Time course of thyrotropin-dependent protein phosphorylation in thyroid slices

The time course of TSH-dependent protein phosphorylation was studied in calf thyroid slices labeled in vitro with 32Pi. Several of the proteins identified by two-dimensional electrophoresis displayed striking increases in 32P labeling in the presence of TSH. Phosphorylation of histones H3 and H1 (two subgroups) was enhanced about 3.7-and 10-fold, respectively, after incubation with TSH (15 mu/ml) for 70 min. Histone phosphorylation showed a lag after exposure to TSH; a major increase occurred only after 30-min incubation but then increased progressively up to 2 h. The lag in histone phosphorylation was also observed with slices prelabeled with 32P before the addition of TSH. In contrast, phosphorylation of a minor basic protein, A5 was also increased by 15 mU/ml TSH but displayed different kinetics, increasing substantially at 10 min. This time course correlates well with the rise in intracellular cAMP levels and protein kinase activity in thyroid slices after TSH.