| TOM70 (TOMM70) is a tetratricopeptide repeat (TPR)–containing receptor of the translocase of the outer mitochondrial membrane (TOM) complex that links cytosolic chaperone–bound preproteins to the general import pore and, in mammalian cells, also serves as an adaptor that couples mitochondrial antiviral signaling to IRF3‑dependent transcription and virus‑induced apoptosis. The cytosolic domain is entirely helical and organized into repeated TPR motifs that form two functionally specialized regions: an N‑terminal clamp that recognizes conserved C‑terminal EEVD motifs in Hsp70/Hsc70 and Hsp90, and a C‑terminal preprotein‑binding groove that engages internal targeting signals and internal MTS‑like segments within carrier‑type and other nonclassical mitochondrial precursors, while a single C‑terminal transmembrane anchor tethers TOM70 to the outer membrane facing the cytosol. In mitochondrial protein biogenesis, TOM70 recognizes hydrophobic or complex preproteins presented as Hsp70/Hsp90–substrate complexes, binds both chaperone and client simultaneously, and positions the preprotein for transfer into the TOM40 channel, enhancing import efficiency especially for carriers and other presequence‑containing substrates that harbor distributed internal targeting information; loss or mutation of TOM70 reduces import of a broad set of hydrophobic and multi‑pass proteins, increasing the burden of mistargeted precursors in the cytosol and triggering mitoprotein stress. Structural and biochemical work indicates that conformational changes between open and closed states of the TPR array, transmitted via specific helices, coordinate chaperone engagement and preprotein handover, and that Tom70‑dependent recruitment of chaperones to the mitochondrial surface is a major determinant of its protective role against proteotoxicity from accumulating mitochondrial precursors. In innate immunity, TOM70 functions as a mitochondrial adaptor for RIG‑I–like receptor signaling: MAVS residing on the outer membrane recruits TOM70, which then binds Hsp90‑associated TBK1 and IRF3 to form a signaling platform that facilitates IRF3 phosphorylation and type I interferon production after infection with RNA viruses such as Sendai virus, integrating mitochondrial interface dynamics with antiviral transcriptional responses. Under the same stimulus, TOM70 also assembles a pro‑apoptotic complex containing HSP90AA1, IRF3, and the proapoptotic BCL‑2 family member BAX on the mitochondrial surface; IRF3 in this complex adopts a non‑transcriptional effector role that cooperates with BAX to permeabilize mitochondria and promote apoptosis, coupling antiviral signaling to elimination of infected cells through a TOM70‑dependent mechanism. Across these pathways, TOM70 occupies a dual position as a core receptor in mitochondrial import and as a scaffold at the mitochondrial outer membrane that recruits chaperones, kinases, transcription factors, and apoptosis regulators. |
