THERMAL UNCOUPLING OF THE CA2+-TRANSPORTING ATPASE IN SARCOPLASMIC-RETICULUM - CHANGES IN SURFACE-PROPERTIES OF LIGHT VESICLES

It is known that the light fraction of rabbit skeletal muscle sarcoplasmic reticulum vesicles can release Ca2+ from the intravesicular space, although the Ca2+-conductive channels are present only in the heavy fraction of sarcoplasmic reticulum vesicles. To study the possible pathways of the Ca2+ leakage from light vesicles we have used a short-term treatment for 4.5 min at 45 degrees C which quickly decreases the efficiency of Ca2+-transporting ATPase operation without any visible effects on the hydrolytic activity of the Ca2+-ATPase in the membranes. The treatment of the vesicles decreased the negative membrane surface potential created by the Ca2+-ATPase. Comparative titration of control and heat-treated preparations of light sarcoplasmic reticulum vesicles by K+, Na+, Mg2+, and Ca2+ revealed clear differences in their surface properties. The short-term heating resulted in release of Ca2+ from the vesicles previously loaded with Ca-45(2+), which indicates an increase in passive membrane permeability to Ca2+. Study of Ca2+-ATPase protein arrangement in the membrane indicated that the heat treatment induced protein oligomerization and some of the Ca2+-ATPase molecules acquired intermolecular and intramolecular covalent bonds. From these data, we have concluded that the changes in the surface and structure properties of the vesicle membranes after the short-term heat treatment were the result of clustering of the Ca2+-ATPase molecules. This protein rearrangement may create channels for calcium leakage from light sarcoplasmic reticulum vesicles.