B. with low nanomolar affinity for other therapeutic antibodies. Keywords: antibody, cyclic peptide, directed evolution, epidermal growth factor receptor (EGFR), protein engineering == Introduction == The development of new antibody conjugation strategies is a fast expanding field with applications in targeted drug delivery and tumor imaging. Classical non-site-specific coupling of drugs or dyes to primary amines or thiols carries the disadvantage of generating a heterogeneous mixture of antibodies with different numbers of attached molecules as well as different sites of attachment. Therefore , in recent years, several interesting new types of biorthogonal chemistry have been explored for antibody conjugation (13). These include the ribosomal incorporation of non-canonical amino acids, chemo-enzymatic conjugation of payloads to the conservedN-glycan of native mAbs (4), the use of SNAP tags and other self-labeling protein and peptide tags (5, 6), or light-controlled covalent attachment of protein G (7, 8). In addition to these covalent antibody conjugation strategies, Donaldsonet al. (9) developed a non-covalent affinity tag for therapeutic antibodies, which they coined a meditope (10). A meditope is a peptide that binds in a large pocket between the four immunoglobulin domains in the antigen-binding fragment (Fab)2of antibodies (Fig. 1A). They identified a disulfide-constrained cyclic peptide (sequence, CQFDLSTRRLKC) that binds in this manner to the therapeutic anti-epidermal growth factor receptor (EGFR) antibody cetuximab (trade name, ErbituxTM). Because the binding site that this meditope recognizes is distinct from the pockets found in natural human antibodies, the tag serves as a specific non-covalent and paratope-independent handle that can be used to specifically bind cetuximab. Intriguingly, the site could be grafted onto another humanized therapeutic antibody (trastuzumab) by mutating 13 key interacting residues into their cetuximab counterparts. == FIGURE 1 . == Improving cyclic peptide binding to cetuximab using yeast display. A, structure of the Fab fragment of cetuximab indicating the position of the meditope in the central cavity. B, schematic representation of the yeast display system. Simultaneous labeling of the HA tag and cetuximab with different fluorophores allows selection for binding to be normalized for variations in expression. At present, many of the potential applications for the cetuximab meditope peptide are limited by its moderate affinity. The original Vasopressin antagonist 1867 meditope was identified in a phage display screen of random 10-mer peptides flanked by cysteines (11). As the sequence space of such a library is 106times larger than the library that was used (12), systematic exploration of the local sequence space is likely to reveal affinity enhancing mutations. Yeast surface display (Fig. 1B) is Vasopressin antagonist 1867 particularly suitable for such affinity maturation (13). In contrast toin vitromolecular display technologies such as phage- or ribosome display, which require panning as a means of selection, Fluorescence Activated Cell Sorting (FACS) is used in yeast display, which offers more refined control of the selection pressure (14). Yeast display is also significantly less vulnerable to unintended selection of the most infectious or fastest-replicating imitations (15). Finally, the candida secretory pathway acts as a filtration system, ensuring that just well folded away full-length healthy proteins and peptides will be exhibited (16). Since subtle affinity enhancements might escape recognition in traditional screens to get a select volume of hit sequences, recently high-throughput protein display technologies have already been used in combination with deep sequencing before and after selection, a combination loosely called deep mutational scanning (1719). This strategy enables the construction of fitness scenery of the chosen property (i. e. affinity) in which delicate effects of person mutations could be discerned by the degree that they are Vasopressin antagonist 1867 enriched or exhausted by the assortment. Deep mutational scanning has Vasopressin antagonist 1867 become applied effectively in combination with computational protein style for the engineering of enzyme inhibitors (20) and antibody joining proteins (21), for the evolution of antibodies (2224) and T-cell receptors (25, 26), meant for epitope mapping (27, 28), for executive immune co-receptors (29), as well Vasopressin antagonist 1867 as establishing proteins structure-function human relationships (30, 31). In this function, we utilized yeast display and deep mutational deciphering to improve the affinity with the meditope peptide for the therapeutic antibody cetuximab simply by screening most single and a select group of double alanine substitutions. The TNFAIP3 consequence of these variations were quantified using fluorescence anisotropy titration experiments, as well as the best version was examined for superior cancer cell targeting. The experimental strategy was likewise compared inside silicomutagenesis with Rosetta using the crystal framework of the meditope-cetuximab complex to determine whether delicate affinity-enhancing variations could also be diagnosed computationally. == Results == == == == == == Display of the Cetuximab Meditope for the Yeast Cell Surface ==.