Effect of phosphorylation on scallop sarcoplasmic reticulum

Fragmented sarcoplasmic reticulum prepared from the cross-striated adductor muscle of the deep sea scallop (Placopecten magellanicus) was phosphorylated with inorganic phosphate to the E2P (ADP-insensitive) form. Negative staining of these preparations showed that the Ca-ATPase was organized into a quasi-crystalline array, which differed from the 'dimer ribbon' structure previously reported for the membrane under relaxing conditions (Castellani & Hardwicke, J. cell. Biol. 97 (1983) 557-61; Castellani et al., J. molec. Biol. 185 (1985) 579-94). In this new form there was only a single Ca-ATPase per unit cell. Dephosphorylation of the E2P membranes and incubation with substrate or substrate analogues in the absence of Ca2+ caused the 'dimer ribbon' structure to appear. These results imply that rotation of at least half of the Ca-ATPase subunits in the scallop sarcoplasmic reticulum may occur about an axis perpendicular to the plane of the membrane on conversion from the E2P state to the state corresponding to that existing in the relaxed muscle.