CCDC38

CCDC38 is a centrosome- and manchette-associated coiled-coil protein essential for sperm flagellum biogenesis, acrosome formation, and male fertility. During spermiogenesis, CCDC38 forms a complex with CCDC42 and CCDC146 on the manchette and sperm tail, where it interacts with the intraflagellar transport protein IFT88 to facilitate delivery of structural cargo such as ODF2 to the growing flagellum; loss of CCDC38 in mice causes distorted manchette morphology, multiple morphological abnormalities of the flagella (MMAF), acrosomal hypoplasia with disorganized fibrous sheaths, and complete male sterility (PMID:35587122, PMID:38038747, PMID:37709195). CCDC38 additionally localizes to centrosomes in mammalian cells and interacts with ubiquitinated histone H2A and the structural protein TEKT3 in the testis, linking it to both cytoskeletal organization and chromatin-associated processes during sperm development (PMID:25074808, PMID:27278724, PMID:37709195).

1. 2014 Medium

   Identification of CCDC38 as a centrosome-associated protein established its connection to the microtubule-organizing center, providing the first subcellular context for its function.

   Evidence Mass spectrometry of bovine sperm centrioles followed by localization assessment in cultured mammalian cells

   PMID:25074808

   Open questions at the time

   • No functional assay performed; centrosomal role inferred solely from localization
   • Relevance to spermatogenesis versus general centrosome biology was unclear
2. 2016 Low

   Discovery of an interaction between CCDC38 and ubiquitinated histone H2A in mouse testes raised the possibility that CCDC38 participates in chromatin-related processes during spermiogenesis, though the functional significance was not resolved.

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

   PMID:27278724

   Open questions at the time

   • Single Co-IP without reciprocal validation or functional follow-up
   • Biological consequence of the H2A interaction remains unknown
   • No loss-of-function experiment to test requirement
3. 2022 High

   Knockout studies and interaction mapping established CCDC38 as a manchette- and flagellum-localized protein that complexes with CCDC42 and the IFT component IFT88 to transport ODF2 to the sperm tail, with its loss causing MMAF and male sterility — answering the central question of its physiological function.

   Evidence Reciprocal Co-IP identifying CCDC42, IFT88, and ODF2 interactions; Ccdc38 knockout mouse showing distorted manchette, MMAF, reduced ODF2 delivery, and complete infertility

   PMID:35587122

   Open questions at the time

   • Direct biochemical reconstitution of the CCDC38–IFT88 transport complex has not been performed
   • Whether CCDC38 is a cargo adaptor versus a structural scaffold for IFT remains unresolved
   • Human genetic validation (MMAF patient mutations in CCDC38) not yet reported
4. 2023 Medium

   Demonstration that CCDC146 is a third core member of the CCDC38–CCDC42 complex and that CCDC146 loss reduces IFT88/IFT20 levels without affecting CCDC38 refined the hierarchical assembly of this flagellum-biogenesis module.

   Evidence Co-IP and atomic-level interaction modeling of CCDC146–CCDC38; Ccdc146 knockout mouse with IFT protein quantification

   PMID:38038747

   Open questions at the time

   • Stoichiometry and assembly order of the trimeric complex are unknown
   • Whether CCDC38 recruits CCDC146 or vice versa has not been tested by sequential depletion
5. 2023 High

   An independent knockout confirmed the flagellar phenotype and extended CCDC38's role to acrosome biogenesis and fibrous sheath assembly, showing acrosomal hypoplasia and aberrant TEKT3 distribution, thereby broadening the functional scope beyond IFT-mediated tail formation.

   Evidence Base-editing Ccdc38 knockout mouse with TEM ultrastructure, immunofluorescence for TEKT3, and ICSI fertility rescue

   PMID:37709195

   Open questions at the time

   • Mechanism linking CCDC38 to acrosomal membrane anchoring is undefined
   • Whether TEKT3 is a direct physical partner or a downstream marker has not been distinguished
   • Relationship between the IFT-transport function and acrosome biogenesis function is unclear

• Key unresolved questions include whether CCDC38 mutations cause human male infertility, how CCDC38 coordinates its dual roles in flagellar IFT and acrosome biogenesis, and the structural basis of the CCDC38–CCDC42–CCDC146 complex.
• No human genetic studies linking CCDC38 variants to MMAF or infertility
• No structural or cryo-EM data for the trimeric complex
• Mechanism coupling manchette-based transport to acrosomal vesicle formation is unknown