The dimeric form of Ca super(2+)-ATPase is involved in Ca super(2+) transport in the sarcoplasmic reticulum

To identify the functional unit of Ca super(2+)-ATPase in the sarcoplasmic reticulum, we assessed Ca super(2+)-transport activities occurring on sarcoplasmic reticulum membranes with different combinations of active and inactive Ca super(2+)-ATPase molecules. We prepared heterodimers, consisting of a native Ca super(2+)-ATPase molecule and a Ca super(2+)-ATPase molecule inactivated by FITC labelling, by fusing vesicles loaded with each type of Ca super(2+)-ATPase. The heterodimers exhibited neither Ca super(2+) transport nor ATP hydrolysis, suggesting that Ca super(2+) transport by the Ca super(2+)-ATPase requires an interaction between functional Ca super(2+)-ATPase monomers. This finding implies that the functional unit of the Ca super(2+)-ATPase is a dimer.