The mitochondrial intermembrane space - a permanently proteostasis-challenged compartment

The mitochondrial intermembrane space (IMS) houses proteins essential for redox regulation, protein import, signaling, and energy metabolism. Protein import into the IMS is mediated by dedicated pathways, including the disulfide relay pathway for oxidative folding. In addition, various IMS-traversing import pathways potentially expose unfolded proteins, representing threats to proteostasis. This trafficking of precursors coincides with unique biophysical challenges in the IMS, including a confined volume, elevated temperature, variable pH and high levels of reactive oxygen species. Ultrastructural properties and import supercomplex formation ameliorate these challenges. Nonetheless, IMS proteostasis requires constant maintenance by chaperones, folding catalysts, and proteases to counteract misfolding and aggregation. The IMS plays a key role in stress signaling, where proteostasis disruptions trigger responses including the integrated stress response (ISR) activated by mitochondrial stress (ISRmt) and responses to cytosolic accumulation of mitochondrial protein precursors. This review explores the biology and mechanisms governing IMS proteostasis, presents models, which have been employed to decipher IMS-specific stress responses, and discusses open questions.