Molecular features of steroid-binding antidins and their use for assaying serum progesterone.

Chicken avidin (Avd) and streptavidin from Streptomyces avidinii are extensively used in bionanotechnology due to their extremely tight binding to biotin (Kd ~ 10-15 M for chicken Avd). We previously reported engineered Avds known as antidins, which have micro- to nanomolar affinities for steroids, non-natural ligands of Avd. Here, we report the 2.8 Å X-ray structure of the sbAvd-2 (I117Y) antidin co-crystallized with progesterone. We describe the creation of new synthetic phage display libraries and report the experimental as well as computational binding analysis of progesterone-binding antidins. We introduce a next-generation antidin with 5 nM binding affinity for progesterone, and demonstrate the use of antidins for measuring progesterone in serum samples. Our data give insights on how to engineer and alter the binding preferences of Avds and to develop better molecular tools for modern bionanotechnological applications.

Structural characterization of core-bradavidin in complex with biotin.

Bradavidin is a tetrameric biotin-binding protein similar to chicken avidin and bacterial streptavidin, and was originally cloned from the nitrogen-fixing bacteria Bradyrhizobium diazoefficiens. We have previously reported the crystal structure of the full-length, wild-type (wt) bradavidin with 138 amino acids, where the C-terminal residues Gly129-Lys138 ("Brad-tag") act as an intrinsic ligand (i.e. Gly129-Lys138 bind into the biotin-binding site of an adjacent subunit within the same tetramer) and has potential as an affinity tag for biotechnological purposes. Here, the X-ray structure of core-bradavidin lacking the C-terminal residues Gly114-Lys138, and hence missing the Brad-tag, was crystallized in complex with biotin at 1.60 Å resolution [PDB:4BBO]. We also report a homology model of rhodavidin, an avidin-like protein from Rhodopseudomonas palustris, and of an avidin-like protein from Bradyrhizobium sp. Ai1a-2, both of which have the Brad-tag sequence at their C-terminus. Moreover, core-bradavidin V1, an engineered variant of the original core-bradavidin, was also expressed at high levels in E. coli, as well as a double mutant (Cys39Ala and Cys69Ala) of core-bradavidin (CC mutant). Our data help us to further engineer the core-bradavidin-Brad-tag pair for biotechnological assays and chemical biology applications, and provide deeper insight into the biotin-binding mode of bradavidin.

Artificial Avidin-Based Receptors for a Panel of Small Molecules.

Proteins with high specificity, affinity, and stability are needed for biomolecular recognition in a plethora of applications. Antibodies are powerful affinity tools, but they may also suffer from limitations such as low stability and high production costs. Avidin and streptavidin provide a promising scaffold for protein engineering, and due to their ultratight binding to D-biotin they are widely used in various biotechnological and biomedical applications. In this study, we demonstrate that the avidin scaffold is suitable for use as a novel receptor for several biologically active small molecules: Artificial, chicken avidin-based proteins, antidins, were generated using a directed evolution method for progesterone, hydrocortisone, testosterone, cholic acid, ketoprofen, and folic acid, all with micromolar to nanomolar affinity and significantly reduced biotin-binding affinity. We also describe the crystal structure of an antidin, sbAvd-2(I117Y), a steroid-binding avidin, which proves that the avidin scaffold can tolerate significant modifications without losing its characteristic tetrameric beta-barrel structure, helping us to further design avidin-based small molecule receptors.

Protein and bacterial interactions with nanostructured polymer coatings.

