May function as a regulator of both motility- and head-associated functions such as capacitation and the acrosome reaction. Binds calcium in vitro (By similarity). Interacts with FSCB. Localizes to fibrous sheath of the principal piece of sperm flagella. Expressed in testis. Phosphorylated on tyrosine residues during in vitro capacitation. Dephosphorylation affects its ability to bind calcium (By similarity). On the 2D-gel the determined pI of this protein is: 4.3 to 4.6, its MW is: 97 kDa.