cAMP-Independent Casein Kinase Mediates Phosphorylation of Many T₃-Dependent Phosphoproteins in Rat Liver Cytosol

Summary

Administration of T₃ (20 μg/100 g BW) for 3 days increases phosphorylation of several proteins in rat liver cytosol in vitro. To help elucidate the mechanism of T₃-induced phosphorylation, we studied which protein kinase(s) mediate phosphorylation of endogenous cytosolic proteins. Five different protein kinases were obtained by DEAE+ cellulose column chromatographic fractionation of liver cytosol. When their ability to phosphorylate heat-inactivated cytosol was investigated, casein kinase, a cAMP independent protein kinase, showed the strongest effect. Casein kinase, purified by phosphocellulose chromatography, phosphorylated more than 10 cytosolic proteins. Several T₃-dependent (and cAMP independent) phosphoproteins were included among these. One protein with Mr 39 × 10³, of which phosphorylation is stimulated by T₃ within five hours after injection, was the most active substrate for casein kinase.

The results suggest that casein kinase is the enzyme responsible for phosphorylation of many rat liver cytosolic proteins and that several phosphoproteins, apparently under T₃-regulation, might be phosphorylated by this enzyme.

Key-Words:

Thyroid Hormone Action - Triiodothyronine - Casein Kinase - Phosphoproteins