Kinase activities in the non-histone chromosomal proteins of resting and proliferating BHK21 C13 cells

Abstract

When BHK21 C13 cells in saturation density are stimulated to proliferate by a change of medium, a synchronized DNA synthesis occurs with a maximum at about 15 h. After stimulation the nonhistone chromosomal proteins (NHC proteins) phosphorylation in vivo increases and reaches a maximum in late G 1 phase. We show that the endogenous phosphorylation in vitro of NHC proteins is 4-fold larger when the NHC proteins are extracted from cells in late G 1 phase than from resting cells. The electrophoretic patterns of [ 32P]distribution show that the [ 32P]incorporation enhancement in NHC proteins from stimulated cells is more significant for the slowly migrating proteins. The NHC protein kinase activity was fractionated on polyacrylamide gel in 10 fractions, in resting as in stimulated cells. Only some of these kinase activities were increased after cell stimulation. The enhancement of phosphorylation may be due in part to an increase in the number of phosphorylation sites and in part to an increase in kinase activities. The use of phosvitin as exogenous substrate allow the conclusion that the activity of some kinase is enhanced after stimulation. Furthermore, by fractionation on polyacrylamide gel five phosvitin kinase activities were found in low molecular weight NHC protein fractions, in control as in stimulated cells. After cell stimulation, two phosvitin kinase activities only are specifically increased. The results show the multiplicity of kinase activities and support the hypothesis that these various kinase activities correspond to specific differentiated functions.