FRIENDLY (FMT) is an RNA binding protein associated with cytosolic ribosomes at the mitochondrial surface.

The spatial organization of protein synthesis in the eukaryotic cell is essential for maintaining the integrity of the proteome and the functioning of the cell. Translation on free polysomes or on ribosomes associated with the endoplasmic reticulum has been studied for a long time. More recent data have revealed selective translation of mRNAs in other compartments, in particular at the surface of mitochondria. Although these processes have been described in many organisms, particularky in plants, the mRNA targeting and localized translation mechanisms remain poorly understood. Here, the Arabidopsis thaliana Friendly (FMT) protein is shown to be a cytosolic RNA binding protein that associates with cytosolic ribosomes at the surface of mitochondria. FMT knockout delays seedling development and causes mitochondrial clustering. The mutation also disrupts the mitochondrial proteome, as well as the localization of nuclear transcripts encoding mitochondrial proteins at the surface of mitochondria. These data indicate that FMT participates in the localization of mRNAs and their translation at the surface of mitochondria.

The interactome of CLUH reveals its association to SPAG5 and its co-translational proximity to mitochondrial proteins.

BACKGROUND: Mitochondria require thousands of proteins to fulfill their essential function in energy production and other fundamental biological processes. These proteins are mostly encoded by the nuclear genome, translated in the cytoplasm before being imported into the organelle. RNA binding proteins (RBPs) are central players in the regulation of this process by affecting mRNA translation, stability, or localization. CLUH is an RBP recognizing specifically mRNAs coding for mitochondrial proteins, but its precise molecular function and interacting partners remain undiscovered in mammals. RESULTS: Here we reveal for the first time CLUH interactome in mammalian cells. Using both co-IP and BioID proximity-labeling approaches, we identify novel molecular partners interacting stably or transiently with CLUH in HCT116 cells and mouse embryonic stem cells. We reveal stable RNA-independent interactions of CLUH with itself and with SPAG5 in cytosolic granular structures. More importantly, we uncover an unexpected proximity of CLUH to mitochondrial proteins and their cognate mRNAs in the cytosol. We show that this interaction occurs during the process of active translation and is dependent on CLUH TPR domain. CONCLUSIONS: Overall, through the analysis of CLUH interactome, our study sheds a new light on CLUH molecular function by revealing new partners and by highlighting its link to the translation and subcellular localization of some mRNAs coding for mitochondrial proteins.

Arabidopsis Voltage-Dependent Anion Channels (VDACs): Overlapping and Specific Functions in Mitochondria.

Voltage-dependent anion channels (VDACs) are essential components of the mitochondrial outer membrane. VDACs are involved in the exchange of numerous ions and molecules, from ATP to larger molecules such as tRNAs, and are supposed to adjust exchanges in response to cell signals and stresses. Four major VDACs have been identified in Arabidopsis thaliana. The goal of this study was to explore the specific functions of these proteins, in particular, in tRNA import into mitochondria and stress response. The main results were: (i) VDACs appeared to differentially interact with tRNAs, and VDAC4 could be the major tRNA channel on the outer membrane, (ii) a VDAC3 mRNA isoform was found induced by different stresses, suggesting that VDAC3 might be specifically involved in early steps of stress response and (iii) an analysis of vdac3 and vdac1 mutant lines showed that VDAC3 and VDAC1 shared some, but not all functions. In conclusion, this work brings new knowledge on VDACs, which do not appear as interchangeable pores of the outer membrane and each VDAC has its own specificity.