Monomeric activin A retains high receptor binding affinity but exhibits low biological activity

Activins are multipotent hormones/growth factors that belong to the transforming growth factor-beta (TGF-beta) superfamily. Like TGF-beta s, activins have 9 conserved cysteine residues and are disulfide-bonded dimers. Based on the three-dimensional structure of TGF-beta 2, we deduced Cys80 in activin A to form the intermolecular disulfide bond. To obtain a monomeric form of activin, Cys80 was exchanged for a serine residue by polymerase chain reaction mutagenesis. The mutant protein was expressed in a baculovirus/insect cell expression system. The molecular mass of this mutant activin was determined to be 13 kDa (consistent with a single chain form of the protein) by SDS-polyacrylamide gel electrophoresis and by laser desorption mass spectroscopy. When this mutant monomeric activin was incubated with cells that expressed either the activin type IIB receptor or both the type I and type IIB receptors, its affinity was found to be 20% of that of native activin on a mass basis. Binding affinity determined using the mouse pituitary cell line AtT 20 was 10% of that of native activin A. Biological potency, however, as determined by the mutant protein's ability to release FSH from anterior pituitary cells in primary culture and by its ability to suppress basal ACTH secretion form AtT 20 cells, was only 1% of that of the native protein. This discrepancy of an order of magnitude between binding and biological activity is consistent with a model in which dimerization of the hormone is not necessary for high affinity binding to its receptor(s) while being essential for efficient signal transduction.