Immunolocalization of heat shock protein 70 in bovine spermatozoa

Heat shock protein 70 (HSP70) is part of a superfamily of molecular chaperones, which protect cells from chemical and heat shock. The objectives of this study were to determine the presence of HSP70 in bovine spermatozoa and its subcellular localization during different stages of spermatogenesis. Analysis of sperm proteins by Western blotting using a monoclonal antibody to the inducible form of HSP70 revealed a single immunoreactive band with an estimated molecular weight of 70 kDa in samples from 18 of 18 bulls. Using immunofluorescence microscopy and the same antibody, HSP70 was localized to the cytoplasm of prophase spermatocytes and elongating spermatids, to cytoplasmic droplets of caput epididymal spermatozoa, and to cytoplasmic droplets, acrosome, post‐acrosomal region and middle piece of corpus and cauda epididymal spermatozoa. The pattern of distribution changed in freshly ejaculated spermatozoa as HSP70 was detected on the acrosome only. During capacitation and acrosome reaction, HSP70 was once again redistributed, and was localized to the equatorial segment, post‐acrosomal region and middle piece. Thus, HSP70 is present in the spermatozoa of mature bulls and redistribution of the protein occurs during capacitation and the acrosome reaction.