TIMM21

TIMM21 (Tim21) is a peripheral subunit of the mitochondrial inner-membrane presequence translocase (TIM23 complex) that physically couples protein import to respiratory-chain biogenesis (PMID:15797382, PMID:23260140). It is specific to the sorting-active, TOM-tethered form of TIM23, where its IMS-exposed C-terminal domain—a novel α/β-mixed fold—directly binds negatively charged residues of the TOM receptor Tom22 to bridge the TOM and TIM23 translocases (PMID:15797382, PMID:17099692). The same domain engages multiple binding sites on the Tim23 IMS region, which it shares with Tim50, defining a dynamic, signal-sensitive interaction network: upon recognition of an incoming presequence, Tim21 is released from Tim50, promoting Pam17 recruitment and conversion to the PAM motor-associated form competent for matrix translocation (PMID:23403928, PMID:25263020). Recruitment of Tim21 to the TIM23 core depends on the Tim23 TM2 helix and on Mgr2, which links Tim21 to the core and to the respiratory chain, while Tim17 G/AXXXG/A motifs govern the switch between sorting and matrix-import forms (PMID:22613836, PMID:24061477, PMID:27994013). Beyond bridging translocases, the sorting-form translocase recruits proton-pumping respiratory-chain complexes to stimulate membrane-potential-dependent inner-membrane insertion of preproteins (PMID:17113393, PMID:18070913). In human cells, TIMM21 is additionally a component of MITRAC respiratory-chain assembly intermediates and, although dispensable for general presequence import, is specifically required for integration of early presequence-containing subunits into respiratory-chain complexes such as cytochrome c oxidase, with its dynamics modulated by ROMO1 (PMID:23260140, PMID:30598479).

1. 2005 High

   Established that the presequence translocase exists in two functionally distinct forms and identified Tim21 as the marker of the TOM-coupled, sorting-active form, framing TIM23 as a modular machine that switches between membrane insertion and matrix translocation.

   Evidence Co-IP, genetic interaction analysis, and import assays in yeast mitochondria

   PMID:15797382

   Open questions at the time

   • Molecular contacts mediating TOM-TIM23 coupling not defined at the residue level
   • Trigger for the form switch not yet identified
2. 2006 High

   Defined the structural and physical basis of TOM-TIM23 coupling by showing the Tim21 IMS domain directly binds Tom22, and solved the domain fold, converting a functional association into an atomic interaction.

   Evidence 1.6 Å X-ray crystallography, binding assays, and Tom22 mutagenesis

   PMID:17099692

   Open questions at the time

   • Stoichiometry and affinity in the intact translocase not measured
   • Does not address how this interaction is regulated during import
3. 2006 High

   Revealed that Tim21 couples the sorting translocase to the respiratory chain, mechanistically linking the membrane potential generated by proton-pumping complexes to preprotein insertion.

   Evidence Co-IP, import assays, native gels, and Tim21-deletion yeast strains

   PMID:17113393

   Open questions at the time

   • Direct binding interface between Tim21 and respiratory complexes not resolved
   • Whether the same coupling operates in mammals not addressed here
4. 2007 High

   Showed the respiratory chain-bound translocase is actively translocating preproteins and that PAM motor subunits assemble stepwise rather than en bloc, refining the order of translocase remodeling during import.

   Evidence Co-IP, cross-linking, and import intermediate trapping in yeast

   PMID:18070913

   Open questions at the time

   • Precise timing of Tim21 departure relative to PAM assembly not pinned down
   • How preproteins are partitioned between inner-membrane and matrix fates not resolved
5. 2012 High

   Extended the function to human cells, placing TIMM21 in MITRAC respiratory-chain assembly intermediates and assigning it a specific role in integrating presequence-containing subunits, establishing a conserved import–respiratory biogenesis link.

   Evidence Mass spectrometry, co-IP, siRNA knockdown, and pulse-chase import assays in human cells

   PMID:23260140

   Open questions at the time

   • Direct human TIM21 binding partners within MITRAC not fully enumerated
   • Whether human TIM21 binds Tom22 as in yeast not tested here
6. 2012 High

   Identified Mgr2 as the bridge linking Tim21 to the TIM23 core and the respiratory chain, defining a TIM23(CORE)–Mgr2/Tim21–respiratory chain binding chain and a structural requirement for the sorting form.

   Evidence Co-IP, deletion mutant analysis, native gels, and import assays in yeast

   PMID:22613836

   Open questions at the time

   • Human ortholog of Mgr2 in this chain not addressed
   • Structural detail of the Mgr2-Tim21 contact unknown
7. 2013 High

   Demonstrated that Tim50 release of Tim21 is signal-sensitive, providing the mechanistic trigger by which presequence recognition converts the sorting form into the matrix-import (PAM-bound) form.

   Evidence Signal-dependent co-IP, cross-linking, and import assays with presequence peptides in yeast

   PMID:23403928

   Open questions at the time

   • Conformational change in Tim23 IMS that drives the switch not visualized
   • Whether the same signal-sensitivity operates in human TIM21 not tested
8. 2013 Medium

   Mapped the determinants on Tim23 governing Tim21 recruitment—TM2 promotes binding while the C-terminus inhibits it—showing how the channel itself controls assembly of the sorting versus motor forms.

   Evidence Site-directed mutagenesis, co-IP, and in vitro binding assays in yeast

   PMID:24061477

   Open questions at the time

   • Single-lab mutagenesis study without structural confirmation of the regulatory contacts
   • Functional consequence in mammalian cells not examined
9. 2014 High

   Resolved at single-residue resolution that the Tim23 IMS domain carries overlapping binding sites for Tim21, Tim50, and Tom22, explaining how the channel acts as a hub coordinating competing interactions during translocase remodeling.

   Evidence NMR spectroscopy with chemical shift perturbation mapping

   PMID:25263020

   Open questions at the time

   • Dynamics measured on isolated domains, not the intact membrane-embedded translocase
   • Order and kinetics of site occupancy during active import not determined
10. 2017 Medium

    Showed that Tim17 G/AXXXG/A transmembrane motifs control TIM23 assembly into either the PAM or Tim21 form, identifying a membrane-helix code that determines translocase fate and Tim21 recruitment.

    Evidence Site-directed mutagenesis, co-IP, viability and import assays in yeast

    PMID:27994013

    Open questions at the time

    • Single-lab study; structural basis of motif-driven assembly not solved
    • Mammalian relevance not tested
11. 2018 Medium

    Linked human TIM21 dynamics during respiratory-chain biogenesis to the TIM23 constituent ROMO1, identifying a new modulator of TIM21 behavior specific to assembly rather than general import.

    Evidence ROMO1 knockout cells, mass spectrometry, co-IP, and import assays in human cells

    PMID:30598479

    Open questions at the time

    • Mechanism by which ROMO1 alters TIM21 dynamics only partially characterized
    • Direct ROMO1-TIM21 contact not structurally defined

• How the regulatory logic and structural transitions defined in yeast map onto the human TIM21 MITRAC function, and the atomic mechanism by which TIM21 integrates presequence-containing subunits into respiratory complexes, remain open.
• No structure of intact human TIM21-containing TIM23 or MITRAC complex
• Direct human binding partners and substrate-handoff mechanism in respiratory assembly not fully resolved