Temperature Sensitivity of Proteoliposomes Reconstituted from a Mixture of Scallop and Rabbit Sarcoplasmic Reticulum Ca2+-ATPases

: We reconstituted proteoliposomes by mixing scallop and rabbit sarcoplasmic reticulum, SR, at different protein weight ratios, and investigated the effects of temperature on their Ca2+-transport activity. When proteoliposomes containing scallop and rabbit SR at a protein ratio of 1:1 were pre-incubated in the presence of Ca2+ at 39 degrees C for 10 min, the Ca2+-transport activity was almost completely lost, whereas the activity of proteoliposomes containing rabbit SR alone decreased only slightly. Essentially the same results were obtained for proteoliposomes reconstituted with Ca2+-ATPases partially purified from scallop and rabbit SR. The susceptibility of the reconstituted proteoliposomes to heat inactivation increased with an increase in the protein weight ratio of scallop to rabbit SR, the maximum being approached at a ratio higher than 1. When the scallop SR was thermally treated before reconstitution, the resulting vesicles showed as high Ca2+-transport activity as that of control vesicles reconstituted from rabbit SR alone. The former vesicles were not inactivated on further treatment at high temperature. In contrast, when the scallop SR was heated in EGTA before reconstitution of vesicles with rabbit SR, the Ca2+-transport activity of the vesicles was strongly inhibited by subsequent treatment at high temperature in the presence of Ca2+. These results can be easily explained if we assume that Ca2+ transport by the reconstituted vesicles can be catalyzed through a dimeric interaction between the scallop and rabbit Ca2+-ATPases in the membrane. Pre-incubation of these vesicles at 39 degrees C for 10 min in the presence of Ca2+ may destroy the dimeric interaction due to denaturation of the scallop Ca2+-ATPase.