Cytoplasmic and nuclear protein kinases during the cell cycle

Abstract

Nuclear and cytoplasmic protein kinases were measured during the traverse of synchronous CHO cultures through G 1 into S phase. Cells were synchronized by selective detachment of cells blocked in metaphase using colcemid. Nuclei were isolated and the protein kinases extracted from the nuclear preparation with 0.6 M NaCl. This procedure solubilized greater than 90% of the total protein kinase activity present in the nuclear preparation. DEAE chromatography of this extract showed 5 apparently different ionic forms of nuclear protein kinases. The nuclear protein kinases preferred casein and phosvitin to histone as substrates and were cyclic AMP-independent. Nuclear protein kinase activities increased greater than two-fold, when expressed as units of activity per cell nucleus, during G 1 phase traverse, concomitant with a 70% increase in nuclear non-histone proteins (those soluble in 0.6 M NaCl). This resulted in only a 40% increase in the specific activities (units/μg protein in 0.6 M NaCl extractable nuclear fraction) of these enzymes as cells progressed through G 1 into S phase. This was in contrast to cytoplasmic cyclic AMP-dependent protein kinase activities which also increased two-fold during progression through G 1 phase while total cellular protein increased less than 20%. Activation of, as well as synthesis of, cyclic AMP-dependent cytoplasmic protein kinases during G 1 phase suggests a regulatory mechanism for precise temporal phosphorylation, whereas the constant specific activity in nuclear kinases during cell cycle is more compatible with the maintenance of bulk phosphorylation processes in the nucleus.