Weak, transient binding of globular protein domains to Short Linear Motifs (SLiMs) in disordered regions of other proteins drive cellular signaling. Mapping the energy landscape of such interactions is essential for deciphering signaling networks and developing therapeutic inhibitors, but is complicated by technical challenges associated with quantitatively measuring weak interactions in high-throughput. Here, we synthesize peptide libraries on spectrally encoded beads with each peptide sequence uniquely linked to a spectral code (MRBLE-pep), allowing high-throughput quantification of protein-peptide binding via imaging. Using computational modeling and MRBLE-pep assays, we map the affinity landscape for human calcineurin (CN), a conserved protein phosphatase essential for the immune response and target of immunosuppressants, interacting with a known SLiM (PxIxIT). We find that PxIxIT recognition depends critically on flanking residues and is regulated by post-translational modifications. Using this information, we designed PxIxIT peptides with unprecedented affinity and therapeutic potential that strongly inhibit CN function in vivo.

Quantitative mapping of protein-peptide affinity landscapes using spectrally encoded beads