Protein kinase activity tightly bound to liver polysomes

Abstract

Rat liver polysomes washed with 0.5–1.5 M KCl at 37°C keep a constant protein kinase activity revealed only by auto-phosphorylation of ribosomal proteins. The enzyme catalyzes the transfer of the γ-phosphate group from ATP to serine (75%) but also to threonine residues (25%). It is released when polysomes are dissociated into subunits using centrifugation through a sucrose gradient containing a high K⁺/Mg⁺⁺ ratio. Its properties have been compared with those of the two other enzymatic activities which are, in contrast, washed out during salt treatment of polysomes. After release upon polysome dissociation, this third activity is able to phosphorylate histone II A. Protection of the enzyme in the polysome structure against salt treatment, suggests that it is located at the junction of the two subunits.