Semisynthetic cyclic peptides containing both non-proteinogenic building blocks, as the synthetic part, and a genetically encoded sequence amenable to DNA-based randomization hold great potential to expand the chemical space in the quest for novel bioactive peptides. Key to an efficient selection of novel binders to biomacromolecules is a robust method to link their genotype and phenotype. A novel bacterial cell surface display technology has been developed to present cyclic peptides composed of synthetic and genetically encoded fragments in their backbones. The fragments were combined by protein trans-splicing and intramolecular oxime ligation. To this end, a split intein half and an unnatural amino acid were displayed with the genetically encoded part on the surface of Escherichia coli. Addition of the synthetic fragment equipped with the split intein partner and an aminooxy moiety, as well as the application of a pH-shift protocol, resulted in the onsurface formation of the semisynthetic cyclic peptide. This approach will serve for the generation of cyclic peptide libraries suitable for selection by fluorescence-activated cell sorting, and more generally enables chemical modification of proteins on the bacterial surface.
The cover feature picture shows a modular connector approach for presenting semisynthetic cyclic peptides on the cell surface of E. coli. Cyclization is achieved upon addition of a synthetic peptide to live cells expressing a recombinant fragment through protein trans‐splicing and oxime ligation. The mild reaction conditions preserve cellular viability and allow sorting by fluorescent‐activated cell sorting (FACS). This unnatural bacterial‐display format holds potential for applications in drug discovery by directed evolution of diverse peptide macrocycles. More information can be found in the communication by H. D. Mootz et al. on page 72 in Issue 1, 2019 (DOI: 10.1002/cbic.201800552).
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