N-Terminal Protein Binding and Disorder-to-Order Transition by a Synthetic Receptor.

We describe the capture and structuring of disordered N-terminal regions by the macrocycle sulfonato-calix[4]arene (sclx(4)). 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 sclx(4) 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 sclx(4) encapsulation of N-terminal methionine. Calixarene capture of intrinsically disordered regions may have applications in regulating protein secondary (and tertiary) structure.