Detection of type-2 casein kinase and its endogenous substrates in the components of the microsomal fraction of rat liver.

Rat liver type-2 casein kinase-TS (Ck-TS) is unevenly distributed among the different components of the rat liver post-mitochondrial particulate fraction: while it accounts for the whole casein kinase activity of protein-glycogen particles, it is absent in the microsomal membranes (exhibiting exclusively casein kinase activity of type-1) and coexists with type-1 casein kinase in ribosomes. At least seven proteins whose phosphorylation is promoted by Ck-TS have been detected in these fractions. The only important target of Ck-TS in protein-glycogen particles is a rather heterogeneous protein of Mr around 85000 which has been identified as glycogen synthase. Two polypeptides of Mr about 16000, possibly identifiable with acidic proteins P1 and P2, and an unidentified protein of Mr about 35000 are the main ribosomal substrates of Ck-TS. Three protein bands of Mr 52000, 79000 and 91000 are also very efficiently phosphorylated whenever the microsomal membranes, devoid of intrinsic casein kinase-2, are incubated with cytosolic Ck-TS. These membrane-bound radiolabeled proteins require deoxycholate for their solubilization: they are very acidic, according to their high affinity for DEAE-cellulose, and give rise to partially superimposable [32P]peptide maps, suggesting extensive homologies among them. They also exhibit a low affinity Ca2+ binding activity comparable to that of calsequestrin.