Thyrotropin stimulation of a thyroid contractile protein phosphorylation

Contractile proteins were isolated from preincubated dog thyroid slices by a procedure identical to the one used for the preparation of muscle actomyosin. At least two of these proteins (molecular weight = 26000 and 15000) were phosphorylated in the intact cell system. After one hour of incubation of the slices with thrytropin (10 mU/ml) and [32P]phosphate, the specific activity of the 26000-Mr protein was increased by a factor of three while no significant change in specific activity was observed in the 1500-Mr contractile protein. This effect of thyrotropin was already observed after 30 min of action, was elicited at hormone concentrations of the same order as those required to induce secretion and has been reproduced by dibutyryl adenosine 3':5'-monophosphate. Cycloheximide (0.35 mM) which almost totally inhibited protein synthesis in thyroid slices, did not affect the thyrotropin stimulation of the phosphorylation of the 26000-Mr protein. The phosphorylation of serine residue(s) has been demonstrated in this protein but the presence of radioactive phosphothreonine could not be detected. On the basis of its molecular weight, the 26000-Mr protein could be similar to the inhibitory component of muscle troponin complex (TN-I).