MitoStores: stress-induced aggregation of mitochondrial proteins

Most mitochondrial proteins are synthesized in the cytosol and post-translationally imported into mitochondria. If the rate of protein synthesis exceeds the capacity of the mitochondrial import machinery, precursor proteins can transiently accumulate in the cytosol. The cytosolic accumulation of mitochondrial precursors jeopardizes cellular protein homeostasis (proteostasis) and can be the cause of diseases. In order to prevent these toxic effects, most non-imported precursors are rapidly degraded by the ubiquitin-proteasome system. However, cells employ a second layer of defense which is the facilitated sequestration of mitochondrial precursor proteins in transient protein aggregates. The formation of such structures is triggered by nucleation factors such as small heat shock proteins. Disaggregases and chaperones can liberate precursors from cytosolic aggregates to pass them on to the mitochondrial import machinery or, under persistent stress conditions, to the proteasome for degradation. Owing to their role as transient buffering systems, these aggregates were referred to as MitoStores. This review articles provides a general overview about the MitoStore concept and the early stages in mitochondrial protein biogenesis in yeast and, in cases where aspects differ, in mammalian cells.