BRCA1 inhibits MYC:MAX heterodimerization by modulating the availability of transient MAX monomers

The breast cancer susceptibility protein 1 (BRCA1) plays a pivotal role in inhibiting the transcriptional activity of the proto-oncogenic intrinsically disordered protein MYC. By direct binding to MYC, BRCA1 blocks its activation by the MYC-associated factor X (MAX) and the formation of the vital MYC:MAX complex. In contrast, BRCA1 is not associated with binding to the MAX. In this study, we demonstrate a previously undisclosed influence of BRCA1 on MYC:MAX activity. By employing simultaneously time- and residue-resolved nuclear magnetic resonance (NMR) spectroscopy, integrated with MD simulations and EPR spectroscopy, we provide evidence for two subtle regulatory mechanisms: 1. BRCA1 competes with MAX:MAX dimers for DNA ligands, which leads to a destabilization of the native DNA-bound form of the homodimer. 2. BRCA1 binds not only MYC to impede its heterodimerization and transcriptional activity, but it also occupies MAX, when dissociated into its uncommon monomeric form. This conformation is an intermediate that occurs transiently before heterodimerization with MYC. We demonstrate that the MAX monomer, much like MYC, forms highly dynamic complexes with BRCA1, which efficiently block both interaction partners. This discovery is rationalized by the fact that MYC and MAX monomers both lack a stable secondary and tertiary structure (so-called intrinsically disordered proteins) but display high sequence similarity. Both the abovementioned mechanisms balance each other. While the competition for DNA ligands (mechanism 1) promotes MYC:MAX formation, occupation of MAX (and MYC) by BRCA1 (mechanism 2) impedes it. Under the in-vitro conditions probed herein, i.e. , an excess of BRCA1, the latter yet dominates, slowing down the MYC:MAX dimerization event and, thus, providing a potential mechanism for downregulation of its transcriptional efficacy. Since the deregulation of BRCA1 activity is directly linked to hereditary breast and ovarian cancer, our findings might open unconventional routes toward novel prevention strategies.