Mitochondrial Nascent Chain Quality Control Determines Organelle Form and Function

Proteotoxicity has long been considered a key factor in mitochondrial dysfunction and human disease. The origin of the endogenous offending toxic substrates and the regulatory pathways to deal with these insults, however, have remained unclear. Mitochondria maintain a compartmentalized gene expression system that in animals is only responsible for synthesis of 1% of the organelle proteome. Because of the relatively small contribution of the mitochondrial genome to the overall proteome, the synthesis and quality control of these nascent chains to maintain organelle proteostasis has long been overlooked. However, recent research has uncovered mechanisms by which defects to the quality control of mitochondrial gene expression are linked to a novel cellular stress response that impinges upon organelle form and function and cell fitness. In this review, we discuss the mechanisms for a key event in the response: activation of the metalloprotease OMA1. This severs the membrane tether of the dynamin-related GTPase OPA1, which is a critical determinant for mitochondrial morphology and function. We also highlight the evolutionary conservation from bacteria of these quality-control mechanisms to maintain membrane integrity, gene expression, and cell fitness.