Polyamine-activated protein phosphatase activity in HeLa cell nuclei.

Protein phosphatase activity towards endogenous nuclear substrates in sonicates of isolated nuclei was activated 2-4-fold by spermine. Exogenous casein was dephosphorylated by these preparations only in the presence of spermine. Activation by spermine was half maximal at about 0.1 mM. Spermidine also activated, with half maximal stimulation at 1mM; putrescine activated poorly. Mg++ and Ca++ appeared to activate the same phosphatase activity but were only 50% as effective as spermine. Spermine activation was inhibited by 200 mM NaCl, 50 mM NaF, or 40 mM beta-glycerol phosphate. Nuclear phosphatase activity, with or without spermine, was inhibited 50% by inhibitor 2 of protein phosphatase 1. These observations suggest that protein phosphatase 1 is a major nuclear protein phosphatase and that its activity against endogenous nuclear substrates is activated by physiological concentrations of spermine.