Kinetics studies of the cardiac Ca-ATPase expressed in Sf21 cells: New insights on Ca-ATPase regulation by phospholamban

Kinetics studies of the cardiac Ca-ATPase expressed in Sf21 cells (Spodoptera frugiperda insect cells) have been carried out to test the hypotheses that phospholamban inhibits Ca-ATPase cycling by decreasing the rate of the E1.Ca to E1'.Ca transition and/or the rate of phosphoenzyme hydrolysis. Three sample types were studied: Ca-ATPase expressed alone, Ca-ATPase coexpressed with wild-type phospholamban (the natural pentameric inhibitor), and Ca-ATPase coexpressed with the L37A-phospholamban mutant (a more potent monomeric inhibitor, in which Leu(37) is replaced by Ala). Phospholamban coupling to the Ca-ATPase was controlled using a monoclonal antibody against phospholamban. Gel electrophoresis and immunoblotting confirmed an equivalent ratio of Ca-ATPase and phospholamban in each sample (1 mol Ca-ATPase to 1.5 mol phospholamban). Steady-state ATPase activity assays at 37 degrees C, using 5 mM MgATP, showed that the phospholamban-containing samples had nearly equivalent maximum activity (similar to 0.75 mu mol.nmol Ca-ATPase(-1).min(-1) at 15 mu M Ca2+), but that wild-type phospholamban and L37A-phospholamban increased the Ca-ATPase K-Ca values by 200 nM and 400 nM, respectively. When steady-state Ca-ATPase phosphoenzyme levels were measured at 0 degrees C, using 1 mu M MgATP, the K-Ca values also shifted by 200 nM and 400 nM, respectively, similar to the results obtained by measuring ATP hydrolysis at 37 degrees C. Measurements of the time course of phosphoenzyme formation at 0 degrees C, using 1 mu M MgATP and 268 nM ionized [Ca2+], indicated that L37A-phospholamban decreased the steady-state phosphoenzyme level to a greater extent (45%) than did wild-type phospholamban (33%), but neither wild-type nor L37A-phospholamban had any effect on the apparent rate of phosphoenzyme formation relative to that of Ca-ATPase expressed alone. Measurements of inorganic phosphate (P-i) release concomitant with the phosphoenzyme formation studies showed that L37A-phospholamban decreased the steady-state rate of P-i release to a greater extent (45%) than did wild-type phospholamban (33%). However, independent measurements of Ca-ATPase dephosphorylation after the addition of 5 mM EGTA to the phosphorylated enzyme showed that neither wild-type phospholamban nor L37A-phospholamban had any effect on the rate of phosphoenzyme decay relative to Ca-ATPase expressed alone. Computer simulation of the kinetics data indicated that phospholamban and L37A-phospholamban decreased twofold and fourfold, respectively, the equilibrium binding of the first Ca2+ ion to the Ca-ATPase E1 intermediate, rather than inhibiting rate of the E.Ca to E'.Ca transition or the rate of phosphoenzyme decay, Therefore, we conclude that phospholamban inhibits Ca-ATPase cycling by decreasing Ca-ATPase Ca2+ binding to the E1 intermediate.