Seminal vesicle autoantigen, a novel phospholipid-binding protein secreted from luminal epithelium of mouse seminal vesicle, exhibits the ability to suppress mouse sperm motility

Seminal vesicle autoantigen (SVA) is a 19 kDa glycoprotein purified from mouse seminal vesicle secretion. It was quantified to be 0.9% (w/v) in the seminal vesicle fluid. We examined its distribution in the accessory sexual gland, characterized its binding sites on the sperm surface and assessed its effect on sperm motility. It was immunolocalized on the epithelium of the primary and secondary folds in the tissue. Mouse spermatozoa collected from caudal epididymis were devoid of SVA. A cytochemical study illustrated the presence of SVA-binding region on the entire cells. The cytochemical staining intensity for the binding of SVA to spermatozoa remained even when the cells were pretreated with protease digestion, acid or heat at 100°C for 10 min. Moreover, the SVA-sperm binding could be inhibited by the dispersed sperm lipid. The specificity of interaction between ¹²⁵I-SVA and phospholipids was studied by TLC overlay techniques. The radiolabelled protein showed strong binding to purified phosphatidylcholine and phosphatidylserine and weak binding to purified sphingomyelin, lysophosphatidylcholine and phosphatidylethanolamine, but did not interact with phosphatidic acid, lysophosphatidic acid or phosphatidylinositol. Among the lipids extracted from spermatozoa, SVA showed strong binding to phosphatidylcholine and weak binding to sphingomyelin and neutral lipids. The assay for SVA-sperm binding with ¹²⁵I-SVA determined the IC₅₀ as being (3.89 ± 0.65) x 10^-5 M^-1, which is compatible with an apparent dissociation constant of (9.10 ± 0.02) x 10^-5 M^-1 estimated by fitting the data of phosphatidylcholine-perturbed SVA fluorescence to a modified Scatchard plot. SVA showed an ability to suppress sperm motility. The average path velocity straight-line velocity and curvilinear velocity of sperm were not delectable by computer-assisted sperm assay after incubation of the cells in the presence of 0.3% SVA at 37°C for more than 40 min.