Seminal vesicle autoantigen, a novel phospholipid-binding protein secretedfrom 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 mo use seminal vesicle secretion. It was quantified to be 0.9% (w/v) in the se minal vesicle fluid. We examined its distribution in the accessory sexual g land, 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 stain ing intensity for the binding of SVA to spermatozoa remained even when the cells were pretreated with protease digestion, acid or heat at 100 degrees C for 10 min. Moreover, the SVA-sperm binding could be inhibited by the dis persed sperm lipid. The specificity of interaction between I-125-SVA and ph ospholipids was studied by TLC overlay techniques. The radiolabelled protei n showed strong binding to purified phosphatidylcholine and phosphatidylser ine and weak binding to purified sphingomyelin, lysophosphatidylcholine and phosphatidylethanolamine, but did not interact with phosphatidic acid, lys ophosphatidic acid or phosphatidylinositol. Among the lipids extracted from spermatozoa, SVA showed strong binding to phosphatidylcholine and weak bin ding to sphingomyelin and neutral lipids. The assay for SVA-sperm binding w ith I-125-SVA determined the IC50 as being (3.89+/-0.65) x 10(-5) M-1, whic h is compatible with an apparent dissociation constant of (9.10+/-0.02) x 1 0(-5) M-1 estimated by fitting the data of phosphatidylcholine-perturbed SV A fluorescence to a modified Scatchard plot. SVA showed an ability to suppr ess sperm motility. The average path velocity, straight-line velocity and c urvilinear velocity of sperm were not detectable by computer-assisted sperm assay after incubation of the cells in the presence of 0.3% SVA at 37 degr ees C for more than 40 min.