The Cby3/ciBAR1 complex positions the annulus along the sperm flagellum during spermiogenesis.

Proper compartmentalization of the sperm flagellum is essential for fertility. The annulus is a septin-based ring that demarcates the midpiece (MP) and the principal piece (PP). It is assembled at the flagellar base, migrates caudally, and halts upon arriving at the PP. However, the mechanisms governing annulus positioning remain unknown. 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.