Selected Proteins of “Prostasome-Like Particles” from Epididymal Cauda Fluid Are Transferred to Epididymal Caput Spermatozoa in Bull1

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Abstract
During epididymal transit, spermatozoa acquire selected proteins secreted by epithelial cells. We recently showed that P25b, a protein with predictive properties for bull fertility, is transferred from prostasome-like particles present in the cauda epididymal fluid (PLPCd) to the sperm surface. To further characterize the interactions between PLPCd and epididymal spermatozoa, PLPCd were prepared by ultracentrifugation of bull epididymal fluid, then surface-exposed proteins were biotinylated and coincubated in different conditions with caput epididymal spermatozoa. Western blot analysis revealed that only selected proteins are transferred from PLPCd to spermatozoa. MALDI-TOF analysis revealed that these transferred proteins are closely related. The pattern of distribution of the PLPCd transferred varied from one sperm cell to the other, with a bias toward the acrosomal cap. This transfer appeared to be temperature sensitive, being more efficient at 32–37°C than at 22°C. Transfer of PLPCd proteins to spermatozoa was also pH dependant, the optimal pH for transfer being 6.0–6.5. The effect of divalent cations on PLPCd protein transfer to caput spermatozoa was investigated. Whereas Mg2+ and Ca2+ have no effect on the amount of proteins remaining associated with spermatozoa following coincubation, Zn2+ had a beneficial effect. These results are discussed with regard to the function of PLPCd in epididymal sperm maturation.