Mutant Preproteins Clog Global Mitochondrial Protein Import to Cause Degenerative Disease

Mitochondrial protein import is essential for mitochondrial function, yet the breadth of physiologically-relevant import stressors is underexplored. Here, we show that mutant mitochondrial preproteins can disrupt global protein import to cause disease. Pathogenic missense mutations in adenine nucleotide translocase 1 (Ant1), and its yeast ortholog Aac2, cause the protein to accumulate along the protein import pathway, thereby obstructing global protein traffic into mitochondria. This impairs mitochondrial respiration, cytosolic proteostasis and cell viability independent of nucleotide transport. We engineered super-clogger Aac2/Ant1 variants that preferentially clog the Translocase of the Outer Membrane (TOM) complex, which confers extreme toxicity in yeast and causes neurodegeneration and a dominant myopathy in mice that phenocopies Ant1-induced disease. These effects are driven by miniscule mutant protein levels across model systems. In sum, we show that a physical incompatibility between a single mitochondrial preprotein and the protein import machinery can robustly derail general protein import to cause disease.