Two rat middle-piece coating antigens are lost upon in vitro capacitation.

The rat sperm-binding antigen, CFS, contains two subunits, RSV IV and RSV V. Sperm-bound RSV IV was previously detected using anti-RSV IV antibodies, while the presence of RSV V on the cell surface was doubtful. The RSV V subunit was purified from rat seminal vesicle secretion by gel filtration and ionic exchange chromatography. Rabbit polyclonal mono-specific antibodies against either RSV IV or RSV V were used to monitor the binding of the CFS complex onto the rat spermatozoa in indirect immunofluorescence experiments with FITC-conjugated goat anti-rabbit antibodies. Caudal epididymal spermatozoa were incubated in vitro with CFS and evidence of the presence of both subunits on the cell surface was obtained as the heterodimeric sperm-coating antigen was detected on the whole tail. Restriction of the CFS binding sites to the middle-piece surface domain of ejaculated spermatozoa was observed. In addition, release of CFS from this plasma membrane region was observed after in vitro capacitation. The localization of CFS before capacitation and its absence from the sperm surface after capacitation are compatible with a role of this factor as an inhibitor of motility in rat decapacitation.