IMMUNOLOCALIZATION AND QUANTITATION OF ACIDIC SEMINAL FLUID PROTEIN (ASFP) IN EJACULATED, SWIM-UP, AND CAPACITATED BULL SPERMATOZOA

Acidic seminal fluid protein (aSFP, 12.9 kDa), a major protein of bull seminal plasma, belongs to the spermadhesin protein family. Boar sper madhesins become bound to the sperm head's surface at ejaculation and are thought to play a role as capacitation factors and/or in gamete re cognition and binding. Here, we have investigated the topographical di stribution and fate of bovine spermadhesin aSFP during sperm capacitat ion in order to assess whether aSFP could be involved in similar aspec ts of the fertilization process as its boar homologous proteins. 5.7 /- 2.1 x 10(6) molecules/spermatozoa were quantitated on the surface o f fresh ejaculated and washed sperm. The binding site of aSFP was rest ricted to a thin coat at the apical part of the acrosomal cap. The amo unt of aSFP in swim-up sperm was 1.8 +/- 1.0 x 10(6) molecules/spermat ozoa, but decreased dramatically to 22 +/- 10 x 10(3) and to undetecta ble levels after incubation of sperm for 1.5h and 18h, respectively, i n capacitation medium. This indicates that the bull spermatozoa surfac e may be completely depleted of spermadhesin aSFP before spermatozoa r each the surroundings of the investing egg. Therefore, our results sug gest that aSFP may act as a decapacitation factor on bull spermatozoa rather than as a zona pellucida binding molecule.