The mitochondrial unfolded protein response: Signaling from the powerhouse

Abstract

Edited by Ruma BanerjeeMitochondria are multifaceted and indispensable organelles required for cell performance. Accordingly, dysfunction to mitochondria can result in cellular decline and possibly the onset of disease. Cells use a variety of means to recover mitochondria and restore homeostasis, including the activation of retrograde pathways such as the mitochondrial unfolded protein response (UPR mt ). In this Minireview, we will discuss how cells adapt to mitochondrial stress through UPR mt regulation. Furthermore, we will explore the current repertoire of biological functions that are associated with this essential stress-response pathway.Mitochondria are double membrane organelles commonly associated with the production of cellular energy via oxidative phosphorylation (OXPHOS).2 Mitochondria are also required for the metabolism of nucleotides, amino acids, and lipids and have an essential function in regulating apoptosis. Maintaining mitochondrial integrity is therefore a key aspect in ensuring cellular and organismal viability. Consequently, a decline in mitochondrial function is frequently associated with the development of numerous diseases (1).Mitochondria are dependent on a diverse compilation of proteins to carry out their vital functions. However, the mitochondrial proteome is faced with various challenges, most notably the partitioning of protein encoding genes between the mitochondrial and nuclear genomes. Remarkably, the human mitochondrial genome only encodes ϳ1% of the total mitochondrial proteome with the remaining proteins being encoded by nuclear genes (2). Sophisticated mechanisms have evolved to efficiently transfer nuclear-encoded mitochondrial proteins to their proper organelle destination following translation on cytosolic ribosomes. To accomplish this complex task, proteins are sorted to mitochondria via targeting sequences that form characteristic amphipathic helices composed of positively charged residues. Such mitochondrial targeting sequences (MTS) are recognized and translocated via the mitochondrial Tom-Tim complex to their respective sub-organelle compartment (3). Exquisite coordination of expression between the mitochondrial and nuclear genomes exists during mitochondrial biogenesis, as failure in genome coordination can disrupt the precise stoichiometry of these OXPHOS complexes resulting in orphan subunit accumulation and proteotoxicity (4). Further contributing to mitochondrial proteotoxicity is the possible damage to the mitochondrial genome from reactive oxygen species (ROS) produced by OXPHOS machinery as well as the ill effects of various environmental toxins. Mechanisms must therefore exist to ensure the protection of the mitochondrial proteome.Quality control of the mitochondrial proteome includes the functions of mitochondrial chaperones that assist in proper protein folding and proteases that promote clearance of misfolded proteins (5). Each sub-compartment of mitochondria houses its own quality control machinery to ensure protein homeostasis and organelle function. ...

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