Mitochondrial protein synthesis quality control

Abstract

Human mitochondrial DNA is one of the most simplified cellular genomes and facilitates compartmentalized gene expression. Within the organelle, there is no physical barrier to separate transcription and translation, nor is there evidence that quality control surveillance pathways are active to prevent translation on faulty mRNA transcripts. Mitochondrial ribosomes synthesize 13 hydrophobic proteins that require co-translational insertion into the inner membrane of the organelle. To maintain the integrity of the inner membrane, which is essential for organelle function, requires responsive quality control mechanisms to recognize aberrations in protein synthesis. In this review, we explore how defects in mitochondrial protein synthesis can arise due to the culmination of inherent mistakes that occur throughout the steps of gene expression. In turn, we examine the stepwise series of quality control processes that are needed to eliminate any mistakes that would perturb organelle homeostasis. We aim to provide an integrated view on the quality control mechanisms of mitochondrial protein synthesis and to identify promising avenues for future research.

Keywords: AFG3L2; MTRFR; OMA1; OPA1; OXA1L; RNA processing; cell stress; co-translational quality control; fusion open reading frames; membrane morphology; mitochondria; non-stop mRNA; post-transcriptional; protein synthesis; proteostasis; ribosome quality control; ribosomes.

Figure 1

Overview of mitochondrial gene expression. (A) A map of human mitochondrial DNA. The two circles represent the heavy (H-strand) and light (L-strand) of the genome with corresponding genetic content colour coded for the oxidative phosphorylation complex, rRNA and tRNA. Mitochondrial tRNA genes are indicated by a single letter abbreviation. Arrows indicate the positions of the major promoter. HSP, H-strand promoter. LSP, L-strand promoter. (B) A table indicating the 3′ non-coding end of the coding sequence (CDS) for the indicated human mitochondrial mRNAs. Those transcripts that do not encode a stop codon are highlighted with a box. (C) A schematic of mitochondrial gene expression from transcription to translation and insertion of nascent chains into the inner membrane. Mitochondrial RNA processing and post-transcriptional events occur in RNA granules, which is shaded. No physical barrier prevents translation initiation on aberrant mRNAs. Image created with BioRender.com.

Figure 2

Translation termination of mitochondrial protein synthesis for wild type and aberrant mRNAs. (A) A schematic of translation termination with an mRNA encoding a stop codon. The mitochondrial class I release factor MTRF1L structurally recognizes the stop codon in the mRNA [31]. (B) A model for translation termination on truncated nonstop mRNAs based upon a working model for the class I release factor MTRFR that is based upon that developed for the bacterial rescue factor ArfB. (C) A model for inducing translation termination when cellular stress induces mitochondrial ribosome stalling in the middle of the mRNA. Image created with BioRender.com.

Figure 3

Membrane-associated quality control of mitochondrial protein synthesis and ensuing stress responses. (A) Wild type setting, OXA1L interacts co-translationally with mitochondrial ribosomes to mediate nascent chain insertion into the inner membrane. Nascent chains that misfold are rapidly degraded by the AFG3L2 protease complex. (B) Defects in nascent chain insertion via OXA1L leads to protein misfolding followed by rapid degradation by the AFG3L2 protease complex. (C) Short-term AFG3L2 dysfunction results in the accumulation of nascent chains, which triggers a proteotoxic stress response on the inner membrane. This activates the OMA1 protease, which cleaves the membrane anchored OPA1 to remodel first the inner membrane followed by the outer membrane morphology. (D) Progressive AFG3L2 dysfunction maintains the membrane stress, which in turn triggers a mitochondrial ribosome and mRNA decay pathway. This negative feedback response leads to the overall reduction of oxidative phosphorylation complexes with time. Image created with BioRender.com.