Sperm Acrosome Associated Protein 4 (SPACA4)

SPACA4 shares highest homology with the Ly6/uPAR superfamily, especially PLAUR, and it has a C-terminal cysteine-rich domain characteristic of Ly6/uPAR proteins. SPACA4 also contains a signal peptide, a transmembrane domain, 4 potential phosphorylation sites, and a transamidase cleavage site, suggesting that SPACA4 is a glycosylphosphatidylinositol (GPI)-anchored protein. Phospholipase C treatment of sperm released SPACA4 into the supernatant, confirming GPI linkage. Northern and dot blot analysis showed a 1.3-kb SPACA4 transcript expressed exclusively in testis. Immunofluorescence analysis localized SPACA4 to the acrosomal region in sperm samples that were stringently washed and in acrosome-reacted sperm. In unwashed sperm samples, SPACA4 localized over the entire sperm.