May function as a regulator of both motility- and head-associated functions such as capacitation and the acrosome reaction. Isoform 1 binds calcium in vitro. Isoform 2 and isoform 6 probably bind calcium. Isoform 3 and isoform 5 do not bind calcium in vitro. Isoform 4 probably does not bind calcium. Interacts with FSCB (By similarity). Isoform 3 self-associates. Isoform 3 and isoform 5 interact with GSK3B. Isoform 1 does not interact with GSK3B. Localized to fibrous sheath including the surface of the longitudinal columns and ribs of the principal piece of sperm flagella. According to PubMed:15752768, isoform 1, isoform 3 and isoform 5 are both nuclear and cytoplasmic. According to PubMed:15752768, isoform 1, isoform 3 and isoform 5 are both nuclear and cytoplasmic. According to PubMed:15752768, isoform 1, isoform 3 and isoform 5 are both nuclear and cytoplasmic. Expressed in elongating spermatids and spermatozoa (at protein level). Isoform 1 is expressed in testis. Isoform 3 and isoform 5 are also expressed in brain, pancreas and numerous brain tumors. Isoform 1 is phosphorylated on tyrosine residues during in vitro capacitation. Isoform 3 and isoform 5 are phosphorylated by GSK3B in vitro. Dephosphorylation affects its ability to bind calcium. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay. May be produced at very low levels due to a premature stop codon in the mRNA, leading to nonsense-mediated mRNA decay.