Anti-apoptotic MCL-1 localizes to the mitochondrial matrix and couples mitochondrial fusion to respiration

MCL-1, an anti-apoptotic BCL-2 family member that is essential for the survival of multiple cell lineages, is also among the most highly amplified genes in cancer. Although MCL-1 is known to oppose cell death, precisely how it functions to promote survival of normal and malignant cells is poorly understood. Here, we report that different forms of MCL-1 reside in distinct mitochondrial locations and exhibit separable functions. On the outer mitochondrial membrane, an MCL-1 isoform acts like other anti-apoptotic BCL-2 molecules to antagonize apoptosis, whereas an amino-terminally truncated isoform of MCL-1 that is imported into the mitochondrial matrix is necessary to facilitate normal mitochondrial fusion, ATP production, membrane potential, respiration, cristae ultrastructure and maintenance of oligomeric ATP synthase. Our results provide insight into how the surprisingly diverse salutary functions of MCL-1 may control the survival of both normal and cancer cells. MCL-1 is an anti-apoptotic BCL-2 family member and is frequently upregulated in cancer, but the mechanism by which it promotes cell survival has been elusive. Opferman and colleagues provide insight into this process by showing that MCL-1 exists in different forms with discrete localizations and functions. MCL-1 variants targeted to the outer mitochondrial membrane antagonize BAX and BAK activation, whereas an N-terminally truncated isoform localizes to the mitochondrial matrix and regulates mitochondrial metabolism.