A ULK1-MTFR1L feedback loop links mitochondrial fission, mitophagy and apoptosis

Mitophagy, the selective degradation of damaged mitochondria, preserves mitochondrial quality, yet how mitochondrial fission is coordinated with autophagy initiation remains unclear. Here we identify the mitochondrial outer membrane protein MTFR1L as a key component of mitophagy initiation hubs after using a synthetic FKBP-FRB system to tether ULK1 kinase to mitochondria independently of damage. We find that MTFR1L is enriched at ULK1 foci together with additional fission factors and constitutive mitochondrial targeting of MTFR1L shifts mitochondrial morphology towards fragmentation. MTFR1L depletion decreases respiratory capacity, elevates apoptosis, and impairs mitophagy flux. Upon mitophagy induction, MTFR1L is phosphorylated in a ULK1 kinase-dependent manner, and reciprocally modulates ULK1 activity, establishing a feedback loop. Moreover, MTFR1L is required for proper ATG13 stability. These findings position MTFR1L as a critical link between mitochondrial fission and the autophagy machinery, coordinating mitophagy initiation and cell survival.