Regulation of Mg2+-independent Ca2+-ATPase by a low molecular mass protein purified from bovine brain

Abstract

he goat sperm microsomal membranes have been found to contain an Mg2+-independent Ca2+-ATPase, a low affinity but highly active enzyme sharing similarities with the SERCA family of ATPases. The present study reports the identification and characterization of a 14 kilodalton cytosolic protein from bovine brain which can act as an endogenous stimulator of the enzyme with an S-50 (concentration producing 50% stimulation) of 0.8 mu molar. Kinetic analysis suggests that the stimulation is noncompetitive with respect to the substrate, and the binding site(s) of the stimulator and substrate are distinct. Binding of the stimulator to the enzyme is reversible. The stimulator increases the affinity of the enzyme for calcium as evident from a decrease inK(0.5) of the enzyme for calcium in presence of the stimulator. Radioactive labeling of the enzyme with [gamma-P-32]-ATP suggests that the stimulator enhances the rate of dephosphorylation of the phosphoenzyme intermediate without altering the phosphorylation reaction step. The stimulatory effect of the protein has been observed only for the Mg2+-independent form of the enzyme, the Mg2+-dependent form being unaffected.