Protein Phosphorylation of the Smooth and Rough Endoplasmic Reticulum in Normal and Neoplastic Liver of the Rat

Abstract

Smooth and rough endoplasmic reticulum from rat liver and hepatomas exhibited endogenous protein kinase activity independent of adenosine 3′:5′‐monophosphate. The phosphorylation of smooth membranes by this process was consistently higher than that of the rough membranes. When histone was added along with the smooth endoplasmic reticulum, cyclic AMP stimulated protein phosphorylation. Analysis of membrane‐phosphorylated proteins by gel electrophoresis showed 5 major bands having estimated molecular weights of 155000, 62000, 50000, 46000 and 43000, whereas only 3 major phosphorylated bands with estimated molecular weights of 62000, 50000 and 43000 were found in membranes of the smooth endoplasmic reticulum of the Morris hepatoma 5123C. Since previous studies in this and other laboratories have demonstrated the similarity of the protein components of membranes of the endoplasmic reticulum of normal liver and hepatoma, our findings indicate an inability of the protein kinase of hepatoma intracellular membranes to phosphorylate protein species that are found in membranes of both liver and the neoplasm.