Practice question

Among cytoskeletal and enzymatic proteins, true paracrine morphogens belong to growth factor families. Activin, a TGF-beta superfamily member, exemplifies paracrine morphogen functioning similar to Xenopus mesoderm induction. Secreted as disulfide-linked dimer, it binds type II and type I serine-threonine kinase receptors causing Smad2/3 phosphorylation, forming complex with Smad4 translocating to nucleus regulating Brachyury, Mix genes. At low concentration induces ventral mesoderm blood, at high concentration induces dorsal organizer notochord. Its activity blocked by Follistatin antagonist, illustrating dosage-dependent positional information analogous to Nodal and BMP roles patterning embryonic fields.