Molecular cloning of complementary DNAs (cDNA) whose expression products bind activin and transforming growth factor beta (TGF-beta 1 and -beta 2) suggests that transmembrane serine/threonine kinases constitute a new class of signaling molecules. A human liver cell cDNA which codes for a new serine/threonine kinase receptor (SKR1) was identified using degenerate oligonucleotide primers complementary to coding sequence for mouse activin and Caenorhabditis elegans daf-1 serine/threonine receptor kinase subdomains VI and VIII in the polymerase chain reaction. The deduced 509-amino acid product consisted of a cysteine-rich extracellular domain and a cytoplasmic serine/threonine kinase domain which are 10-20 and 40% homologous to the respective domains in the activin and transforming growth factor beta receptor kinases. Cells overexpressing SKR1 exhibited no increase in binding of activin, inhibin, TGF-beta 1, TGF-beta 2, or bone morphogenic factor type 2B. Except for its absence in bone and spleen, SKR1 exhibits a tissue expression pattern similar to the TGF-beta receptor II gene. Similarly, SKR1 is expressed in normal parenchymal cells, endothelial cells, fibroblasts, and tumor-derived epithelial cells. The expression pattern and lack of binding to prototypic members of the TGF-beta 1-5 branch of the TGF-beta superfamily suggests that SKR1 is potentially a receptor for a new member of the TGF-beta branch of the ligand superfamily.