Structural remodeling of the mitochondrial protein biogenesis machinery under proteostatic stress.

Cells have evolved organelle-specific responses to maintain protein homeostasis (proteostasis). During proteostatic stress, mitochondria down-regulate translation and enhance protein folding, yet the underlying mechanisms remain poorly defined. Here, we used cryo-electron tomography to observe the structural consequences of mitochondrial proteostatic stress within human cells. We detected protein aggregates within the mitochondrial matrix, accompanied by a marked remodeling of cristae architecture. Concomitantly, the number of mitochondrial ribosome complexes was significantly reduced. Mitochondrial Hsp60 (mHsp60), a key protein folding machine, underwent major conformational changes to favor complexes with its co-chaperone mHsp10. We visualized the interactions of mHsp60 with native substrate proteins and determined in vitro mHsp60 cryo-electron microscopy structures enabling nucleotide state assignment of the in situ structures. These data converge on a model of the mHsp60 functional cycle and its essential role in mitochondrial proteostasis. More broadly, our findings reveal structural mechanisms governing mitochondrial protein biosynthesis and their remodeling under proteostatic stress.