Phosphorylation of Acid-soluble Nucleolar Proteins of Novikoff Hepatoma Ascites Cells in Vivo

Nucleolar phosphoproteins are extensively labeled 2 hours after injection of 50 mCi per rat of [32P]orthophosphate into rats bearing Novikoff hepatoma ascites tumor cells. The nucleolar proteins soluble in 0.4 n H2SO4 were analyzed by two-dimensional electrophoresis and autoradiography. Of the 97 proteins visualized by staining, phosphorylation was detected in approximately 40 proteins by autoradiography and by scintillation counting of spots cut from the gels. Several high molecular weight protein spots designated C23, C24, C26, and C27 incorporated as much as 500 times more 32P per spot as some low molecular weight spots. Analysis for phosphoamino acids in 30 spots cut out of the gel slab indicated that radioactive phosphoserine was present in all the spots analyzed.