Proteostasis at the mitochondrial outer membrane: Quality control of mitochondrial protein transport

Mitochondria are enclosed by a double-membrane structure composed of the outer and inner membranes, and this architectural organization underlies their diverse cellular functions. In particular, the mitochondrial outer membrane serves as an essential interface between the cytosol and the mitochondrial interior, regulating the flux of proteins, lipids, small molecules, and ions through the coordinated activities of its resident proteome. Consequently, structural and functional defects of outer membrane proteins are subject to continuous surveillance, and aberrant proteins are rapidly recognized and degraded. Defects in precursor translocation or translation can lead to the stalling of precursor proteins at the primary protein import gate, the TOM complex. Such situations are resolved by multiple quality control systems operating across both the mitochondria and the cytosol. In addition, proteins normally destined for the endoplasmic reticulum or peroxisomes may be mistargeted to mitochondria, and these mislocalized proteins are likewise managed through dedicated mechanisms that promote their degradation or re-targeting. In this review, we summarize current insights into the molecular factors and mechanisms that maintain proteostasis at the mitochondrial outer membrane.