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Subcellular localization

flagella, Nucleoplasm

Functional category

• Ciliary assembly/disassembly
• Cilia–cytoskeleton/adhesion links

Function

We report that a Chibby3 (Cby3)/Cby1-interacting BAR domain-containing 1 (ciBAR1) complex is required for this process. Ablation of either gene in mice results in male fertility defects, caused by kinked sperm flagella with the annulus mispositioned in the PP. Cby3 and ciBAR1 interact and colocalize to the annulus near the curved membrane invagination at the flagellar pocket. In the absence of Cby3, periannular membranes appear to be deformed, allowing the annulus to migrate over the fibrous sheath into the PP. Collectively, our results suggest that the Cby3/ciBAR1 complex regulates local membrane properties to position the annulus at the MP/PP junction.Prior to S9, ciBAR1 is detectable at the distal appendages of basal bodies. Cby3 is then recruited to the distal appendages around S9, while annulus assembly initiates independently. The Cby3/ciBAR1 complex remains at the flagellar base, and the annulus continues to mature. At S15, the annulus migrates caudally to the MP/PP junction. Throughout this process, the Cby3/ciBAR1 complex localizes to the proximal curved membranes of the flagellar pocket and likely confers local membrane rigidity. When the annulus reaches its destination, the increased membrane rigidity prevents membrane deformation and aids the annulus to halt at the proximal point of the fully developed fibrous sheath. In Cby3−/− spermatids, ciBAR1 is recruited to the flagellar base but fails to stably associate with the periannular membrane during annulus migration, while ciBAR1−/− spermatids show no detectable Cby3 recruitment at the flagellar base. In both mutants, the curved periannular membrane exhibits decreased rigidity, allowing the annulus to migrate over the fibrous sheath into the PP(38197861).