Structural study of a small molecule receptor bound to dimethyllysine in lysozymeElectronic supplementary information (ESI) available: Fig. S1: crystals of the lysozyme-KMe2:sclx4 complex grown at different sclx4 concentrations. Fig. S2: crystals of the complex grown in the presence of chloride- and sulfate-containing salts. Fig. S3: 1D 1H NMR spectra of lysozyme-KMe2 in buffer and DMSO mixtures. Table S1: summary of crystallization conditions, data collection and refinement statistics. Movie S1

Lysine is a ubiquitous residue on protein surfaces. Post translational modifications of lysine, including methylation to the mono-, di- or trimethylated amine result in chemical and structural alterations that have major consequences for protein interactions and signalling pathways. Small molecules that bind to methylated lysines are potential tools to modify such pathways. To make progress in this direction, detailed structural data of ligands in complex with methylated lysine is required. Here, we report a crystal structure of p -sulfonatocalix[4]arene (sclx 4 ) bound to methylated lysozyme in which the lysine residues were chemically modified from Lys-NH 3 + to Lys-NH(Me 2 ) + . Of the six possible dimethyllysine sites, sclx 4 selected Lys116-Me 2 and the dimethylamino substituent was deeply buried in the calixarene cavity. This complex confirms the tendency for Lys-Me 2 residues to form cation-π interactions, which have been shown to be important in protein recognition of histone tails bearing methylated lysines. Supporting data from NMR spectroscopy and MD simulations confirm the selectivity for Lys116-Me 2 in solution. The structure presented here may serve as a stepping stone to the development of new biochemical reagents that target methylated lysines. X-ray crystallography reveals how a calixarene can bind to dimethyllysine to form a complex with features similar to the aromatic cage motif of a chromodomain bound to a histone tail.