Adsorption of proteins and adhesion of bacteria to a surface is affected by chemical and physical interactions. In this study, polymer coatings and their ability to adsorb avidin and Staphylococcus aureus were investigated. The surface chemistry and topography of the polymer coatings was modified by changing the weight ratio of the hydrophobic polystyrene (PS) and the hydrophilic acrylonitrile butadiene styrene (ABS) components in the polymer blend. Avidin adsorbed less to the ABS phase compared with the PS phase. The side-on orientation of avidin on the ABS surface, however, resulted in a higher specific binding of biotinylated bovine serum albumin. Steric effects and hydrophobic protein-surface interactions decreased the activity of avidin on the PS phase. The increased hydrophobicity and roughness of the polymer coatings enhanced the adhesion of S. aureus. The avidin-coated latex surface with 55% relative surface coverage of the PS phase showed anti-microbial behavior.

His-tagged norovirus-like particles: A versatile platform for cellular delivery and surface display.

In addition to vaccines, noninfectious virus-like particles (VLPs) that mimic the viral capsid show an attractive possibility of presenting immunogenic epitopes or targeting molecules on their surface. Here, functionalization of norovirus-derived VLPs by simple non-covalent conjugation of various molecules is shown. By using the affinity between a surface-exposed polyhistidine-tag and multivalent tris-nitrilotriacetic acid (trisNTA), fluorescent dye molecules and streptavidin-biotin conjugated to trisNTA are displayed on the VLPs to demonstrate the use of these VLPs as easily modifiable nanocarriers as well as a versatile vaccine platform. The VLPs are able to enter and deliver surface-displayed fluorescent dye into HEK293T cells via a surface-attached cell internalization peptide (VSV-G). The ease of manufacturing, the robust structure of these VLPs, and the straightforward conjugation provide a technology, which can be adapted to various applications in biomedicine.

Efficient preparation of shuffled DNA libraries through recombination (Gateway) cloning.

Efficient and robust subcloning is essential for the construction of high-diversity DNA libraries in the field of directed evolution. We have developed a more efficient method for the subcloning of DNA-shuffled libraries by employing recombination cloning (Gateway). The Gateway cloning procedure was performed directly after the gene reassembly reaction, without additional purification and amplification steps, thus simplifying the conventional DNA shuffling protocols. Recombination-based cloning, directly from the heterologous reassembly reaction, conserved the high quality of the library and reduced the time required for the library construction. The described method is generally compatible for the construction of DNA-shuffled gene libraries.

Switchavidin: reversible biotin-avidin-biotin bridges with high affinity and specificity.

Switchavidin is a chicken avidin mutant displaying reversible binding to biotin, an improved binding affinity toward conjugated biotin, and low nonspecific binding due to reduced surface charge. These properties make switchavidin an optimal tool in biosensor applications for the reversible immobilization of biotinylated proteins on biotinylated sensor surfaces. Furthermore, switchavidin opens novel possibilities for patterning, purification, and labeling.

pH-dependent deformations of the energy landscape of avidin-like proteins investigated by single molecule force spectroscopy.

Avidin and avidin-like proteins are widely used in numerous techniques since the avidin-biotin interaction is known to be very robust and reliable. Within this study, we investigated this bond at the molecular level under harsh conditions ranging from very low to very high pH values. We compared avidin with streptavidin and a recently developed avidin-based mutant, chimeric avidin. To gain insights of the energy landscape of these interactions we used a single molecule approach and performed the Single Molecule Force Spectroscopy atomic force microscopy technique. There, the ligand (biotin) is covalently coupled to a sharp AFM tip via a distensible hetero-bi-functional crosslinker, whereas the receptor of interest is immobilized on the probe surface. Receptor-ligand complexes are formed and ruptured by repeatedly approaching and withdrawing the tip from the surface. Varying both pulling velocity and pH value, we could determine changes of the energy landscape of the complexes. Our results clearly demonstrate that avidin, streptavidin and chimeric avidin are stable over a wide pH range although we could identify differences at the outer pH range. Taking this into account, they can be used in a broad range of applications, like surface sensors at extreme pH values.

Molecular engineering of avidin and hydrophobin for functional self-assembling interfaces.

