JOURNALS - THE PLANT JOURNAL

A FRIENDLY CONNECTION: HOW MRNAS GET RECRUITED TO THE MITOCHONDRIAL SURFACE THE PLANT JOURNAL BY GWENDOLYN KIRSCHNER

Mickaele Hemono, Thalia Salinas-Giegé, Jeanne Roignant, Audrey Vingadassalon, Philippe Hammann, Elodie Ubrig, Patryk Ngondo, AnneMarie Duchêne. FRIENDLY (FMT) is an RNA binding protein associated with cytosolic ribosomes at the mitochondrial surface. The Plant Journal 2022; 112: 309¬–321. https://doi. org/10.1111/tpj.15962

Biogenesis of organelles requires the synthesis and import of new proteins. Secreted and transmembrane proteins are synthesised from mRNA transcripts that are translated by ribosomes associated with endoplasmic reticulum membranes, whereas soluble cytoplasmic proteins are synthesised on free polysomes. For the correct targeting of the translated proteins to the organelle, 30–50% of all mRNAs for nuclear-encoded mitochondrial proteins are found at the mitochondrial surface. Nucleotide sequence motifs in the encoding transcript and signals in the synthesised polypeptide can be responsible for mRNA delivery to the mitochondrial surface. But which proteins direct these mRNAs here?

Some of the proteins involved in recruiting cytosolic ribosomes to the mitochondrial membrane have been identified in yeast and mammals, but to date none have been identified in plants. The group of Anne-Marie Duchêne (University of Strasbourg) set out to address this question. For their report in The Plant Journal, her team purified cytosolic ribosomes from an Arabidopsis mitochondrial extract and identified enriched proteins in this ribosomal fraction (Hemono et al., 2022). Among them, they detected the cytosolic protein friendly mitochondria (FMT). The fmt mutant had previously been identified from a screen of ethyl methanesulfonate-mutagenised Arabidopsis seedlings for mutants in mitochondrial morphology. In wild-type plants, mitochondria are distributed evenly throughout the cytoplasm, but in the fmt mutant, the majority of mitochondria were arranged in clusters of tens of organelles, and therefore the mutant was named friendly mitochondria. FMT is a member of the clustered mitochondria (CLU) superfamily of conserved eukaryotic proteins and had been characterised regarding its role in the regulation of mitochondrial association and fusion.

Figure 1: FMT (green) binds ribosomes and RNA and localizes in the cytosol and at the mitochondrial surface.

Hemono et al. used translational GFP fusion constructs and found that FMT localised to the cytosol, where it was diffuse and enriched around mitochondria or concentrated as foci that moved together with the mitochondria. Co-immunoprecipitation coupled with mass spectrometry identified many ribosomal proteins as interactors of FMT. To test if FMT binds directly to RNA, the authors used an oligo(dT) capture experiment: tissue was irradiated with UV to bind RNA to interacting proteins, and poly(A) RNAs were isolated from the tissue extracts, together with the covalently linked proteins. FMT was found in the eluate, confirming its RNA-binding property. The authors analysed the knock-out mutant phenotype and found that, in addition to the clustering of the mitochondria, the mitochondrial proteome of mutant seedlings was affected, and stress-related proteins such as alternative oxidase (AOX) had increased abundance. Also, most of the identified mitochondrially encoded proteins were slightly decreased in the mutant, suggesting that mitochondrial translation might be inhibited. The authors selected three mRNAs whose orthologs in Solanum tuberosum are targeted to the mitochondrial surface, along with VDAC3 mRNA that has been identified on the surface of mitochondria in Arabidopsis, and found that they were no longer enriched at the mitochondrial surface in the mutant. Taken together, these findings suggest that FMT is directly or indirectly involved in mRNA targeting to the mitochondrial surface, in their anchorage or in localised translation.

The study by Hemono et al. provides a basis to delve into the co-translational mitochondrial protein import pathway, which is still largely unexplored in plants. To date, only a single mRNA has been identified at the mitochondrial surface in Arabidopsis. Given that FMT localises both in the cytosol as well as in foci associated with the mitochondria, and is able to bind both ribosomes and mRNA, this protein could play a role in mRNA transport from the nucleus to the mitochondrial surface.