Interaction of the Intrinsically Disordered c-Myc Oncoprotein with Racemic and Enantiopure Small Molecules

Abstract

The prevalence of intrinsically disordered proteins (IDPs) in cell signaling and disease makes them significant targets. Despite the absence of defined tertiary structure, small molecules can bind IDPs at sites determined by a short, linear segment of the protein's primary sequence. The oncoprotein c-Myc, a transcription factor that must undergo coupled folding and binding to its obligate partner Max in order to interact with DNA, is an ideal system for understanding specificity in small-molecule binding to IDPs. Three small molecule interaction sites exist in the bHLHZip region of c-Myc, the segment necessary for coupled folding and binding to Max. The chiral small molecule 10074-A4 interacts with one of these sites (Myc372-389). The presence of Myc372-389 induces small molecule circular dichroism of racemic 10074-A4, indicating an enantiospecific interaction. We have synthesized the pure R and S enantiomers of 10074-A4 (as well as pure enantiomers of derivatives) and found that at 10 uM and above the compound can undergo a transition upon addition to water from an aggregate, to a dispersed molecule, to an assembled chiral complex with a strong CD signature. SPR measurements indicate interaction between Myc and 10074-A4. These data suggest multiple possible binding modes. We also report the hydrodynamic radius of the bHLHZip region of c-Myc, as determined by fluorescence correlation spectroscopy and dynamic light scattering, under different conditions including in the presence of various small molecules and c-Myc's obligate partner Max.