TRA98 is a testis-specific nuclear protein, but its biological role is unclear. We analyzed the localization of TRA98 indeveloping spermatogenic cells using immunofluorescence (IF) and immunoelectron microscopy (IEM). TRA98 was localized exclusively to the nuclei. In spermatocytes, IF staining was associated with certain sub-nuclear structures to show a reticular pattern; the XY body was strongly stained. In spermatids, both reticular and punctate staining patterns were observed. In late spermatids, staining decreased as cell differentiation proceeded. However, epididymal sperm were strongly stained when smear preparations were not fixed, or followed by treatment with4 Murea or 2% mercaptoethanol. IEM showed that gold signals were closely associated with electron-dense masses but not with the nucleoli. We then investigated the expression of TRA98 indifferentiating spermatocytes using a quantitative IEM technique. A small expression peak was observed around stage II-III and a second large peak was noted at stage XI. In spermatids, a single expression peak was observed at step 5; labeling density then decreased gradually but did not reach zero. In early spermatids, heterochromatin was stained much more than euchromatin. The results suggest that the function of TRA98 increases at three points during spermatogenesis. In addition, TRA98 is maintained in the sperm head and carried into the egg after fertilization.
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