Single cysteine to tyrosine transition inactivates the growth inhibitory function of Piedmontese myostatin

Myostatin, amember of the transforming growth factor- superfamily, is a secretedgrowth factor that is proteolytically processed to give COOH-terminalmature myostatin and NH₂-terminal latency-associated peptide in myoblasts. Piedmontese cattle are a heavy-muscled breed thatexpress a mutated form of myostatin in which cysteine(313) is substituted with tyrosine. Here we havecharacterized the biology of this mutated Piedmontese myostatin.Northern and Western analyses indicate that there is increasedexpression of myostatin mRNA and precursor myostatin protein in theskeletal muscle of Piedmontese cattle. In contrast, a decrease inmature myostatin was observed in Piedmontese skeletal muscle. However,there is no detectable change in the circulatory levels of maturemyostatin in Piedmontese cattle. Myoblast proliferation assay performedwith normal and Piedmontese myostatin indicated that mature wild-typemyostatin protein inhibited the proliferation ofC₂C₁₂ myoblasts. Piedmontese myostatin, bycontrast, failed to inhibit myoblast proliferation. In addition, whenadded in molar excess, Piedmontese myostatin acted as a potent"competitive inhibitor" molecule. These results indicate that, inPiedmontese myostatin, substitution of cysteine with tyrosineresults in the distortion of the "cystine knot" structure and aloss of biological activity of the myostatin. This mutation alsoappears to affect either processing or stability of mature myostatinwithout altering the secretion of myostatin.