Nuclear-membrane-associated protein kinase and substrates. The effect of growth state on activity and specificity.

Nuclear membrane was isolated from cultured cells by two different techniques. The first technique employed sonication to lyse the nuclei, followed by treatment with citrate buffer to strip away the chromatin. The second procedure involved incubation with the polyanion heparin to lyse the nuclei. In both procedures, the nuclear membrane was purified by isopycnic centrifugation on discontinuous sucrose gradients. Both preparations contained endogenous protein kinase activity and phosphorylated endogenous membrane proteins. The phosphoprotein profiles and characteristics of the phosphorylation reaction were very similar for the two preparations, except that the heparin-prepared membranes lacked two major phosphorpoteins which were present in membranes prepared by sonication. The growth state of the culture had a dramatic effect on nuclear-membrane protein phosphorylation. Proliferating cells exhibited a 3-5-fold greater extent of phosphorylation of nuclear-membrane proteins than did quiescent cells. The increased phosphorylation observed in proliferating cells implies that regulation at the level of the nuclear membrane may be an important site for regulation of cell-cycle events.