Evidence for an epididymal origin of bovine sperm forward motility protein

Forward mobility protein (FMP) is a glycoprotein found in bovine seminal plasma that can induce forward motion in immotile caput spermatozoa under conditions of elevated cyclic AMP levels. This paper presents evidence that this seminal plasma protein is of epididymal origin. FMP activity is present in decreasing amounts in caudal epididymal fluid, seminal plasma and rete testis fluid. The FMP of caudal epididymal fluid displays properties identical to those of seminal plasma FMP after heat, freeze-thaw, trypsin or β-galactosidase treatment. Further physical evidence for the similar character of epididymal and seminal plasma FMP is shown by the fact that the complex Sepharose 6B elution profiles of the forward motility inducing activity in the 2 fluids are nearly identical. The typical motility inducing profile seen in the semen of the normal monkey was not observed in the ejaculate of the vasectomized animal. This indicates that FMP is not produced by the accessory glands. These data indicate an epididymal origin of FMP and suggest a physiological role for this protein in the acquisition of sperm motility.