CCDC38

CCDC38 is a testis-specific coiled-coil protein that functions in sperm flagellum and acrosome biogenesis during spermiogenesis, where it localizes to the manchette and sperm tail (PMID:35587122). It acts as a scaffold within the intraflagellar transport machinery, physically interacting with CCDC42, IFT88, and ODF2, and its loss reduces delivery of ODF2 to the flagellum and produces a distorted manchette, multiple morphological abnormalities of the flagella (MMAF), and male sterility in knockout mice (PMID:35587122). CCDC38 is additionally required for acrosome biogenesis and fibrous sheath assembly; its loss causes acrosomal hypoplasia, loosely anchored acrosomal membrane, disorganized fibrous sheaths, and reduced and mislocalized TEKT3 (PMID:37709195). It further interacts with CCDC146 and lies upstream of or parallel to it in the flagellum biogenesis pathway, since CCDC38 levels are unaffected by Ccdc146 loss (PMID:38038747). An earlier proteomic study placed CCDC38 at the centrosome/sperm centriole (PMID:25074808), and a single report links it to ubiquitinated histone H2A in testes (PMID:27278724); beyond these interactions and the structural-transport role, no further mechanistic detail has been characterized in the available corpus.

1. 2014 Medium

   Establishing where CCDC38 acts first required identifying its subcellular home, which proteomics of sperm centrioles placed at the centrosome.

   Evidence Mass spectrometry of sperm centrioles with localization assessment in cultured cells

   PMID:25074808

   Open questions at the time

   • No functional role assigned at this stage
   • Centrosomal localization not linked to a spermiogenesis phenotype
   • Single lab, limited follow-up for CCDC38 specifically
2. 2016 Low

   A candidate molecular link was tested by asking whether CCDC38 associates with chromatin marks, identifying an interaction with ubiquitinated histone H2A in testes.

   Evidence Co-immunoprecipitation and immunofluorescence co-localization in mouse testes

   PMID:27278724

   Open questions at the time

   • Single Co-IP without mutagenesis or functional rescue
   • Functional significance of the H2A association unresolved
   • Not reconciled with later cytoskeletal/flagellar role
3. 2022 High

   The core function was defined by knockout, showing CCDC38 is testis-specific, localizes to the manchette and sperm tail, and is required for intraflagellar transport of ODF2 and normal flagellum formation.

   Evidence Ccdc38 knockout mice with reciprocal Co-IP (CCDC42, IFT88, ODF2), immunofluorescence, and Western blot for flagellar ODF2

   PMID:35587122

   Open questions at the time

   • Direct demonstration that CCDC38 itself moves as IFT cargo not shown
   • Molecular basis of partner selectivity in the complex unresolved
   • Stoichiometry and assembly order of the CCDC42/IFT88/ODF2 interactions undefined
4. 2023 High

   The scope of CCDC38 function was extended beyond the flagellum to acrosome biogenesis and fibrous sheath assembly, with TEKT3 identified as a downstream cargo affected by its loss.

   Evidence Base-editing-derived Ccdc38 knockout mice analyzed by EM, immunofluorescence, Western blot, with ICSI rescue

   PMID:37709195

   Open questions at the time

   • Whether TEKT3 is a direct CCDC38 interactor not established
   • Mechanism connecting manchette transport to acrosome anchoring unclear
   • Single lab
5. 2023 Medium

   Genetic and physical epistasis was tested against CCDC146, placing CCDC38 upstream of or parallel to it in flagellum biogenesis.

   Evidence Co-immunoprecipitation, atomic-level interaction modeling with validation, and Western blot in Ccdc146-KO mice

   PMID:38038747

   Open questions at the time

   • Directionality (upstream vs parallel) not fully resolved
   • Functional consequence of the CCDC38–CCDC146 interaction untested by double perturbation
   • Single lab

• How CCDC38 mechanically couples manchette-based transport to both flagellar structural protein delivery and acrosome anchoring, and whether it functions as IFT cargo or adaptor, remains unresolved.
• No structural model of CCDC38 within an assembled transport complex
• Direct substrate/cargo binding versus indirect stabilization not distinguished
• No human patient mutation evidence in the corpus