OMA1 protease eliminates arrested protein import intermediates upon depolarization of the inner mitochondrial membrane.

Abstract

Most mitochondrial proteins originate from the cytosol and require active transport into the organelle. Such precursor proteins must be largely unfolded to pass through translocation channels in mitochondrial membranes. Misfolding of transported proteins can result in their arrest and translocation failure. Arrested proteins block further import, disturbing mitochondrial functions and cellular proteostasis. Cellular responses to translocation failure have been defined in yeast. To discover molecular mechanisms that resolve failed import events in human cells, we developed the translocase clogging model using a fusion protein with a rigid domain. The mechanism we uncover differs significantly from these described in fungi, where ATPase-driven extraction of blocked protein is directly coupled with proteasomal processing. We found human cells to rely primarily on mitochondrial factors to clear translocation channel blockage. The mitochondrial membrane depolarization triggered proteolytic cleavage of the stalled protein, which involved mitochondrial protease OMA1. The cleavage allowed releasing the protein fragment that blocked the translocase. The released fragment was further cleared in the cytosol by the valosin containing protein (VCP)/p97 and proteasome.