Influence of the 53 kDa glycoprotein on the cooperativity of the Ca(2+)-ATPase of the sarcoplasmic reticulum.

Previous results from this laboratory suggest that the 53 kDa glycoprotein (GP-53) of rabbit skeletal muscle sarcoplasmic reticulum membrane (SR) may influence coupling between Ca2+ transport and ATP hydrolysis by the Ca(2+)-ATPase. Here we report evidence that GP-53 may influence the cooperative behavior of the Ca(2+)-ATPase. The ATPase activity of the Ca(2+)-ATPase displays negative cooperative dependence (Hill coefficient n less than 1) on [MgATP] and has positive cooperative dependence (n greater than 1) on [Ca2+]free. We have determined the degree of cooperativity for native SR vesicles, SR preincubated with antiserum against GP-53 or preimmune serum, and SR partially extracted with KCl-cholate. Our results show that SR preincubated with preimmune serum or SR treated with cholate in 50 mM KCl (yielding membranes rich in GP-53) demonstrate a cooperative dependence of Ca(2+)-ATPase activity on both [ATP] and [Ca2+] similar to that of untreated SR. SR preincubated with anti-GP-53 antiserum (which causes an uncoupling of Ca2+ transport from ATP hydrolysis) or SR extracted with cholate in 1 M KCl (yielding membranes depleted of GP-53) displays decreased positive cooperative dependence on [Ca2+] and decreased negative cooperative dependence on [ATP]. The results are consistent with the interpretation that GP-53 may influence the cooperative behavior of the Ca(2+)-ATPase.