m-Calpain requires DNA for activity on nuclear proteins at low calcium concentrations

m-Calpain (calpain II, m-CANP), which normally requires millimolar Ca2+ for activity in vitro, was capable of proteolyzing a number of matrix proteins in isolated rat liver nuclei at Ca2+ concentrations as low as 3 microM (Mellgren, R. L. (1991) J. Biol. Chem. 266, 13920-13924). Treatment of nuclei with deoxyribonuclease I eliminated the activity of m-calpain at low Ca2+ concentrations, while ribonuclease A and phospholipase C had no effect. Addition of DNA to DNase-treated nuclei restored m-calpain activity at low Ca2+. RNA had little if any effect. Eukaryotic and prokaryotic DNA were equally effective, and synthetic polydeoxyribonucleotides were also activators. m-Calpain did not bind to a DNA-cellulose column in the presence of 200 microM Ca2+, and m-calpain preincubated in the presence of DNA and 200 microM Ca2+ was not activated at low Ca2+ concentrations following removal of the DNA. DNA did not alter the Ca2+ requirement for m-calpain-catalyzed cleavage of casein. These results demonstrate that the Ca2+ requirement for proteolysis of nuclear matrix proteins by m-calpain can be dramatically decreased in the presence of DNA. Activation did not seem to be a result of DNA binding directly to calpain but appeared to require interaction of DNA, calpain, and calpain substrates in the nuclear matrix.