Mitochondrial protein aggregates recruit key chaperones and are sequestered in a fission- and fusion-dependent process.

The potential proteotoxicity of mitochondrial aggregates in yeast cells is reduced by a sequestration of affected polypeptides into a mitochondrial protein quality control compartment (IMiQ). Based on the expression of an aggregation-prone protein in the mitochondrial matrix, we determined the effect of organelle dynamics on aggregate sequestration. Fusion-deficient cells were unable to accumulate the aggregates in the IMiQ, resulting in a stress-sensitive phenotype. In contrast, fission-deficient cells could not separate the aggregate from the mitochondrial network. In these mitochondria, the aggregates were neutralized by the formation of a shell formed by mitochondrial chaperones. We also performed quantitative mass spectrometry to analyse the mitochondrial proteome and the extent of co-aggregation of mitochondrial proteins. Although only minor changes of the total proteome were detected in response to aggregate accumulation, we found a recruitment of proteins of the respiratory chain complexes and of the protein quality control system (PQC). In particular, members of the Hsp70 chaperone family were prominently associated with the aggregate. We conclude that this chaperone-dependent neutralization prevents a major co-aggregation of endogenous mitochondrial proteins.