"Redirecting an anti-IL-1ß antibody to bind a new, unrelated and computationally predicted epitope on hIL-17A".

Antibody engineering technology is at the forefront of therapeutic antibody development. The primary goal for engineering a therapeutic antibody is the generation of an antibody with a desired specificity, affinity, function, and developability profile. Mature antibodies are considered antigen specific, which may preclude their use as a starting point for antibody engineering. Here, we explore the plasticity of mature antibodies by engineering novel specificity and function to a pre-selected antibody template. Using a small, focused library, we engineered AAL160, an anti-IL-1ß antibody, to bind the unrelated antigen IL-17A, with the introduction of seven mutations. The final redesigned antibody, 11.003, retains favorable biophysical properties, binds IL-17A with sub-nanomolar affinity, inhibits IL-17A binding to its cognate receptor and is functional in a cell-based assay. The epitope of the engineered antibody can be computationally predicted based on the sequence of the template antibody, as is confirmed by the crystal structure of the 11.003/IL-17A complex. The structures of the 11.003/IL-17A and the AAL160/IL-1ß complexes highlight the contribution of germline residues to the paratopes of both the template and re-designed antibody. This case study suggests that the inherent plasticity of antibodies allows for re-engineering of mature antibodies to new targets, while maintaining desirable developability profiles.

The x-ray crystal structure of the first RNA recognition motif and site-directed mutagenesis suggest a possible HuR redox sensing mechanism.

Hu-antigen R (HuR) is a ubiquitous RNA-binding protein that comprises three RNA recognition motifs (RRMs). The first two tandem RRMs are known to bind to AU-rich elements (AREs) in the 3'-untranslated region of many mRNAs. The third RRM is connected to the second RRM through a basic hinge region that contains a localization signal termed HuR nucleocytoplasmic shuttling. Binding of HuR to the ARE in the 3'-untranslated region of mRNA leads to nuclear export, stabilization, and/or translational de-repression of the mRNA, resulting in upregulation of the encoded protein. Among the various ARE binding proteins known to date, HuR is still the only known ubiquitous antagonist of posttranscriptional gene silencing by AREs. Given the wide repertoire of known and suspected targets of HuR, it is considered to be a central node in the ARE pathway. Here, the x-ray crystal structure of the first RRM of HuR (amino acids 18-99) at 2.0 A resolution is presented. The overall fold consists of two alpha-helices and a four-stranded beta-sheet, with a beta1-alpha1-beta2-beta3-alpha2-beta4 topology and a beta-hairpin between alpha2 and beta4. The asymmetric unit consists of four chains. The large crystal contact interfaces observed between chains A/B and C/D contain hydrophobic residues located at the alpha-helix side of the fold, opposite to the RNA-binding interface. This hydrophobic region structurally resembles the protein-protein interaction site of RRM domains of other proteins. Because the nature of the assumed HuR homodimerization is mechanistically not well understood to date, we used site-directed mutagenesis, analytical size-exclusion chromatography and multiangle light scattering to investigate HuR interactions via the RRM hydrophobic region. Our data indicate that in vitro, HuR RRM1 and RRM1,2 homodimerization involves a disulfide bond at cysteine 13. This homodimerization mode may have a functional significance in redox modulation of HuR activity in response to oxidative stress. Because HuR is involved in many diseases (e.g., cancer, cachexia, and inflammatory bowel disease), the presented structure may provide a basis for rational drug design.

Rapid and easy thermodynamic optimization of the 5'-end of mRNA dramatically increases the level of wild type protein expression in Escherichia coli.

Low levels of expression in Escherichia coli are often observed when using wild type proteins. The addition of an N-terminal His-tag to these same proteins dramatically improves the level of expression. We therefore concluded that post-transcriptional regulation and in particular translational regulation are probably influenced by the presence of the tag. The RNAfold program was used to analyze the 5'-end of the encoding mRNA, and more precisely the area encompassing the Shine-Dalgarno region and the initiation codon ATG. We observed that hairpin loops can be formed and that the stability of these loops correlates with the level of protein expression in E. coli. Our recently developed cloning technology by PCR fragment integration allows us to easily and rapidly introduce mutations anywhere within a gene. In our studies, we used this technology to destabilize the predicted hairpin by introducing silent mutations within the first 72 nucleotides of the coding sequence. As a result of the decreased stability of the RNA hairpins, we could significantly increase the level of expression of wild type proteins and without having to rely on the use of tags in E. coli. In addition, our studies allow us to predict whether or not a protein will be expressed without additional engineering of its encoding gene.

Engineering and functional immobilization of opioid receptors.

Opioid receptors, like many G protein-coupled receptors (GPCRs), are notoriously unstable in detergents. We have now developed a more stable variant of the mu-opioid receptor (MOR) and also a method for the immobilization of solubilized, functional opioid receptors on a solid phase (magnetic beads). Starting with the intrinsically more stable kappa-opioid receptor (KOR), we optimized the conditions (i.e. detergents and stabilizing ligands) for receptor extraction from lipid bilayers of HEK293T cells to obtain maximal amounts of functional, immobilized receptor. After immobilization, the ligand binding profile remains the same as observed for the membrane-embedded receptor. For the immobilized wild-type mu-opioid receptor, however, no conditions were found under which ligand binding capacity was retained. To solve this problem, we engineered the receptor chimera KKM where the N-terminus and the first transmembrane helix (TM1) of wild-type MOR is exchanged for the homologous receptor parts of the wild-type KOR. This hybrid receptor behaves exactly as the wild-type MOR in functional assays. Interestingly, the modified MOR is expressed at six times higher levels than wild-type MOR and is similarly stable as wild-type KOR after immobilization. Hence the immobilized MOR, represented by the chimera KKM, is now also amenable for biophysical characterization. These results are encouraging for future stability engineering of GPCRs.