May function as a regulator of both motility- and head-associated functions such as capacitation and the acrosome reaction. May bind calcium in vitro (By similarity). Interacts with FSCB. Localized to fibrous sheath including the surface of the longitudinal columns and ribs of the principal piece of sperm flagella. Phosphorylated on tyrosine residues during in vitro capacitation. Dephosphorylation affects its ability to bind calcium (By similarity).