Functions of J‐domain proteins in mitochondrial protein biogenesis

Mitochondrial biogenesis and functions depend on the import and assembly of more than 1000 proteins that are made as precursors on cytosolic ribosomes. The majority of these precursor proteins are transported from the ribosome to the translocase of the outer membrane (TOM complex), which constitutes the main entry site for mitochondrial precursors. The transient localization of mitochondrial precursor proteins in the cytosol represents a major burden for cellular proteostasis since these proteins can aggregate and accumulate in different cellular compartments, causing proteotoxic stress. Inside mitochondria, protein translocases sort the precursor proteins into the mitochondrial subcompartments-outer and inner membrane, the intermembrane space and matrix. The imported proteins have to be folded and efficiently assembled into functional protein complexes. Molecular chaperones such as Hsp70 monitor these processes to minimize proteotoxic stress. J-domain proteins stimulate the ATPase activity of Hsp70 and recruit the chaperones to their clients in the biogenesis of mitochondrial proteins. They ensure protein targeting to mitochondria, drive protein import into mitochondria, as well as folding and assembly of mitochondrial proteins. Here, we summarize the emerging view of how J-domain proteins guide mitochondrial precursor proteins from their synthesis in the cytosol until their folding into a mature protein and assembly into protein complexes in mitochondria.