Control over the functionality of interfaces through biomolecular engineering is a central tool for nanoscale technology as well as many current applications of biology. In this work we designed fusion proteins that combined the surface adhesion and interfacial activity of a hydrophobin-protein together with the high affinity biotin-binding capability of an avidin-protein. We found that an overall architecture that was based on a circularly permuted version of avidin, dual-chain avidin, and hydrophobin gave a highly functional combination. The protein was produced in the filamentous fungus Trichoderma reesei and was efficiently purified using an aqueous two-phase partitioning procedure. The surface adhesive properties were widely different compared to wild-type avidin. Functional characterization showed that the protein assembled on hydrophobic surfaces as a thin layer even at very low concentrations and efficiently bound a biotinylated compound. The work shows how the challenge of creating a fusion protein with proteins that form multimers can be solved by structural design and how protein self-assembly can be used to efficiently functionalize interfaces.

Chimeric Avidin--NMR structure and dynamics of a 56 kDa homotetrameric thermostable protein.

Chimeric avidin (ChiAVD) is a product of rational protein engineering remarkably resistant to heat and harsh conditions. In quest of the fundamentals behind factors affecting stability we have elucidated the solution NMR spectroscopic structure of the biotin-bound form of ChiAVD and characterized the protein dynamics through 15N relaxation and hydrogen/deuterium (H/D) exchange of this and the biotin-free form. To surmount the challenges arising from the very large size of the protein for NMR spectroscopy, we took advantage of its high thermostability. Conventional triple resonance experiments for fully protonated proteins combined with methyl-detection optimized experiments acquired at 58°C were adequate for the structure determination of this 56 kDa protein. The model-free parameters derived from the 15N relaxation data reveal a remarkably rigid protein at 58°C in both the biotin-bound and the free forms. The H/D exchange experiments indicate a notable increase in hydrogen protection upon biotin binding.

A novel chimeric avidin with increased thermal stability using DNA shuffling.

Avidins are a family of proteins widely employed in biotechnology. We have previously shown that functional chimeric mutant proteins can be created from avidin and avidin-related protein 2 using a methodology combining random mutagenesis by recombination and selection by a tailored biopanning protocol (phage display). Here, we report the crystal structure of one of the previously selected and characterized chimeric avidin forms, A/A2-1. The structure was solved at 1.8 Å resolution and revealed that the protein fold was not affected by the shuffled sequences. The structure also supports the previously observed physicochemical properties of the mutant. Furthermore, we improved the selection and screening methodology to select for chimeric avidins with slower dissociation rate from biotin than were selected earlier. This resulted in the chimeric mutant A/A2-B, which showed increased thermal stability as compared to A/A2-1 and the parental proteins. The increased stability was especially evident at conditions of extreme pH as characterized using differential scanning calorimetry. In addition, amino acid sequence and structural comparison of the chimeric mutants and the parental proteins led to the rational design of A/A2-B I109K. This mutation further decreased the dissociation rate from biotin and yielded an increase in the thermal stability.

Coxsackievirus B3 VLPs purified by ion exchange chromatography elicit strong immune responses in mice.

Coxsackievirus B3 (CVB3) is an important cause of acute and chronic viral myocarditis, and dilated cardiomyopathy (DCM). Although vaccination against CVB3 could significantly reduce the incidence of serious or fatal viral myocarditis and various other diseases associated with CVB3 infection, there is currently no vaccine or therapeutic reagent in clinical use. In this study, we contributed towards the development of a CVB3 vaccine by establishing an efficient and scalable ion exchange chromatography-based purification method for CVB3 virus and baculovirus-insect cell-expressed CVB3 virus-like particles (VLPs). This purification system is especially relevant for vaccine development and production on an industrial scale. The produced VLPs were characterized using a number of biophysical methods and exhibited excellent quality and high purity. Immunization of mice with VLPs elicited a strong immune response, demonstrating the excellent vaccine potential of these VLPs.

