Identification of a hamster epididymal region-specific secretory glycoprotein that binds nonviable spermatozoa

Even though the epididymis produces an environment promoting sperm maturati on and viability, some sperm do not survive transit through the epididymal tubule. Mechanisms that segregate the epididymal epithelium and/or the viab le sperm population from degenerating spermatozoa are poorly understood. We report here the identification and characterization of HEP64, a 64-kDa gly coprotein secreted by principal cells of the corpus and proximal cauda epid idymidis of the hamster that specifically binds to and coats dead/dying spe rmatozoa. The HEP64 monomer contains similar to 12 kDa carbohydrate and, fo llowing chemical deglycosylation, migrates as a similar to 52-kDa polypepti de. Both soluble (luminal fluid) and sperm-associated HEP64 are assembled i nto disulfide-linked high molecular weight oligomers that migrate as a doub let band of 260/280 kDa by nonreducing SDS-PAGE. In the epididymal lumen, H EP64 is concentrated into focal accumulations containing aggregates of stru cturally abnormal or degenerating spermatozoa, and examination of sperm sus pensions reveals that HEP64 forms a shroudlike coating surrounding abnormal spermatozoa. The HEP64 glycoprotein firmly binds degenerating spermatozoa and is not released by either nonionic detergent or high salt extraction. E lectron microscopic immunocytochemistry demonstrates that HEP64 localized t o an amorphous coating surrounding the abnormal spermatozoa. The potential mechanisms by which this epididymal secretory protein binds dead spermatozo a as well as its possible functions in the sperm storage function of the ca uda epididymidis are discussed.