Transforming growth factor-beta (TGF-beta) and activin signal primarily through interaction with type I and type II receptors, which are transmembrane serine-threonine kinases. Tsk 7L is a type I receptor for TGF-beta and requires coexpression of the type II TGF-beta receptor for ligand binding. Tsk 7L also specifically bound activin, when coexpressed with the type IIA activin receptor. Tsk 7L could associate with either type II receptor and the ligand binding specificity of Tsk 7L was conferred by the type II receptor. Tsk 7L can therefore act as type I receptor for both activin and TGF-beta, and possibly other ligands.