bims-mitran 2022-06-12 papers

Issue of 2022‒06‒12
five papers selected by
Andreas Kohler

Nature. 2022 Jun 08.

Mechanism of mitoribosomal small subunit biogenesis and preinitiation.

Yuzuru Itoh, Anas Khawaja, Ivan Laptev, Miriam Cipullo, Ilian Atanassov, Petr Sergiev, Joanna Rorbach, Alexey Amunts.

Mitoribosomes are essential for the synthesis and maintenance of bioenergetic proteins. Here we use cryo-electron microscopy to determine a series of the small mitoribosomal subunit (SSU) intermediates in complex with auxiliary factors, revealing a sequential assembly mechanism. The methyltransferase TFB1M binds to partially unfolded rRNA h45 that is promoted by RBFA, while the mRNA channel is blocked. This enables binding of METTL15 that promotes further rRNA maturation and a large conformational change of RBFA. The new conformation allows initiation factor mtIF3 to already occupy the subunit interface during the assembly. Finally, the mitochondria-specific ribosomal protein mS37 (ref. 1) outcompetes RBFA to complete the assembly with the SSU-mS37-mtIF3 complex2 that proceeds towards mtIF2 binding and translation initiation. Our results explain how the action of step-specific factors modulate the dynamic assembly of the SSU, and adaptation of a unique protein, mS37, links the assembly to initiation to establish the catalytic human mitoribosome.

DOI: https://doi.org/10.1038/s41586-022-04795-x

Cell. 2022 May 30. pii: S0092-8674(22)00590-6. [Epub ahead of print]

Non-coding 7S RNA inhibits transcription via mitochondrial RNA polymerase dimerization.

Xuefeng Zhu, Xie Xie, Hrishikesh Das, Benedict G Tan, Yonghong Shi, Ali Al-Behadili, Bradley Peter, Elisa Motori, Sebastian Valenzuela, Viktor Posse, Claes M Gustafsson, B Martin Hällberg, Maria Falkenberg.

The mitochondrial genome encodes 13 components of the oxidative phosphorylation system, and altered mitochondrial transcription drives various human pathologies. A polyadenylated, non-coding RNA molecule known as 7S RNA is transcribed from a region immediately downstream of the light strand promoter in mammalian cells, and its levels change rapidly in response to physiological conditions. Here, we report that 7S RNA has a regulatory function, as it controls levels of mitochondrial transcription both in vitro and in cultured human cells. Using cryo-EM, we show that POLRMT dimerization is induced by interactions with 7S RNA. The resulting POLRMT dimer interface sequesters domains necessary for promoter recognition and unwinding, thereby preventing transcription initiation. We propose that the non-coding 7S RNA molecule is a component of a negative feedback loop that regulates mitochondrial transcription in mammalian cells.

Keywords: 7S RNA; POLRMT; SUV3; cryo-EM; dimer; mitochondria; mtDNA; mtEXO; non-coding RNA; transcription

DOI: https://doi.org/10.1016/j.cell.2022.05.006

Int J Mol Sci. 2022 May 30. pii: 6141. [Epub ahead of print]23(11):

How RNases Shape Mitochondrial Transcriptomes.

Jérémy Cartalas, Léna Coudray, Anthony Gobert.

Mitochondria are the power houses of eukaryote cells. These endosymbiotic organelles of prokaryote origin are considered as semi-autonomous since they have retained a genome and fully functional gene expression mechanisms. These pathways are particularly interesting because they combine features inherited from the bacterial ancestor of mitochondria with characteristics that appeared during eukaryote evolution. RNA biology is thus particularly diverse in mitochondria. It involves an unexpectedly vast array of factors, some of which being universal to all mitochondria and others being specific from specific eukaryote clades. Among them, ribonucleases are particularly prominent. They play pivotal functions such as the maturation of transcript ends, RNA degradation and surveillance functions that are required to attain the pool of mature RNAs required to synthesize essential mitochondrial proteins such as respiratory chain proteins. Beyond these functions, mitochondrial ribonucleases are also involved in the maintenance and replication of mitochondrial DNA, and even possibly in the biogenesis of mitochondrial ribosomes. The diversity of mitochondrial RNases is reviewed here, showing for instance how in some cases a bacterial-type enzyme was kept in some eukaryotes, while in other clades, eukaryote specific enzymes were recruited for the same function.

Keywords: RNA degradation; RNA maturation; endoribonuclease; exoribonuclease; mitochondrial gene expression

DOI: https://doi.org/10.3390/ijms23116141

Biomol NMR Assign. 2022 Jun 06.

1H, 13C, and 15N resonance assignments and solution structures of the KH domain of human ribosome binding factor A, mtRbfA, involved in mitochondrial ribosome biogenesis.

Kanako Kuwasako, Sakura Suzuki, Nobukazu Nameki, Masayuki Takizawa, Mari Takahashi, Kengo Tsuda, Takashi Nagata, Satoru Watanabe, Akiko Tanaka, Naohiro Kobayashi, Takanori Kigawa, Peter Güntert, Mikako Shirouzu, Shigeyuki Yokoyama, Yutaka Muto.

Ribosome biogenesis is a complicated, multistage process coordinated by ribosome assembly factors. Ribosome binding factor A (RbfA) is a bacterial one, which possesses a single structural type-II KH domain. By this domain, RbfA binds to a 16S rRNA precursor in small ribosomal subunits to promote its 5'-end processing. The human RbfA homolog, mtRbfA, binds to 12S rRNAs in the mitoribosomal small subunits and promotes its critical maturation process, the dimethylation of two highly conserved consecutive adenines, which differs from that of RbfA. However, the structural basis of the mtRbfA-mediated maturation process is poorly understood. Herein, we report the 1H, 15N, and 13C resonance assignments of the KH domain of mtRbfA and its solution structure. The mtRbfA domain adopts essentially the same α1-β1-β2-α2(kinked)-β3 topology as the type-II KH domain. Comparison with the RbfA counterpart showed structural differences in specific regions that function as a putative RNA-binding site. Particularly, the α2 helix of mtRbfA forms a single helix with a moderate kink at the Ser-Ala-Ala sequence, whereas the corresponding α2 helix of RbfA is interrupted by a distinct kink at the Ala-x-Gly sequence, characteristic of bacterial RbfA proteins, to adopt an α2-kink-α3 conformation. Additionally, the region linking α1 and β1 differs considerably in the sequence and structure between RbfA and mtRbfA. These findings suggest some variations of the RNA-binding mode between them and provide a structural basis for mtRbfA function in mitoribosome biogenesis.

Keywords: Mitochondria; Mitoribosome biogenesis; RBFA; Type-II KH domain; mtRbfA

DOI: https://doi.org/10.1007/s12104-022-10094-3