Purification, calcium binding properties, and ultrastructural localization of the 53,000- and 160,000 (sarcalumenin)-dalton glycoproteins of the sarcoplasmic reticulum

The 53-kDa glycoprotein and sarcalumenin (160-kDa glycoprotein) were extracted from rabbit skeletal muscle sarcoplasmic reticulum with EGTA and purified by fractionation on DEAE-Sephadex A-25 and lentil lectin-Sepharose 4B. Sarcalumenin was shown to bind up to 400 nmol of Ca2+/mg of protein at pH 7.5, which is equivalent to binding of approximately 35 mol of Ca2+/mol of protein. The apparent dissociation constant was 300 microM in the presence of 20 mM KCl and 600 microM in 150 mM KCl. The 53-kDa glycoprotein did not bind any Ca2+ under the conditions examined. Immunoblot analysis of isolated sarcoplasmic reticulum subfractions demonstrated the presence of the two glycoproteins in both the longitudinal sarcoplasmic reticulum and the terminal cisternae. Their concentrations were higher, however, in the longitudinal sarcoplasmic reticulum vesicles. Comparative immunoelectron microscopic studies using monoclonal antibodies revealed a codistribution of the 53-kDa glycoprotein with the Ca2(+)-ATPase in all regions of the free sarcoplasmic reticulum. A similar distribution was found for sarcalumenin, although immunolabeling was much weaker. The colocalization of the 53-kDa glycoprotein and sarcalumenin with the Ca2(+)-ATPase and the Ca2+ binding properties of sarcalumenin suggest that the glycoproteins may be involved in the sequestration of Ca2+ in the nonjunctional regions of the sarcoplasmic reticulum.