Increased proteolysis in chromatin of terminally differentiated and quiescent cells

Endogenous proteolysis in chromatin of terminally differentiated, quiescent, and actively proliferating cells was studied by measuring the released acid-soluble radioactivity of [ 3H]tryptophan-prelabelled nuclear proteins, and by following the specific quantitative and qualitative changes in electrophoregrams of chromosomal proteins. The experiments suggest that the chromatin of differentiated mouse kidney and liver cells, as well as chromatin from Friend cells induced to commit terminal differentiation, exhibit increased proteolysis in comparison with that of chromatin isolated from actively proliferating cells. Enhanced proteolysis was found also for the slowly renewing and quiescent cells from adult mice. The control experiments designated to discriminate between the two possible alternatives explaining the difference—increased activity of the proteolytic enzymes associated with chromatin, or increased susceptibility of the chromosomal proteins to proteases—supported the latter alternative.