The Mitochondrial Outer Membrane Protein Tom70-Mediator in Protein Traffic, Membrane Contact Sites and Innate Immunity

Abstract

Tom70 is a versatile adaptor protein of 70 kDa anchored in the outer membrane of mitochondria in metazoa, fungi and amoeba. The tertiary structure was resolved for the Tom70 of yeast, showing 26 α-helices, most of them participating in the formation of 11 tetratricopeptide repeat (TPR) motifs. Tom70 serves as a docking site for cytosolic chaperone proteins and co-chaperones and is thereby involved in the uptake of newly synthesized chaperone-bound proteins in mitochondrial biogenesis. In yeast, Tom70 additionally mediates ER-mitochondria contacts via binding to sterol transporter Lam6/Ltc1. In mammalian cells, TOM70 promotes endoplasmic reticulum (ER) to mitochondria Ca2+ transfer by association with the inositol-1,4,5-triphosphate receptor type 3 (IP3R3). TOM70 is specifically targeted by the Bcl-2-related protein MCL-1 that acts as an anti-apoptotic protein in macrophages infected by intracellular pathogens, but also in many cancer cells. By participating in the recruitment of PINK1 and the E3 ubiquitin ligase Parkin, TOM70 can be implicated in the development of Parkinson’s disease. TOM70 acts as receptor of the mitochondrial antiviral-signaling protein (MAVS) and thereby participates in the corresponding system of innate immunity against viral infections. The protein encoded by Orf9b in the genome of SARS-CoV-2 binds to TOM70, probably compromising the synthesis of type I interferons.

A comprehensive library of fluorescent constructs of SARS‐CoV‐2 proteins and their initial characterisation in different cell types

“…SARS-CoV-2 Orf9b-GFP fusion protein displayed a typical mitochondrial pattern in all tested cell lines (A549, BEAS-2B, Caco-2 and HeLa) (Figures 7A and S7A), and was specifically associated with mitochondria in A549 cells as assessed by its colocalisation with the mitochondrial marker Tom20 (Figure 7A), Mitotracker and DsRed-Mito-7 (Figure S7B), similarly to what has been described for the cognate protein Orf9b from SARS-CoV-1 (Shi et al, 2014). Recent investigations indicate that Orf9b from SARS-CoV-2 interacts with the mitochondrial protein TOM70 (Jiang et al, 2020;Kreimendahl and Rassow, 2020). Consistent with this observation, mutating an Orf9b residue in the interacting region with TOM70 (Gordon et al, 2020) (L52D) abolished the mitochondrial localisation of Orf9b (Figures 7A and S7C).…”

“…Because the structure of isolated Orf9b strongly differs from the structure of Orf9b in complex with TOM70 and a direct mechanism for lipid membrane recognition has been suggested (Meier et al, 2006), we next tested whether Orf9b associated with mitochondria via protein-protein or protein-lipid interactions. To this end, we expressed Orf9b-GFP in S. cerevisiae, which harbours a TOM70 protein homologue (Chan et al, 2006;Kreimendahl and Rassow, 2020). Here, Orf9b-GFP appeared entirely soluble (Figure 7C), suggesting that mitochondrial localisation of Orf9b relies on a specific interaction between Orf9b and human TOM70 and not on direct interaction with the lipid bilayer.…”

Role of Mitochondria in Viral Infections

“…SARS-CoV-2 main protease Mpro (nsp5) impairs both the virus-induced type I IFN production and the induction of downstream antiviral interferon-stimulated genes (ISGs) [ 108 ]. Another protein, Orf9b, localizes to mitochondria, binds to TOM70, an adaptor protein of the mitochondrial outer membrane, and suppresses the antiviral type I IFN response [ 109 , 110 ]. However, the molecular consequences of Orf9b binding to TOM70 are not yet clear.…”

The Diversity of the Mitochondrial Outer Membrane Protein Import Channels: Emerging Targets for Modulation

“…The membrane import complexes—known as translocases—are composed of receptor subunits that recognise incoming precursor proteins, proteins that form channels for transport across membranes or insertion into membranes and protein subunits that modulate the activity and stability of complexes. In the mitochondrial outer membrane, four complexes have been identified ( Figure 1 ): translocase of the outer mitochondrial membrane (TOM) [ 6 , 7 ], topogenesis of the mitochondrial outer membrane β-barrel proteins/sorting and assembly machinery (TOB/SAM) [ 8 , 9 , 10 , 11 ], mitochondrial import complex (MIM) [ 12 , 13 ] and endoplasmic reticulum-mitochondria encounter structure (ERMES) [ 14 ]. The TOM complex recognises, translocates and segregates most of the precursor proteins delivered to different mitochondrial locations.…”