THE CALCIUM-PUMP OF THE LIVER NUCLEAR-MEMBRANE IS IDENTICAL TO THAT OF ENDOPLASMIC-RETICULUM

The envelope membrane of rat liver nuclei contains a P-type Ca2+-transporting pump, revealed by the presence of a Ca2+-stimulated phosphoenzyme. The level of the nuclear phosphoenzyme in autoradiographed polyacrylamide gels was decreased by lanthanum, as typically observed in the endoplasmic reticulum Ca2+ pump. It was also decreased by thapsigargin and 2,5-di-(tert-butyl)-1,4-benzohydroquinone, two accepted inhibitors of the endoplasmic reticulum Ca2+-ATPase. Comparative proteolysis of the phosphorylated enzyme of liver microsomes (endoplasmic reticulum) and nuclear membranes revealed an identical cleavage pattern. In addition, antibodies raised against the endoplasmic reticulum Ca2+ pump cross-reacted with the pump in the nuclear membranes. The findings show that nuclear membranes contain a Ca2+-transporting pump closely related to that of the endoplasmic reticulum, if not identical to it. The pump is likely to be involved in the control of nuclear free calcium.