mRNA Display in Cell Lysates Enables Identification of Cyclic Peptides Targeting the BRD3 Extraterminal Domain.

mRNA display is a powerful technology to screen libraries of >1012 cyclic peptides against a protein target, enabling the rapid discovery of high affinity ligands. These cyclic peptides are particularly well suited to challenging protein targets that have been difficult to drug with small molecules. However, target choice can still be limited as screens are typically performed against purified proteins which often demands the use of isolated domains and precludes the use of aggregation-prone targets. Here, we report a method to perform mRNA display selections in mammalian cell lysates without the need for prior target purification, vastly expanding the potential target scope of mRNA display. We have applied the methodology to identify low to sub-nanomolar peptide binders for two targets: a NanoLuc subunit (LgBiT) and full-length bromodomain-containing protein 3 (BRD3). Our cyclic peptides for BRD3 were found to bind to the extraterminal (ET) domain of BRD3 and the closely related BRD proteins, BRD2 and BRD4. While many chemical probes exist for the bromodomains of BRD proteins, the ET domain is relatively underexplored, making these peptides valuable additions to the BRD toolbox.

Affinity maturation of the RLIP76 Ral binding domain to inform the design of stapled peptides targeting the Ral GTPases.

Ral GTPases have been implicated as critical drivers of cell growth and metastasis in numerous Ras-driven cancers. We have previously reported stapled peptides, based on the Ral effector RLIP76, that can disrupt Ral signaling. Stapled peptides are short peptides that are locked into their bioactive form using a synthetic brace. Here, using an affinity maturation of the RLIP76 Ral-binding domain, we identified several sequence substitutions that together improve binding to Ral proteins by more than 20-fold. Hits from the selection were rigorously analyzed to determine the contributions of individual residues and two 1.5 Å cocrystal structures of the tightest-binding mutants in complex with RalB revealed key interactions. Insights gained from this maturation were used to design second-generation stapled peptides based on RLIP76 that exhibited vastly improved selectivity for Ral GTPases when compared with the first-generation lead peptide. The binding of second-generation peptides to Ral proteins was quantified and the binding site of the lead peptide on RalB was determined by NMR. Stapled peptides successfully competed with multiple Ral-effector interactions in cellular lysates. Our findings demonstrate how manipulation of a native binding partner can assist in the rational design of stapled peptide inhibitors targeting a protein-protein interaction.

A QM/MM study of the nature of the entatic state in plastocyanin.

Plastocyanin is a copper containing protein that is involved in the electron transfer process in photosynthetic organisms. The active site of plastocyanin is described as an entatic state whereby its structure represents a compromise between the structures favored by the oxidized and reduced forms. In this study, the nature of the entatic state is investigated through density functional theory-based hybrid quantum mechanics/molecular mechanics (QM/MM) molecular dynamics simulations. The strain energy is computed to be 12.8 kcal/mol and 14.5 kcal/mol for the oxidized and reduced forms of the protein, indicating that the active site has an intermediate structure. It is shown that the energy gap between the oxidized and reduced forms varies significantly with the fluctuations in the structure of the active site at room temperature. An accurate determination of the reorganization energy requires averaging over conformation and a large region of the protein around the active site to be treated at the quantum mechanical level. © 2016 The Authors. Journal of Computational Chemistry Published by Wiley Periodicals, Inc.