Bovine sperm forward motility protein: Binding to epididymal spermatozoa

Immature spermatozoa from the bovine caput epididymis develop an uncoordinated thrashing motion in response to an elevation in cyclic AMP levels and convert to strong progressive motility upon the subsequent addition of bovine forward motility protein. We show that this protein activates caput sperm in a switchlike, all-or-none manner and that the activation is not easily reversible. We also present evidence that the forward motility protein binds to caput sperm as they pass through the epididymis. First, the continued presence of free forward motility protein is not necessary. Second, the activation does not involve a simple enzymatic mechanism. Third, the forward motility protein can be depleted from solution by caput, but not caudal, epididymal sperm. Fourth, forward motility protein can be extracted from caudal, but not caput, epididymal sperm with Triton X-100.