Iodination of Calsequestrin in the Sarcoplasmic Reticulum of Rabbit Skeletal Muscle: A Re-Examination

Abstract

The exposed proteins of sarcoplasmic reticulum (SR) vesicles from skeletal muscle were iodinated with the use of Sepharose 4B-bound lactoperoxidase, so that the location of the proteins in the membrane could be determined. It was found that the pattern of protein labelling could be modified simply by changing the constituents of the incubation media. This implies that the position or configuration of a particular protein in the membrane can be altered by the local environment. When the reaction was performed in the presence of 25 mM tris-maleate, pH 7 ·0, alone, the Ca2+ pump ATPase (molecular weight 105000) and calsequestrin (47000) were both heavily labelled, suggesting they are at least partially exposed on the outer surface of the membrane. By contrast the high affinity calcium-binding protein (55000) was not labelled. However, when the vesicles were iodinated under conditions that were suitable for ATPase activity and Ca2+ accumulation, namely in the presence of 25 mM tris-maleate, pH 7 ·0, 5 mM ATP, 5 mM Mg2+ and 0·05 mM Ca2+, a different pattern of labelling was obtained. No labelling of calsequestrin was observed whereas the extent of labelling of the Ca2+ pump ATPase remained about the same. The inclusion of anyone of the additives mentioned was effective in inhibiting the iodination of calsequestrin in the SR vesicle. When added alone, Ca2+ was more effective than Mg2+ in preventing labelling of calsequestrin. Half-maximal inhibition was observed at concentrations of approximately 0·05 mM Ca 2+ and 0·2-0·3 mM Mg2+ . Although much reduced, significant labelling of calsequestrin was observed even in the presence of 5 mM ATP. Investigations with partially purified calsequestrin revealed that the iodination of calsequestrin was the same in both the presence and absence of 1 mM Ca2 +. Therefore the reduction in label observed in intact SR vesicles probably represents a change in the location of calsequestrin within the membrane, rather than inhibition by Ca2+ of the iodination sites of the protein itself.