N‑TerminalProtein Binding and Disorder-to-OrderTransition by a Synthetic Receptor

We describe the capture and structuring of disordered N-terminal regions by the macrocycle sulfonato-calix[4]arene (<b>sclx</b>_<b>4</b>). Using the trimeric β-propeller Ralstonia solanacearum lectin (RSL) as a scaffold, we generated a series of mutants with extended and dynamic N-termini. Three of the mutants feature an N-terminal methionine-lysine motif. The fourth mutant contains the disordered 8-residue N-terminus of Histone 3, a component of the nucleosome. X-ray crystallography and NMR spectroscopy provide evidence for <b>sclx</b>_<b>4</b> binding to the flexible N-terminal regions. Three crystal structures reveal that the calixarene recognizes the N-terminal Met-Lys motif, capturing either residue. We provide crystallographic proof for <b>sclx</b>_<b>4</b> encapsulation of N-terminal methionine. Calixarene capture of intrinsically disordered regions may have applications in regulating protein secondary (and tertiary) structure.