Endogenous phosphate acceptor proteins for rat liver cytosolic casein kinases.

Highly purified preparations of rat liver cytosol casein kinase TS (Ck-TS) still contain a phosphorylatable protein (Mr = 25,000 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis) which is also detectable in crude cytosol and which is not phosphorylated by casein kinase S from the same source. When purified Ck-TS is added back to crude cytosol, it promotes the phosphorylation of at least three protein bands (Mr = 89,000, 49,000, and 40,000) besides the 25,000 band. The phosphorylation of the 50,000 and 25,000 bands is greatly enhanced whenever enzymatically dephosphorylated and/or heated (70 degrees C, 5 min) cytosol replaces native cytosol as a substrate for Ck-TS. The electrophoretic mobilities of the 80,000, 49,000, and 25,000 phosphorylatable proteins are consistent with their identification as glycogen synthase, calsequestrin, and protein phosphatase inhibitor-1, respectively. Actually, in vitro all these three proteins readily undergo a Ck-TS-dependent phosphorylation.