The ER-SURF pathway uses ER-mitochondria contact sites for protein targeting to mitochondria

Most mitochondrial proteins are synthesized on cytosolic ribosomes and imported into mitochondria in a post-translational reaction. Mitochondrial precursor proteins which use the ER-SURF pathway employ the surface of the endoplasmic reticulum (ER) as an important sorting platform. How they reach the mitochondrial import machinery from the ER is not known. Here we show that mitochondrial contact sites play a crucial role in the ER-to-mitochondria transfer of precursor proteins. The ER mitochondria encounter structure (ERMES) and Tom70, together with Djp1 and Lam6, are part of two parallel and partially redundant ER-to-mitochondria delivery routes. When ER-to-mitochondria transfer is prevented by loss of these two contact sites, many precursors of mitochondrial inner membrane proteins are left stranded on the ER membrane, resulting in mitochondrial dysfunction. Our observations support an active role of the ER in mitochondrial protein biogenesis. ER-mitochondria contact sites are essential for the import of hydrophobic mitochondrial precursor proteins to the inner mitochondrial membrane via the ER-SURF pathway.Mitochondrial precursors use ER-mitochondria contact sites for targeting. ERMES and Tom70 serve as alternative transfer structures for precursor proteins. Hydrophobic proteins of the inner membrane are the predominant ER-SURF clients. ER-mitochondria contact sites are essential for the import of hydrophobic mitochondrial precursor proteins to the inner mitochondrial membrane via the ER-SURF pathway.