Calcium-dependent ATPase activities (open symbols) or stoichiometries of fluorophore (FITC, panel A; IAF, panel B) labeling (closed symbols) were measured for cardiac SR microsomes in the absence (squares, circles) or presence (triangles) of 6 mM AMPPCP to block access to the nucleotide binding cleft. Specific labeling of Lys514, by FITC, or Cys674, by IAF in SERCA2a of cardiac SR membranes was performed and calcium-dependent ATPase activities were measured (25°C) as described in Materials and Methods.