Here we present a biophysical, structural, and computational analysis of the directed evolution of the human DNA repair protein O 6 -alkylguanine-DNA alkyltransferase (hAGT) into SNAP-tag, a self-labeling protein tag. Evolution of hAGT led not only to increased protein activity but also to higher stability, especially of the alkylated protein, suggesting that the reactivity of the suicide enzyme can be influenced by stabilizing the product of the irreversible reaction. Whereas wild-type hAGT is rapidly degraded in cells after alkyl transfer, the high stability of benzylated SNAP-tag prevents proteolytic degradation. Our data indicate that the intrinstic stability of a key α helix is an important factor in triggering the unfolding and degradation of wild-type hAGT upon alkyl transfer, providing new insights into the structure−function relationship of the DNA repair protein.T he specific labeling of proteins with synthetic probes is a powerful approach for studying protein function. One way to achieve such a specific labeling is based on so-called selflabeling protein tags.1 In this approach, the protein of interest is expressed as a fusion protein with a peptide or protein (i.e., tag) whose role is to specifically bind to a synthetic probe in vitro or in vivo. A well-established example of a self-labeling protein tag is SNAP-tag.2 SNAP-tag specifically reacts with substituted O 6 -benzylguanine derivatives and thereby permits the labeling of SNAP-tag fusion proteins with a wide variety of different synthetic probes. Recent applications include its use for the analysis of protein complexes, 3 super-resolution microscopy, 4 the identification of protein−protein interactions, 5 drug target identification, 6 and the determination of protein half-life in animals. 7 The appeal of self-labeling tags such as SNAP-tag is the ease with which fusion proteins can be labeled with synthetic probes even in living cells. A conceptual limitation of the approach is the fact that the tag can affect the properties of its fusion partner. It is therefore important that the properties of the tag be as thoroughly characterized as possible.SNAP-tag was generated in a stepwise manner from human O 6 -alkylguanine-DNA alkyltransferase (hAGT) by introduction of a total of 19 point mutations (Figure 1) and deletion of 25 C-terminal residues. Saturation mutagenesis of four active-site residues followed by phage display and selection for activity against BG derivatives resulted in GE AGT, a mutant with 20-fold increased activity toward such substrates ( Figure 1B). 8Subsequent saturation mutagenesis of four additional residues involved in substrate binding followed by phage selection resulted in AGT-54, a mutant with 1.5-fold higher activity than GE AGT. To further optimize the protein for applications in protein labeling, mutations were introduced to suppress DNA binding and reactivity toward nucleosides, to remove nonessential cysteines, and to truncate the last 25 residues. 9The resulting mutant M AGT displayed relatively low activ...

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We introduce a strategy for evolving protein substrate specificity by the insertion of random amino acid loops into the protein backbone. Application of this strategy to human O6-alkylguanine-DNA alkyltransferase (AGT) led to the isolation of mutants that react with the non-natural substrate O6-propargylguanine. Libraries generated by conventional random or targeted saturation mutagenesis, by contrast, did not yield any mutants with activity towards this new substrate. The strategy of loop insertion to alter enzyme specificity should be general and applicable to other classes of proteins. An important application of the isolated AGT mutant is in molecular imaging, where the mutant and parental AGTs are used to label two different AGT fusion proteins with different fluorophores in the same living cell or in vitro . This allowed the establishment of fluorescence-based assays to detect protein-protein interactions and measure enzymatic activities.

revealed some unique properties of the WrbA family: lower affinity for its cofactor-the flavin mononucleotide (FMN)-and the multimeric character of protein in solution. WrbA protein is apparently the first characterized case in which multimerization is associated directly with the flavodoxin-like domain itself. In all other multimeric flavodoxins the flavodoxin-like domain is fused to a multimerization domain [3]. WrbA protein and its homologs thus present a unique family among the typical flavodoxin-like proteins. Structural analysis may aid in understanding these unique properties and may reveal the physiological role of WrbA in the living organisms. This was a motivation for searching of diffraction-quality crystals. WrbA apoprotein crystals grown by standard and advanced crystallization techniques consisted of twinned plates. The quality of crystals was successfully improved by using additives and gelling protein solution for crystallization. Crystals suitable for X-ray diffraction measurement were measured at synchrotron DESY, beamline X13 (Hamburg), at cryotemperature. Crystals diffracted to 2.2 Å. Solving of protein structure is in progress. Limited proteolysis [4] of WrbA apoprotein led to preliminary identification of folded substructures and flexible parts of protein structure. Acknowledgements: This work is supported by grant of the Ministry of Education of the CR (KONTAKT ME640) to I.K.S. and by NSF grant INT-03-09049 to J.C. Grants MSM6007665808 and AVOZ60870520 are also acknowledged.

Fast track microfluidic screening of the antibody repertoires of 12 convalescent COVID-19 donors comprising 2.8mio antibodies yielded MTX-COVAB, a human-derived monoclonal antibody with low picomolar neutralization IC50 of SARS-CoV-2. COVAB neutralization potency is on par with the Regeneron cocktail as demonstrated in a comparative neutralization assay. MTX-COVAB shows strong efficacy in vivo and binds to all currently identified clinically relevant variants of SARS-CoV-2. MTX-COVAB completes GMP manufacturing by the end of this year and will be tested in the clinic in March 2021.