αβ′‐NAC cooperates with Sam37 to mediate early stages of mitochondrial protein import

Abstract

The mitochondrial proteome is mostly composed of nuclear-encoded proteins. Such polypeptides are synthesized with signals that guide their intracellular transport to the surface of the organelle and later within the different mitochondrial subcompartments until they reach their functional destination. It has been suggested that the nascent-polypeptide associated complex (NAC) - a cytosolic chaperone that recognizes nascent chains on translationally active ribosomes - has a role in the import of nuclear-encoded mitochondrial proteins. However, the molecular mechanisms that regulate the NAC-mediated cotranslational import are still not clear. Here, we show that a particular NAC heterodimer formed by subunits α and β' in Saccharomyces cerevisiae is specifically involved in the process of mitochondrial import and functionally cooperates with Sam37, an outer membrane protein subunit of the sorting and assembly machinery complex. Mutants in both components display growth defects, incorrectly accumulate precursor forms of mitochondrial proteins in the cytosol, and have an altered mitochondrial protein content. We propose that αβ'-NAC and Sam37 are members of the system that recognizes mitochondrial proteins at early stages of their synthesis, escorting them to the import machinery of mitochondria.

Integrating Rio1 activities discloses its nutrient-activated network in Saccharomyces cerevisiae

“…These proteins were selected firstly because they localize to different organelles and contribute to dissimilar activities: Rvb2 is a nuclear ATPase with helicase activity; Sky1 is a nuclear and cytoplasmic protein kinase, while Sam 37 is mitochondrial membrane component (46). Second, because some of these proteins perform multiple roles: Rvb2 contributes to the transcriptional regulation of ribosome biogenesis and ribonucleoprotein complex genes, to the assembly of box C/D snoRNP complexes (70–72) but also promotes RNA polymerase II assembly, DNA damage repair, telomerase complex assembly, mitotic spindle formation, the heat response and phosphatidylinositol three kinase related signaling (71–74), Sky1 regulates mRNA export, cation homeostasis, sulfur metabolism, mitophagy and apoptosis (74–77), while Sam37 is required for mitochondrial protein import (78,79). Third, because the e -values of these three interactors cover the entire range of our hit list (1.0E-02 to 1.0E-24; Supplementary Table S1).…”

Molecular Mechanism of the Specificity of Protein Import into Chloroplasts and Mitochondria in Plant Cells

“…Import factors at the surfaces of chloroplasts and mitochondria play a crucial role in the import of cargo by binding to incoming precursors associated with chaperones from the cytosol (Young et al, 2003;Smith et al, 2004;Qbadou et al, 2006;Ponce-Rojas et al, 2017). To test whether the import receptor AtToc159 is involved in specificity determination, we examined protein subcellular localization in ppi2 mutant protoplasts.…”

mRNA Targeting, Transport and Local Translation in Eukaryotic Cells: From the Classical View to a Diversity of New Concepts

“…It was also demonstrated in yeast that OM14, a protein of the outer membrane of mitochondria, functions as a NAC receptor, and the knockout of its gene leads to a decrease in the proportion of ribosomes associated with the outer membrane of mitochondria, and to a disturbance of the import of proteins into the organelle [ 84 ]. Another mitochondrial protein, SAM37, a component of the yeast membrane protein sorting and assembly complex (SAM-complex), dynamically associated with the TOM translocon, was also reported to be a NAC partner [ 85 ].…”

“…In yeast, it has been observed that different NACs (heterodimers of αβ-NAC and αβ'-NAC) may be involved in the biogenesis of different sets of proteins through the binding of different substrates. The αβ'-NAC complex has a specific role in the biogenesis of mitochondrial proteins, since it is predominantly associated with ribosomes that actively synthesize these proteins [ 85 , 306 ].…”