A protein activator of the plasma membrane Ca++-ATPase of heart sarcolemma

Stimulation of Mg2+-independent form of Ca2+-ATPase by a low molecular mass protein purified from goat testes cytosol

A low molecular mass protein purified from goat (Capra hircus) testes cytosol following gel filtration and anion exchange chromatographic separation stimulates Mg(2+)-independent Ca(2+)-ATPase activity without any significant effect on Mg(2+)-dependent Ca(2+)-ATPase. Stimulation of the ATPase is due to an increase in the rate of dephosphorylation of the overall reaction step of the enzyme. Binding of the stimulator increases the affinity of Ca(2+)-ATPase for Ca(2+). An analysis of enzyme kinetics reveals a reversible type of binding of the stimulator to the ATPase and non-competitive type of stimulation with respect to the substrate. Stimulation seems due to binding of the protein at a single site following Michaelis-Menten model. The protein can also counter the effect of calcium antagonists exerted on the ATPase. The pI of the protein is 6.2 and its molecular mass has been determined to be 13, 961 by Q-TOF-MS.

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

The 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 S50 (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 in K0.5 of the enzyme for calcium in presence of the stimulator. Radioactive labeling of the enzyme with [gamma-32P]-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.