TIMM17B

TIMM17B is a core subunit of the human TIM23 presequence translocase in the inner mitochondrial membrane, where it is required for efficient import of nuclear-encoded proteins into mitochondria (PMID:40631992). Knockdown of TIMM17B in intact human cells reduces mitochondrial protein import, as measured by orthogonal pulse-chase (mitoRUSH) and nascent-protein labeling assays (PMID:40631992). The stability of the TIMM17B-containing TIM23 complex is maintained by the prohibitin (PHB1/PHB2) complex; in contrast to the paralogous TIMM17A variant, the TIMM17B variant is not protected from YME1L protease degradation by OCIAD1, defining a regulatory axis that differentially controls the two TIM23 variants (PMID:39630581). Beyond these findings, structural and biochemical detail of the TIMM17B translocase has not been experimentally characterized in the available corpus.

1. 2024 Low

   Whether human TIM23 contains a distinct TIMM17B-based variant and how it is organized was unclear; comparative modeling established TIMM17B as a core subunit of one of two human TIM23 translocase variants conserved with the yeast Tim17/Tim23 core.

   Evidence AlphaFold2/ColabFold structural prediction with comparative analysis against the yeast TIM23 complex

   PMID:38837610

   Open questions at the time

   • Computational prediction only, no experimental structural or biochemical validation
   • Channel-like ROMO1/TIMM17B architecture not confirmed in vitro
   • Functional distinction between TIMM17A and TIMM17B variants not addressed
2. 2024 High

   How the two TIM23 variants are stabilized and differentially regulated was unknown; this work showed the prohibitin complex stabilizes both variants while OCIAD1 specifically shields the TIMM17A (not TIMM17B) variant from YME1L degradation.

   Evidence Reciprocal Co-IP, OCIAD1/prohibitin KO/KD, protease inhibition, blue-native PAGE and quantitative proteomics in human cells

   PMID:39630581

   Open questions at the time

   • Mechanism of how OCIAD1 selectively distinguishes TIMM17A from TIMM17B is not resolved
   • Physiological consequence of losing TIMM17B-variant stability not defined
   • Whether prohibitin acts directly or via membrane organization is unresolved
3. 2025 Medium

   Whether TIMM17B is functionally required for import in living human cells (rather than predicted) was open; depletion experiments established that TIMM17B is needed for efficient mitochondrial protein import.

   Evidence mitoRUSH pulse-chase import assay and azidohomoalanine/click nascent-protein labeling with siRNA knockdown in human cells

   PMID:40631992

   Open questions at the time

   • Substrate selectivity of the TIMM17B versus TIMM17A variant not defined
   • Single-lab demonstration
   • Does not resolve which precursor classes depend on TIMM17B

• It remains unknown whether the TIMM17B-containing translocase imports a distinct substrate repertoire from the TIMM17A variant and what the physiological role of variant specialization is.
• No substrate-resolved comparison of the two variants
• No experimental structure of the human TIMM17B translocase
• Tissue/condition-specific roles of TIMM17B unexplored