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Mechanism of membrane-tethered mitochondrial protein synthesis

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Science  19 Feb 2021:
Vol. 371, Issue 6531, pp. 846-849
DOI: 10.1126/science.abe0763

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Making the energy makers

Within a mitochondrion, the powerhouse of eukaryotic cells, synthesis of the specialized transmembrane proteins of the electron transport chain is performed by dedicated mitoribosomes. The mechanism by which mitoribosomes couple protein synthesis with membrane insertion is poorly understood. Itoh et al. determined structures of the human mitoribosome during nascent chain synthesis while bound to its membrane insertase. These structures revealed a series of coordinated conformational changes within the polypeptide exit tunnel. The gating mechanism offers a fundamental molecular insight into how membrane proteins are synthesized in human mitochondria.

Science, this issue p. 846

Abstract

Mitochondrial ribosomes (mitoribosomes) are tethered to the mitochondrial inner membrane to facilitate the cotranslational membrane insertion of the synthesized proteins. We report cryo–electron microscopy structures of human mitoribosomes with nascent polypeptide, bound to the insertase oxidase assembly 1–like (OXA1L) through three distinct contact sites. OXA1L binding is correlated with a series of conformational changes in the mitoribosomal large subunit that catalyze the delivery of newly synthesized polypeptides. The mechanism relies on the folding of mL45 inside the exit tunnel, forming two specific constriction sites that would limit helix formation of the nascent chain. A gap is formed between the exit and the membrane, making the newly synthesized proteins accessible. Our data elucidate the basis by which mitoribosomes interact with the OXA1L insertase to couple protein synthesis and membrane delivery.

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