze disulfide bond formation in cytoplasmic AP, a reversal of their normal role. In addition, we show that the recently discovered thioredoxin 2 can perform many of the roles of thioredoxin 1 in vivo, and thus is able to reduce certain essential cytoplasmic enzymes. Our results suggest that the three most effective cytoplasmic disulfide-reducing proteins are thioredoxin 1, thioredoxin 2 and glutaredoxin 1; expression of any one of these is sufficient to support aerobic growth. Our results help to explain how the reducing environment in the cytoplasm is maintained so that disulfide bonds do not normally occur.     

Immunomodulation of cucumber mosaic virus infection by intrabodies selected <Emphasis Type="Italic">in vitro</Emphasis> from a stable single-framework phage display library

Immunomodulation by the ectopic expression of intracellular antibodies (‚intrabodies’) has a great potential for interfering with physiological or pathological functions in vivo in a highly specific manner. One of the major obstacles of this technology is the inability of most antibodies to properly fold and function in the reducing environment of the cytoplasm, which prevents the formation of essential disulfide bonds. We wished to assess the intracellular performance of antibodies derived from a semi-synthetic single-chain variable fragment (scFv) phage display library (‚F8 library’) built on a thermodynamically stable single-framework scaffold. To this purpose, we chose to modulate the infection of a pandemic plant pathogen, the cucumber mosaic virus (CMV). After in vitro ‚biopanning’ on immobilized virions, two scFvs were biochemically characterized, showing high affinity toward the antigen. They were transiently expressed at high yields as soluble molecules in the cytoplasm of Nicotiana benthamiana plants. Subsequently, they were expressed in the cytoplasm of transgenic tomato plants. Challenge with high viral dose showed that both scFvs were able to elicit a phenotypic effect and led to the identification of a transgenic line fully resistant to infection. In these plants, the scFv binds the virus in the inoculated leaves preventing viral long distance movement. This work represents the first demonstration that the ‚F8 library’ can be directly screened in vitro to rapidly isolate antigen-specific scFvs that act as effective intrabodies in vivo. These antibodies represent powerful tools to interfere with several intracellular targets, modulating pathogen infectivity and/or cellular metabolism.We dedicate this work to Piero Roggero who prematurely passed away on June 19th 2003.     

The Active-Site Cysteines of the Periplasmic Thioredoxin-Like Protein CcmG of Escherichia coli Are Important but Not Essential for Cytochrome c Maturation In Vivo

A new member of the family of periplasmic protein thiol:disulfide oxidoreductases, CcmG (also called DsbE), was characterized with regard to its role in cytochrome c maturation in Escherichia coli. The CcmG protein was shown to be membrane bound, facing the periplasm with its C-terminal, hydrophilic domain. A chromosomal, nonpolar in-frame deletion in ccmG resulted in the complete absence of all c-type cytochromes. Replacement of either one or both of the two cysteine residues of the predicted active site in CcmG (WCPTC) led to low but detectable levels of Bradyrhizobium japonicum holocytochrome c₅₅₀ expressed in E. coli. This defect, but not that of the ccmG null mutant, could be complemented by adding low-molecular-weight thiol compounds to growing cells, which is in agreement with a reducing function for CcmG.     

Production of correctly folded Fab antibody fragment in the cytoplasm of Escherichia coli trxB gor mutants via the coexpression of molecular chaperones.

Disulfide bonds are normally formed after a polypeptide has been exported from the reducing environment of the cytoplasm into a more oxidizing compartment, such as the bacterial periplasm. Recently, we showed that in Escherichia coli trxB gor mutants, in which the reduction of thioredoxin and glutathione is impaired, the redox potential of the cytoplasm becomes comparable to that of the mammalian endoplasmic reticulum, thus allowing the formation of disulfide bonds in certain complex proteins (P. H. Bessette et al., 1999, Proc. Natl. Acad. Sci. USA 96, 13703-13708]. Here, we investigate the expression of a Fab antibody fragment in the bacterial cytoplasm. The effect of coexpressing cytoplasmic chaperones (GroEL/ES, trigger factor, DnaK/J), as well as signal sequenceless versions of periplasmic chaperones (DsbC and Skp), was examined. Skp coexpression was shown to have the most significant effect (five- to sixfold increase) on the yield of correctly folded Fab. A maximum yield of 0.8 mg Fab/L/OD(600) Fab was obtained, indicating that cytoplasmic expression may be a viable alternative for the preparative production of antibody fragments.     

