Deciphering molecular specificity in MCL-1/BAK interaction and its implications for designing potent MCL-1 inhibitors

The intricate interplay among BCL-2 family proteins governs mitochondrial apoptosis, with the anti-apoptotic protein MCL-1 primarily exerting its function by sequestering the pore-forming effector BAK. Understanding the MCL-1/BAK complex is pivotal for the sensitivity of cancer cells to BH3 mimetics, yet the precise molecular mechanism underlying their interaction remains elusive. Herein, we demonstrate that a canonical BH3 peptide from BAK inadequately binds to MCL-1 proteins, whereas an extended BAK-BH3 peptide with five C-terminal residues exhibits a remarkable 65-fold increase in affinity. By elucidating the complex structures of MCL-1 bound to these two BAK-BH3 peptides at 2.08 Å and 1.98 Å resolutions, we uncover their distinct binding specificities. Notably, MCL-1 engages in critical hydrophobic interactions with the extended BAK-BH3 peptide, particularly at an additional p5 sub-pocket, featuring a π-π stacking interaction between MCL-1 Phe319 and BAK Tyr89. Mutations within this p5 sub-pocket substantially disrupt the MCL-1/BAK protein-protein interaction. Furthermore, the p5 sub-pocket of MCL-1 significantly influences the efficacy of MCL-1 inhibitors. Overall, our findings elucidate the molecular specificity underlying MCL-1 binding to BAK and underscore the significance of the p5 hydrophobic sub-pocket in their high-affinity interaction, thus providing novel insights for the development of BH3 mimetics targeting the MCL-1/BAK interaction as potential therapeutics for cancer treatment.