The Phosphoprotein Intermediate of a Ca2+ Transport ATPase in Rat Liver Endoplasmic Reticulum

Smooth endoplasmic reticulum vesicles from rat liver display an ATP-supported Ca2+ transport which is mediated by a (Ca2+ + Mg2+)-ATPase. During the catalytic cycle the terminal phosphate from ATP is incorporated to form an acid-precipitable reaction product(118 000-Mr in SDS-gel electrophoresis) with stability characteristics of an acylphosphate. Comparative studies with sarcoplasmic reticulum vesicles from fast-twitch skeletal muscle suggest that the 118 000-Mr phosphopeptide may be identified with the phosphorylated reaction intermediate of a Ca2+ transport ATPase in endoplasmic reticulum, similar to that in sarcoplasmic reticulum of muscle.