Nucleolin (C23), a physiological substrate for casein kinase II

Nucleolin (C23), a 110 kDa phosphoprotein, which is mainly found in the nucleolus has been shown to be a physiological substrate for casein kinase II (CKII). Nucleolin was identified and characterized by immunodetection using an anti-nucleolin antibody. Phosphopeptide patterns from nucleolin phosphorylated by purified casein kinase II and of phosphorylated nucleolin which had been isolated from tumor cells grown in the presence of [ 32P]-o-phosphate, were identical. The partial tryptic digest revealed nine phosphopeptides. Nucleolin isolated from Krebs II mouse ascites cells was phosphorylated by purified casein kinase II with about two moles phosphate per one mole of nucleolin.