AJ-p90: a novel protein of the perinuclear theca in human sperm subacrosome

A perinuclear theca protein of the human sperm subacrosome was detected using LH43 monoclonal antibody which was originally raised against human keratinocytes. Using indirect immunofluorescence, the antibody stained the acrosomal zone (AZ) of dried ejaculated spermatozoa but did not react with viable cells, thus suggesting that the antigen was intracellular. This was confirmed by immunogold electron microscopy which also revealed the ultrastructural localisation of the antigen to the subacrosomal fibrils. Throughout spermatogenesis the antigen was only detected on the AZ of developed testicular spermatozoa and its expression was continued during their epididymal passage. Biochemically, the protein was insoluble in Triton, and dithiothreitol (DTT) was required for its solubilisation. In Western blotting of sperm and keratinocyte lysates, the antibody detected similar 90-kDa protein doublets (AJ-p90). These biochemical features exclude the identity of AJ-p90 with various cyto- and karyo-skeletal antigens, including the intermediate filaments and microfilaments. AJ-p90 therefore represents a novel product of the subacrosomal perinuclear theca. The significance of these data is discussed together with the importance of the antibody for probing the perinuclear theca in normal and abnormal germ cells, including multinucleated spermatids which also showed reactivity with the antibody.