CYLC1

CYLC1 encodes a testis-specific, lysine-rich cytoskeletal protein that is essential for perinuclear theca (calyx) assembly and acrosome-nucleus attachment during spermiogenesis. CYLC1 localizes to the acrosomal region in round spermatids and to the calyx of the perinuclear theca in mature sperm, where it self-associates and interacts with inner acrosomal membrane protein SPACA1 and nuclear envelope protein FAM209, forming an IAM–cylicins–NE sandwich structure that mechanically anchors the acrosome to the sperm nucleus (PMID:38573307). CRISPR/Cas9-mediated Cylc1 knockout in mice causes male subfertility characterized by impaired calyx assembly, acrosome-nuclear detachment, severe sperm head deformities, defective cytoplasmic shedding, and reduced motility, and missense variants in human CYLC1 similarly disrupt acrosome morphology and CYLC1 localization, establishing CYLC1 deficiency as a cause of human male infertility (PMID:38013430, PMID:41075067).

1. 2016 Medium

   Prior to mammalian functional work, C. elegans homologues of CYLC1 were shown to regulate microtubule dynamics, establishing that this family of small basic KKD/E-repeat proteins interacts with the cytoskeleton to control tubulin polymerization at the cell periphery.

   Evidence RNAi/loss-of-function genetics and live imaging of microtubule dynamics in C. elegans smooth muscle cells

   PMID:26985017

   Open questions at the time

   • C. elegans homologue function in microtubule regulation has not been shown to be conserved in mammalian CYLC1
   • Mechanism by which KKD/E repeats modulate tubulin dynamics is undefined at a structural level
2. 2023 High

   The first mammalian loss-of-function study demonstrated that CYLC1 is required for perinuclear theca calyx assembly and normal sperm morphogenesis, resolving the long-standing question of whether this testis-specific protein is functionally important for male fertility.

   Evidence CRISPR/Cas9 Cylc1 knockout mouse with phenotypic characterization of sperm morphology, motility, and count; immunofluorescence localization in spermatids and mature sperm; whole-exome sequencing identifying CYLC1 variants in infertile men

   PMID:38013430

   Open questions at the time

   • Molecular mechanism by which CYLC1 loss impairs cytoplasmic shedding was not defined
   • Relative contributions of CYLC1 versus paralog CYLC2 to calyx function were not resolved
3. 2024 High

   Protein interaction mapping revealed that CYLC1 self-associates and bridges the inner acrosomal membrane (via SPACA1) to the nuclear envelope (via FAM209), establishing the molecular architecture of the IAM–cylicins–NE sandwich that anchors the acrosome to the nucleus.

   Evidence Co-immunoprecipitation/protein interaction assays, CRISPR/Cas9 knockout mice, immunofluorescence, human whole-exome sequencing

   PMID:38573307

   Open questions at the time

   • Stoichiometry and direct binding interfaces between CYLC1, SPACA1, and FAM209 have not been structurally resolved
   • Whether CYLC1 and CYLC2 form heteromeric complexes within the calyx is not established
4. 2025 Medium

   Human missense variants in CYLC1 that do not affect protein expression but alter its subcellular localization confirmed that correct CYLC1 targeting, not merely its presence, is required for acrosome integrity, and revealed secondary displacement of PLCZ1 as a downstream consequence.

   Evidence Western blotting, immunofluorescence, SEM/TEM, and Sanger sequencing of patient sperm; Cylc1 KO mouse confirmation

   PMID:41075067

   Open questions at the time

   • Functional consequence of PLCZ1 mislocalization for fertilization competence is not tested
   • Findings from a single patient cohort await independent replication
   • Structural basis for how K328E and K324M disrupt CYLC1 targeting is unknown

• It remains unknown how CYLC1 is initially recruited to the acrosomal region in round spermatids, what post-translational modifications regulate its assembly into the calyx, and whether its microtubule-regulatory activity observed in C. elegans homologues plays any role during mammalian spermiogenesis.
• No structural model of CYLC1 or the calyx complex exists
• Mechanism of initial CYLC1 recruitment to the perinuclear theca is undefined
• Relationship between invertebrate microtubule regulation and mammalian calyx function is unresolved