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.

We describe the development of a reference biosensor surface, based upon a binary mixture of oligo-ethylene glycol thiols, one of which has biotin at the terminus, adsorbed onto gold as self-assembled monolayers (SAMs). These surfaces were analyzed in detail by X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) to establish the relationship between the thiol solution composition and the surface composition and structure. We report the use of argon cluster primary ions for the analysis of PEG-thiols, establishing that the different thiols are intimately mixed and that SIMS may be used to measure surface composition of thiol SAMs on gold with a detection limit better than 1% fractional coverage. The adsorption of neutralized chimeric avidin to these surfaces was measured simultaneously using ellipsometry and QCM-D. Comparison of the two measurements demonstrates the expected nonlinearity of the frequency response of the QCM but also reveals a strong variation in the dissipation signal that correlates with the surface density of biotin. These variations are most likely due to the difference in mechanical response of neutralized chimeric avidin bound by just one biotin moiety at low biotin density and two biotin moieties at high density. The transition between the two modes of binding occurs when the average spacing of biotin ligands approaches the diameter of the avidin molecule.

Label-free biosensors detect binding of prey molecules (″analytes″) to immobile bait molecules on the sensing surface. Numerous methods are available for immobilization of bait molecules. A convenient option is binding of biotinylated bait molecules to streptavidin-functionalized surfaces, or to biotinylated surfaces via biotin-avidin-biotin bridges. The goal of this study was to find a rapid method for reversible immobilization of biotinylated bait molecules on biotinylated sensor chips. The task was to establish a biotin-avidin-biotin bridge which was easily cleaved when desired, yet perfectly stable under a wide range of measurement conditions. The problem was solved with the avidin mutant M96H which contains extra histidine residues at the subunit-subunit interfaces. This mutant was bound to a mixed self-assembled monolayer (SAM) containing biotin residues on 20% of the oligo(ethylene glycol)-terminated SAM components. Various biotinylated bait molecules were bound on top of the immobilized avidin mutant. The biotin-avidin-biotin bridge was stable at pH ≥3, and it was insensitive to sodium dodecyl sulfate (SDS) at neutral pH. Only the combination of citric acid (2.5%, pH 2) and SDS (0.25%) caused instantaneous cleavage of the biotin-avidin-biotin bridge. As a consequence, the biotinylated bait molecules could be immobilized and removed as often as desired, the only limit being the time span for reproducible chip function when kept in buffer (2-3 weeks at 25 °C). As expected, the high isolectric pH (pI) of the avidin mutant caused nonspecific adsorption of proteins. This problem was solved by acetylation of avidin (to pI < 5), or by optimization of SAM formation and passivation with biotin-BSA and BSA.

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.

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.

Stringent regulation of tyrosine kinase activity is essential for normal cellular function. In humans, the tyrosine kinase Src is inhibited via phosphorylation of its C-terminal tail by another kinase, C-terminal Src kinase (Csk). Although Src and Csk orthologs are present across holozoan organisms, including animals and protists, the Csk-Src negative regulatory mechanism appears to have evolved gradually. For example, in choanoflagellates, Src and Csk are both active, but the negative regulatory mechanism is reportedly absent. In filastereans, a protist clade closely related to choanoflagellates, Src is active, but Csk is apparently inactive. In this study, we use a combination of bioinformatics, kinase assays, and yeast-based growth assays to characterize holozoan Src and Csk orthologs. We show that, despite appreciable differences in domain architecture, Csk from, a highly diverged holozoan marine protist, is active and can inhibit Src. However, in comparison with other Csk orthologs, Csk displays broad substrate specificity and inhibits Src in an activity-independent manner. Furthermore, in contrast to previous studies, we show that Csk from the filasterean is active and that the Csk-Src negative regulatory mechanism is present in Csk and Src proteins from and the choanoflagellate Our results suggest that negative regulation of Src by Csk is more ancient than previously thought and that it might be conserved across all holozoan species.

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.

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