Investigating the binding behaviour of two avidin-based testosterone binders using molecular recognition force spectroscopy.

Molecular recognition force spectroscopy, a biosensing atomic force microscopy technique allows to characterise the dissociation of ligand-receptor complexes at the molecular level. Here, we used molecular recognition force spectroscopy to study the binding capability of recently developed testosterone binders. The two avidin-based proteins called sbAvd-1 and sbAvd-2 are expected to bind both testosterone and biotin but differ in their binding behaviour towards these ligands. To explore the ligand binding and dissociation energy landscape of these proteins, we tethered biotin or testosterone to the atomic force microscopy probe while the testosterone-binding protein was immobilized on the surface. Repeated formation and rupture of the ligand-receptor complex at different pulling velocities allowed determination of the loading rate dependence of the complex-rupturing force. In this way, we obtained the molecular dissociation rate (k(off)) and energy landscape distances (x(ß)) of the four possible complexes: sbAvd-1-biotin, sbAvd-1-testosterone, sbAvd-2-biotin and sbAvd-2-testosterone. It was found that the kinetic off-rates for both proteins and both ligands are similar. In contrast, the x(ß) values, as well as the probability of complex formations, varied considerably. In addition, competitive binding experiments with biotin and testosterone in solution differ significantly for the two testosterone-binding proteins, implying a decreased cross-reactivity of sbAvd-2. Unravelling the binding behaviour of the investigated testosterone-binding proteins is expected to improve their usability for possible sensing applications.

Zebavidin--an avidin-like protein from zebrafish.

The avidin protein family members are well known for their high affinity towards D-biotin and high structural stability. These properties make avidins valuable tools for a wide range of biotechnology applications. We have identified a new member of the avidin family in the zebrafish (Danio rerio) genome, hereafter called zebavidin. The protein is highly expressed in the gonads of both male and female zebrafish and in the gills of male fish, but our data suggest that zebavidin is not crucial for the developing embryo. Biophysical and structural characterisation of zebavidin revealed distinct properties not found in any previously characterised avidins. Gel filtration chromatography and native mass spectrometry suggest that the protein forms dimers in the absence of biotin at low ionic strength, but assembles into tetramers upon binding biotin. Ligand binding was analysed using radioactive and fluorescently labelled biotin and isothermal titration calorimetry. Moreover, the crystal structure of zebavidin in complex with biotin was solved at 2.4 Å resolution and unveiled unique ligand binding and subunit interface architectures; the atomic-level details support our physicochemical observations.

The highly dynamic oligomeric structure of bradavidin II is unique among avidin proteins.

Bradavidin II is a biotin-binding protein from Bradyrhizobium japonicum that resembles chicken avidin and bacterial streptavidin. A biophysical characterization was carried out using dynamic light scattering, native mass spectrometry, differential scanning calorimetry, and isothermal titration calorimetry combined with structural characterization using X-ray crystallography. These observations revealed that bradavidin II differs from canonical homotetrameric avidin protein family members in its quaternary structure. In contrast with the other avidins, bradavidin II appears to have a dynamic (transient) oligomeric state in solution. It is monomeric at low protein concentrations but forms higher oligomeric assemblies at higher concentrations. The crystal structure of bradavidin II revealed an important role for Phe42 in shielding the bound ligand from surrounding water molecules, thus functionally replacing the L7,8 loop essential for tight ligand binding in avidin and streptavidin. This bradavidin II characterization opens new avenues for oligomerization-independent biotin-binding protein development.

Resonance assignments of the 56 kDa chimeric avidin in the biotin-bound and free forms.

