Bicyclic Peptide Library Screening for the Identification of Gαi Protein Modulators.

Noncanonical G protein activation and inactivation, particularly for the Gαi/s protein subfamilies, have long been a focus of chemical research. Combinatorial libraries were already effectively applied to identify modulators of the guanine-nucleotide exchange, as can be exemplified with peptides such as KB-752 and GPM-1c/d, the so-called guanine-nucleotide exchange modulators. In this study, we identified novel bicyclic peptides from a combinatorial library screening that show prominent properties as molecular switch-on/off modulators of Gαi signaling. Among the series of hits, the exceptional paradigm of GPM-3, a protein and state-specific bicyclic peptide, is the first chemically identified GAP (GTPase-activating protein) modulator with a high binding affinity for Gαi protein. Computational analyses identified and assessed the structure of the bicyclic peptides, novel ligand-protein interaction sites, and their subsequent impact on the nucleotide binding site. This approach can therefore lead the way for the development of efficient chemical biological probes targeting Gαi protein modulation within a cellular context.