Influence of the 53 kDa glycoprotein on the cooperativity of the Ca2+-ATPase of the sarcoplasmic reticulum

Abstract 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 Ca 2+ 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 n >1) on [Ca 2+ ] free . We have determined the degree of cooperativity for native SR vesicles, SR preincubated with antiserum against GP-53 or preimmume 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 coperative dependence of Ca 2+ -ATPase activity on both [ATP] and [Ca 2+ ] similar to that of untreated SR. SR preincubated with anti-GP-53 antiserum (which causes an uncoupling of Ca 2+ 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 [Ca 2+ ] 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.