Avidin is a homotetrameric ~56 kDa protein found in chicken egg white. Avidin's ability to bind biotin with a very high affinity has widely been exploited in biotechnological applications. Protein engineering has further diversified avidin's feasibility. ChiAVD(I117Y) is a product of rational protein engineering. It is a hyperthermostable synthetic hybrid of avidin and avidin-related protein 4 (AVR4). In this chimeric protein a 23-residue segment in avidin has been replaced with the corresponding sequence found in AVR4, and a point mutation at subunit interface 1-3 (and 2-4) has been introduced. Here we report the backbone and sidechain resonance assignments of the biotin-bound form of ChiAVD(I117Y) as well as the backbone resonance assignments of the free form.

Tuning the "roadblock" effect in kinesin-based transport.

Major efforts are underway to harness motor proteins for technical applications. Yet how to best attach cargo to microtubules that serve as kinesin-driven "molecular shuttles" without compromising transport performance remains challenging. Furthermore, microtubule-associated proteins (MAPs) can block motor protein-powered transport in neurons, which can lead to neurodegenerative diseases. Again it is unclear how different physical roadblock parameters interfere with the stepping motion of kinesins. Here, we employ a series of MAPs, tailored (strept)avidins, and DNA as model roadblocks and determine how their geometrical, nanomechanical, and electrochemical properties can reduce kinesin-mediated transport. Our results provide insights into kinesin transport regulation and might facilitate the choice of appropriate cargo linkers for motor protein-driven transport devices.

Structure of bradavidin-C-terminal residues act as intrinsic ligands.

Bradavidin is a homotetrameric biotin-binding protein from Bradyrhizobium japonicum, a nitrogen fixing and root nodule-forming symbiotic bacterium of the soybean. Wild-type (wt) bradavidin has 138 amino acid residues, whereas the C-terminally truncated core-bradavidin has only 118 residues. We have solved the X-ray structure of wt bradavidin and found that the C-terminal amino acids of each subunit were uniquely bound to the biotin-binding pocket of an adjacent subunit. The biotin-binding pocket occupying peptide (SEKLSNTK) was named "Brad-tag" and it serves as an intrinsic stabilizing ligand in wt bradavidin. The binding of Brad-tag to core-bradavidin was analysed by isothermal titration calorimetry and a binding affinity of ∼25 µM was measured. In order to study the potential of Brad-tag, a green fluorescent protein tagged with Brad-tag was prepared and successfully concentrated from a bacterial cell lysate using core-bradavidin-functionalized Sepharose resin.

Purification of norovirus-like particles (VLPs) by ion exchange chromatography.

Recombinant expression of the norovirus capsid protein VP1 leads to self-assembly of non-infectious virus-like particles (VLPs), which are recognized as promising vaccine candidates against norovirus infections. To overcome the scalability issues connected to the ultracentrifugation-based purification strategies used in previous studies, an anion exchange-based purification method for norovirus VLPs was developed in this study. The method consists of precipitation by polyethylene glycol (PEG) and a single anion exchange chromatography step for purifying baculovirus-expressed GII.4 norovirus VLPs, which can be performed within one day. High product purity was obtained using chromatography. The purified material also contained fully assembled monodispersed VLPs, which were recognized by human sera containing polyclonal antibodies against norovirus GII.4.

Production and characterization of virus-like particles and the P domain protein of GII.4 norovirus.

Noroviruses are an important cause of epidemic acute gastroenteritis in humans. In this study the production and characterization of GII.4 norovirus virus-like particles (VLPs) in insect cells is reported. Furthermore, the expression of corresponding norovirus polyhistidine-tagged P domain protein in Escherichia coli is described. The protruding P domain of the norovirus capsid is known to contain determinants for antibody and receptor binding. Therefore, P domain proteins were studied as an alternative diagnostic tool for evaluating norovirus infection. Analyses by dynamic light scattering and cryo-electron microscopy revealed the presence of intact VLPs with an average diameter of about 40 nm. Immunostaining and ELISA assays using norovirus-specific human sera revealed that VLPs and the P domain are recognized by norovirus-specific antibodies and by their putative receptor. The VLPs and P domain protein are potentially useful in the development of diagnostic and vaccination tools for noroviruses.