Fibrous sheath CABYR (Calcium Binding Tyrosine-(Y)-Phosphorylation Regulated) binding protein (FSCB) is a testis- and sperm-specific protein that is expressed post-meiotically and localised in mouse sperm flagella. FSCB homologues are present in other mammals, including rat and human. Conserved motifs in FSCB include PXXP, proline-rich and extensin-like regions [].FSCB is regulated by protein kinase A (PKA)-mediated tryrosine phosphorylation. Its capacity of being phosphorylated and binding to calcium enhances the spermatozoa flagellar movement, spermatozoa capacitation and activation. FSCB phosphorylation activates spermatozoa motility and inhibits SUMOylation of two crucial proteins ROPN1/ROPN1L that are associated with PKA/A kinase activity and spermatozoa motility [].
