The activin and TGF-β type II receptors are members of a separate subfamily of transmembrane receptors with intrinsic protein kinase activity, which also includes the recently cloned TGF-β type I receptor. We have isolated and characterized a cDNA clone (C14) encoding a new member of this subfamily. The domain structure of the C14-encoded protein corresponds with the structure of the other known transmembrane serine/threonine kinase receptors. It also contains the two inserts in the kinase domain that are characteristic for this subfamily. Using in situ hybridization, C14 mRNA was detected in the mesenchymal cells located adjacent to the müllerian ducts of males and females at day 15 (E15) of embryonic development. Marked C14 mRNA expression was also detected in the female gonads. In female E16 embryos, the C14 mRNA expression pattern remained similar to that in E15 embryos. However, in male E16 embryos C14 mRNA was detected in a circular area that includes the degenerating müllerian duct. The expression of C14 mRNA was also studied using RNase protection assays. At E15 and E16, C14 mRNA is expressed in the female as well as in the male urogenital ridge. However, at E19, a high C14 mRNA level in the female urogenital ridge contrasts with a lack of C14 mRNA in the male urogenital ridge. This correlates with the almost complete degeneration of the müllerian ducts in male embryos at E19. C14 mRNA expression was also detected in embryonic testes at E15, E16 and E19 using RNase protection assays, but at much lower levels than those found in the developing ovaries. In eleven other tissues no C14 mRNA was observed.

The results point to anti-müllerian hormone (AMH) being the most likely candidate ligand for C14. The embryonic C14 mRNA expression pattern in the urogenital ridge correlates with the expected site of AMH action, and C14 mRNA expression in the fetal ovary is in agreement with known effects of AMH on gonadal differentiation.

Postnatal C14 mRNA expression in rats was found to be confined mainly to the gonads. In the testis, C14 mRNA expression occurs in Sertoli cells. This testicular expression markedly increases during the first 3 weeks after birth, concurrent with the onset of spermatogenesis.