Human mtRF1 terminates COX1 translation and its ablation induces mitochondrial ribosome-associated quality control

Abstract

Translation termination requires release factors that read a STOP codon in the decoding center and subsequently facilitate the hydrolysis of the nascent peptide chain from the peptidyl tRNA within the ribosome. In human mitochondria eleven open reading frames terminate in the standard UAA or UAG STOP codon, which can be recognized by mtRF1a, the proposed major mitochondrial release factor. However, two transcripts encoding for COX1 and ND6 terminate in the non-conventional AGA or AGG codon, respectively. How translation termination is achieved in these two cases is not known. We address this long-standing open question by showing that the non-canonical release factor mtRF1 is a specialized release factor that triggers COX1 translation termination, while mtRF1a terminates the majority of other mitochondrial translation events including the non-canonical ND6. Loss of mtRF1 leads to isolated COX deficiency and activates the mitochondrial ribosome-associated quality control accompanied by the degradation of COX1 mRNA to prevent an overload of the ribosome rescue system. Taken together, these results establish the role of mtRF1 in mitochondrial translation, which had been a mystery for decades, and lead to a comprehensive picture of translation termination in human mitochondria.

Molecular basis of translation termination at noncanonical stop codons in human mitochondria

“…These results show that mtRF1 is the release factor responsible for the recognition of noncanonical stop codons. This conclusion is supported by the work of two independent research groups who published their findings during the revision of this manuscript ( 25 , 29 ). However, these results also raise the question of how the unusual codon-recognition domain of mtRF1 accomplishes this task.…”

“…The observed residual production of COX1 in the absence of mtRF1 (Fig. 1C) is likely because of the activity of rescue factor mtRF-R ( 20 , 25 ). Deletion of mtRF-R or ICT1 resulted in marked and general reduction of mitochondrial protein synthesis (fig.…”

“…Several in vitro studies did not detect binding of mtRF1 to ribosomes programmed with AGA or AGG codons ( 13 , 15 , 17 , 19 , 24 ). Recently, mtRF1 has been shown to participate in the translation termination of COX1 but not of ND6; however, the mechanistic role of the factor in translation termination remained elusive ( 25 ).…”

Cytochrome c oxidase biogenesis – from translation to early assembly of the core subunit COX1

“…Three studies reported that mtRF1 is specifically responsible for translation termination of COX1, and thus for the biogenesis of complex IV (Fig. 1, left side) [8][9][10]. Loss of mtRF1 leads to a selective reduction in newly synthesized COX1 and to isolated complex IV deficiency.…”

“…However, as these patients with usually undetectable levels of C12ORF65 display a general translation defect, mtRQC mediated by C12ORF65 seems to be generally involved in ribosome rescue and not selectively responsible for the release of stalled COX1‐translating ribosomes. Interestingly, the reduction in COX1 translation termination due to mtRF1 ablation specifically reduces the level of COX1 mRNA while all other mitochondrial transcripts remained unaffected [9,10]. The specific reduction in COX1 mRNA might reflect a feedback mechanism that prevents an overload of the mtRQC if mtRF1 is missing.…”