Spermine inhibits the phosphorylation of the 11,000- and 10,000-dalton nuclear proteins catalyzed by nuclear protein kinase NI in NB-15 mouse neuroblastoma cells.

The nuclear protein kinase NI (NI kinase) was purified from NB-15 mouse neuroblastoma cells by phosphocellulose column and casein affinity column chromatography. The purified NI kinase exhibited (i) an apparent subunit molecular weight of about 37,000, (ii) autophosphorylation, and (iii) insensitivity to inhibition by heparin. When NI kinase was added to heat-treated neuroblastoma nuclei in the presence of [gamma-32P] ATP, two proteins with apparent subunit molecular weights of 11,000 and 10,000 were prominently phosphorylated. Other protein kinases tested including the nuclear protein kinase NII, Type I cAMP-dependent protein kinase, and protein kinase C did not catalyze the phosphorylation of these two proteins. The NI kinase-catalyzed phosphorylation of these two proteins was completely inhibited by 1 mM spermine. In contrast, 10 mM putrescine, 2 mM spermidine, 5 mM arginine, and 10 mM NH4Cl, had no inhibitory effect on this phosphorylation reaction. Our study also indicated that the phosphorylation of the 11,000- and 10,000-dalton proteins occurred in the nuclear matrix fraction but not in heterogeneous nuclear ribonucleoproteins, high mobility group proteins, or histone fractions. We have previously reported that spermine specifically inhibits the endogenous phosphorylation of an 11,000-dalton nuclear protein in various mammalian cell lines (Chen, K. Y., and Verma, R. (1984) Biochem. Biophys. Res. Commun. 118, 710-716). The present study suggests that the 11,000- and 10,000-dalton nuclear proteins may be native substrates of nuclear protein kinase NI and that their phosphorylation can be affected by physiological concentrations of spermine.