Designing small-molecule switches for protein-protein interactions.

Mutations introduced into human growth hormone (hGH) (Thr175 --> Gly-hGH) and the extracellular domain of the hGH receptor (Trp104 --> Gly-hGHbp) created a cavity at the protein-protein interface that resulted in binding affinity being reduced by a factor of 10(6). A small library of indole analogs was screened for small molecules that bind the cavity created by the mutations and restore binding affinity. The ligand 5-chloro-2-trichloromethylimidazole was found to increase the affinity of the mutant hormone for its receptor more than 1000-fold. Cell proliferation and JAK2 phosphorylation assays showed that the mutant hGH activates growth hormone signaling in the presence of added ligand. This approach may allow other protein-protein and protein-nucleic acid interactions to be switched on or off by the addition or depletion of exogenous small molecules.