Antibodies against the 53 kDa glycoprotein inhibit the rotational dynamics of both the 53 kDa glycoprotein and the Ca2+-ATPase in the sarcoplasmic reticulum membrane

Abstract The purpose of this study is to better define the relationship of the 53 kDa glycoprotein (GP-53) of the sarcoplasmic reticulum (SR) to other SR proteins. Towards that end the effects of antibodies against GP-53 on the rotational dynamics of maleimide spin-labeled proteins of SR of rabbit skeletal muscle were investigated. The labeling protocol used in this study provided 1.6±0.3 moles spin label incorporated per 10 5 g SR protein. Labeling specificity studies indicated that nearly 70% of the label bound specifically to the Ca 2+ -ATPase, with the remainder bound to GP-53. Using saturation-transfer electron paramagnetic resonance (ST-EPR), it was determined that the rotational mobility (i.e., the rate of rotation) of the spin-labeled SR proteins decreased greater than 5-fold upon preincubation of MSL-SR with an antiserum against the GP-53, while preincubation of MSL-SR with preimmune serum had no effect. Preincubation of MSL-SR with a monoclonal antibody against the GP-53 produced a 4-fold decrease in the rotational mobility of the MSL-SR proteins compared to control measurements. Further, these effects showed a marked calcium dependence: the decrease in the rotational mobility of the MSL-SR proteins preincubated with anti-GP-53 antibodies in 500 μM Ca 2+ was 3–6-fold greater than that of MSL-SR preincubated with antibodies in 5 mM EGTA. While MSL was bound to both Ca 2+ -ATPase and GP-53, model calculations indicated that the decreases observed in the rotational mobility of the MSL-SR proteins caused by the anti-GP-53 monoclonal antibodies were too large to be accounted for by effects on GP-53 alone. The calculation suggest that the rotational rate of Ca 2+ -ATPase was also diminished by anti-GP-53 monoclonal antibodies, indicating an interaction between GP-53 and Ca 2+ -ATPase in the SR membrane.