[ATP-dependent structural changes in chromatin]

Incubation (1 hour, 37 degrees C) of nuclei and chromatin from guinea pig spleen (but not from thymus or the liver) resulted in a proteolysis of H1 and H2A histones and accumulation of specific hydrolytic products. Sodium dodecyl sulfate gel electrophoresis revealed a decline in H1 and H2A and the appearance of new protein bands under histone H10 and between H2A and H4. ATP (10 mM) (but not cAMP or PPi) added to the incubation mixture prevented the H1 and H2A digestion and accumulation of the aforementioned products. The ATP, which protects the histones in the nucleus and chromatin from proteinases, promoted the cleavage of cytosolic low molecular weight proteins. The mechanisms of ATP-dependent chromatin structural rearrangements determining the resistance of nuclear proteins to proteolysis are discussed.