Identification of a nuclear localization signal in activin/inhibin beta(A)subunit; intranuclear beta(A) in rat spermatogenic cells

Activin is a dimeric glycoprotein hormone that was initially characterized by its ability to stimulate pituitary FSH secretion and was subsequently re cognized as a growth factor with diverse biological functions in a large va riety of tissues. In the testis, activin has been implicated in the auto/pa racrine regulation of spermatogenesis through its cognate cell membrane rec eptors on Sertoli and germ cells. In this study we provide evidence for int ranuclear activin/inhibin beta(A) subunit and show its distribution in the rat seminiferous epithelium. We have shown by transient expression in HeLa cells of beta-galactosidase fusion proteins that the beta(A) subunit precur sor contains a functional nuclear localization signal within the lysine-ric h sequence corresponding to amino acids 231-244. In all stages of the rat s eminiferous epithelial cycle, an intense immunohistochemical staining of nu clear beta(A) was demonstrated in intermediate or type B spermatogonia or p rimary spermatocytes in their initial stages of the first meiotic prophase, as well as in pachytene spermatocytes and elongating spermatids primarily in stages IX-XII. In some pachytene spermatocytes, the pattern of beta(A) i mmunoreactivity was consistent with the characteristic distribution of pach ytene chromosomes. In the nuclei of round spermatids, beta(A) immunoreactiv ity was less intense, and in late spermatids it was localized in the residu al cytoplasm, suggesting disposal of beta(A) before spermatozoal maturation . Immunoblot analysis of a protein extract from isolated testicular nuclei revealed a nuclear beta(A) species with a molecular mass of approximately 2 4 kDa, which is more than 1.5 times that of the mature activin beta(A) subu nit present in activin dimers. These results suggest that activin/inhibin b eta(A) may elicit its biological functions through two parallel signal tran sduction pathways, one involving the dimeric molecule and cell surface rece ptors and the other an alternately processed beta(A) sequence acting direct ly within the nucleus. According to our immunohistochemical data, beta(A) m ay play a significant role in the regulation of nuclear functions during me iosis and spermiogenesis.