Obstacles overcome: Gene‐specific transcription factors are emerging therapeutic targets, but their direct inhibition is considered to be difficult because of the absence of enzymatic activities. This manuscript describes the discovery of chromone‐based inhibitors of the protein–protein interactions required for activity of the transcription factor STAT5, a therapeutic target for the treatment of human cancers.

bZip and bHLHZip protein family members comprise a large fraction of eukaryotic transcription factors and need to bind DNA in order to exert most of their fundamental biological roles. Their binding to DNA requires homo- or heterodimerization via alpha-helical domains, which generally do not contain obvious binding sites for small molecules. We have identified two small molecules, dubbed Mycro1 and Mycro2, which inhibit the protein-protein interactions between the bHLHZip proteins c-Myc and Max. Mycros are the first inhibitors of c-Myc/Max dimerization, which have been demonstrated to inhibit DNA binding of c-Myc with preference over other dimeric transcription factors in vitro. Mycros inhibit c-Myc-dependent proliferation, gene transcription, and oncogenic transformation in the low micromolar concentration range. Our data support the idea that dimeric transcription factors can be druggable even in the absence of obvious small-molecule binding pockets.

c‐Myc/Max Dimerization. The interaction between the oncoprotein c‐Myc and its activation partner Max is considered to be a desirable target for cancer drug development. However, the interaction is mediated by large α‐helical interfaces without obvious binding sites for small molecule inhibitors. This paper describes the identification of a substitution pattern for pyrazolopyrimidines which appears to be associated with activity against this challenging target.

The catalytic activity of Src-family kinases is regulated by association with its SH3 and SH2 domains. Activation requires displacement of intermolecular contacts by SH3/SH2 binding ligands resulting in dissociation of the SH3 and SH2 domains from the kinase domain. To understand the contribution of the SH3-SH2 domain pair to this regulatory process, the binding of peptides derived from physiologically relevant SH2 and SH3 interaction partners was studied for Lck and its relative Fyn by NMR spectroscopy. In contrast to Fyn, activating ligands do not induce communication between SH2 and SH3 domains in Lck. This can be attributed to the particular properties of the Lck SH3-SH2 linker which is shown to be extremely flexible thus effectively decoupling the behavior of the SH3 and SH2 domains. Measurements on the SH32 tandem from Lck further revealed a relative domain orientation that is distinctly different from that found in the Lck SH32 crystal structure and in other Src kinases. These data suggest that flexibility between SH2 and SH3 domains contributes to the adaptation of Src-family kinases to specific environments and distinct functions.