Time Course of Thyrotropin-Dependent Protein Phosphorylation in Thyroid Slices*

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Abstract
The time course of TSH[thyrotropin]-dependent protein phosphorylation was studied in calf thyroid slices labeled in vitro with 32Pi. Several of the proteins identified by 2-dimensional electrophoresis displayed striking increases in 32P labeling in the presence of TSH. Phosphorylation of histones H3 and H1 (2 subgroups) was enhanced about 3.7- and 10-fold, respectively, after incubation with TSH (15 mU[units]/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. 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 c[cyclic]AMP levels and protein kinase activity in thyroid slices after TSH.