Dephosphorylation of 19K and 21K polypeptides in response to thyroid-stimulating hormone in cultured thyroid cells.

Cultured dog thyroid cells incubated with [32P] phosphate contain at least two phosphoproteins of 19 and 21 kDalton (K), as determined by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. Myosin light chain appears to be a component of the 19K and 21K phosphoproteins by the following criteria: 1) coextraction with myosin heavy chain from Triton-insoluble cytoskeletons with KCl-ATP, 2) coisolation with myosin heavy chain by immunoprecipitation, and 3) purification of undenatured myosin with pyrophosphate-agarose gel electrophoresis. The phosphorylation state of these proteins is decreased by incubation of cells with TSH. In the basal state, the 19K and 21K proteins from Triton-insoluble cytoskeleton fractions contain 0.86 +/- 0.07 (+/- SE) mol phosphate/mol protein, which is reduced to 0.34 +/- 0.03 in TSH-treated cells. TSH-induced dephosphorylation occurs in 1 min with 2.5 mU/ml TSH and reaches a maximum at 15 min. This TSH effect appears to be mediated by cAMP, since it is mimicked by (Bu)2cAMP, forskolin, cholera toxin, and prostaglandin E1 and is potentiated by isobutylmethylxanthine. Carbamylcholine, ionophore A23187, and norepinephrine, which inhibit TSH stimulation of cAMP, have no effect on basal phosphorylation of the 19K and 21K proteins, but do inhibit the effect of TSH.