The Disulfide Bonding System Suppresses CsgD-Independent Cellulose Production in Escherichia coli

The bacterial extracellular matrix encases cells and protects them from host-related and environmental insults. The Escherichia coli master biofilm regulator CsgD is required for the production of the matrix components curli and cellulose. CsgD activates the diguanylate cyclase AdrA, which in turn stimulates cellulose production through cyclic di-GMP (c-di-GMP). Here, we identified and characterized a CsgD- and AdrA-independent cellulose production pathway that was maximally active when cultures were grown under reducing conditions or when the disulfide bonding system (DSB) was compromised. The CsgD-independent cellulose activation pathway was dependent on a second diguanylate cyclase, called YfiN. c-di-GMP production by YfiN was repressed by the periplasmic protein YfiR, and deletion of yfiR promoted CsgD-independent cellulose production. Conversely, when YfiR was overexpressed, cellulose production was decreased. Finally, we found that YfiR was oxidized by DsbA and that intraprotein YfiR disulfide bonds stabilized YfiR in the periplasm. Altogether, we showed that reducing conditions and mutations in the DSB system caused hyperactivation of YfiN and subsequent CsgD-independent cellulose production.     

Generation of an intrabody‐based reagent suitable for imaging endogenous proliferating cell nuclear antigen in living cancer cells

Intrabodies, when expressed in cells after genetic fusion to fluorescent proteins, are powerful tools to study endogenous protein dynamics inside cells. However, it remains challenging to determine the conditions for specific imaging and precise labelling of the target antigen with such intracellularly expressed antibody fragments. Here, we show that single‐chain Fv (scFv) antibody fragments can be generated that specifically recognize proliferating cell nuclear antigen (PCNA) when expressed in living cancer cells. After selection by phage display, the anti‐PCNA scFvs were screened in vitro after being tagged with dimeric glutathione‐S‐transferase. Anti‐PCNA scFvs of increased avidity were further engineered by mutagenesis with sodium bisulfite and error‐prone PCR, such that they were almost equivalent to conventional antibodies in in vitro assays. These intrabodies were then rendered bifunctional by fusion to a C‐terminal fragment of p21 protein and could thereby readily detect PCNA bound to chromatin in cells. Finally, by linking these optimized peptide‐conjugated scFvs to an enhanced green fluorescent protein, fluorescent intrabody‐based reagents were obtained that allowed the fate of PCNA in living cells to be examined. The approach described may be applicable to other scFvs that can be solubly expressed in cells, and it provides a unique means to recognize endogenous proteins in living cells with high accuracy. Copyright © 2014 John Wiley & Sons, Ltd.     

Functional expression of monomeric streptavidin and fusion proteins in Escherichia coli: applications in flow cytometry and ELISA

Monomeric streptavidin (mSA) offers a combination of structural and binding properties that are useful in many applications, including a small size and monovalent biotin binding. Because mSA contains a structurally important disulfide bond, the molecule does not fold correctly when expressed inside the cell. We show that mSA can be expressed in a functional form in Escherichia coli by fusing the OmpA signal sequence at the amino terminus. Expressed mSA is exported to the periplasm, from which the molecule leaks to the medium under vigorous shaking. Purified mSA can be conjugated with FITC and used to label microbeads and yeast cells for analysis by flow cytometry, further expanding the scope of mSA-based applications. Some applications require recombinant fusion of mSA with another protein. mSA fused to EGFP cannot be secreted to the medium but was successfully expressed in an engineered cell line that supports oxidative folding in the cytoplasm. Purified mSA-EGFP and mSA-mCherry bound biotin with high affinity and were successfully used in conventional flow cytometry and imaging flow cytometry. Finally, we demonstrate the use of mSA in ELISA, in which horseradish peroxidase-conjugated mSA and biotinylated secondary antibody are used together to detect primary antibody captured on an ELISA plate. Engineering mSA to introduce additional lysine residues can increase the reporter signal above that of wild-type streptavidin. Together, these examples establish mSA as a convenient reagent with a potentially unique role in biotechnology.