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Tim9 is a small TIM family soluble IMS chaperone (holdase) whose primary role is chaperone-like binding and transfer of hydrophobic precursors, not catalysis or transport.
"Tim9 is best viewed as a **specialized, soluble IMS holdase chaperone**"
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Tim9 is located in the mitochondrial intermembrane space, between TOM-mediated entry and TIM22-mediated inner-membrane insertion.
"Tim9 is located in the **mitochondrial IMS**"
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Tim9 forms the canonical Tim9-Tim10 heterohexamer (3:3 stoichiometry, ~70 kDa soluble complex) and a Tim9-Tim10-Tim12 (3:2:1) docking complex linked to TIM22.
"Tim9:Tim10 complex stoichiometry is **3:3**; the TIM22-associated complex is **Tim9–Tim10–Tim12 = 3:2:1**; the soluble Tim9/Tim10 complex is about **~70 kDa**"
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Tim9 acts as a chaperone/escort factor that prevents aggregation of hydrophobic precursors in the aqueous IMS and transfers them from TOM to TIM22.
"Tim9 acts as a **chaperone/escort factor** that prevents aggregation of hydrophobic precursors in the aqueous IMS and transfers them from **TOM** to **TIM22**."
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Within the complex, Tim10 provides the key substrate-binding elements; Tim9 alone does not bind the AAC carrier (consistent with UniProt noting Tim9 has a strong structural role relative to Tim10).
"Biochemical substrate-binding evidence indicates that **Tim10** (alone) can bind the **ATP/ADP carrier (AAC)** similarly to the Tim9–Tim10 complex, whereas **Tim9 alone does not bind AAC**, supporting a model where Tim10 provides key substrate-binding elements within the complex"
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The Tim9-Tim10 complex recognizes hydrophobic clients through a hydrophobic cleft formed by chaperone helices.
"TIM9·10 recognizes hydrophobic clients primarily through a **hydrophobic cleft** formed by chaperone helices"
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Small TIMs including Tim9 can also assist trafficking of some outer-membrane beta-barrel proteins toward the SAM complex.
"Small TIMs can also assist trafficking of some outer-membrane β-barrel proteins toward the SAM complex"
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Tim9 is itself imported via TOM and oxidatively folded in the IMS by the MIA pathway, which introduces the two intramolecular disulfides in its conserved CX3C motifs (rather than stable metal coordination in the mature form).
"Small TIM proteins (including Tim9) are imported through TOM and undergo oxidative folding in the IMS via the **MIA (mitochondrial import and assembly) pathway**, which introduces disulfides in the conserved CX3C motifs"
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Tim9 and Tim10 assemble into an essential heterohexameric Tim9/10 complex; unassembled small TIMs can be degraded by Yme1, with assembly being protective.
"Tim9 and Tim10 assemble into an essential heterohexameric Tim9/10 complex** that chaperones hydrophobic preproteins from TOM to TIM22"
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Conserved-residue mutants tim9-19 (E52G) and tim9-3 (V40A + S60P) abolish a detectable Tim9-Tim10 heterohexamer, linking these residues to complex integrity and import function.
"tim9-19 (E52G)** and **tim9-3 (V40A + S60P)** show **no detectable Tim9–Tim10 heterohexamer** in detergent-solubilized mitochondria"