Mitochondrial dynamics and mtDNA homeostasis have been linked to specialized mitochondrial subdomains known as small MTFP1-enriched mitochondria (SMEM), though the underlying molecular mechanisms remain unclear. Here we identified MISO (mitochondrial inner membrane subdomain organizer), a conserved protein that regulates both mitochondrial dynamics and SMEM formation in Drosophila and mammalian cells. MISO inhibits fusion by recruiting MTFP1 and promotes fission through FIS1-DRP1. Furthermore, MISO drives SMEM biogenesis and facilitates their peripheral fission that promotes lysosomal degradation of mtDNA. Genetic ablation of MISO abolishes SMEM generation, confirming that MISO is both necessary and sufficient for SMEM formation. Inner mitochondrial membrane stresses, including mtDNA damages, OXPHOS dysfunction and cristae disruption, stabilize the otherwise short-lived MISO protein, thereby triggering SMEM assembly. This process depends on the C-terminal domain of MISO, likely mediated by oligomerization. Together, our findings reveal a molecular pathway through which inner mitochondrial membrane stresses modulate mitochondrial dynamics and mtDNA homeostasis via MISO-orchestrated SMEM organization.
