The dimeric form of Ca2+-ATPase is involved in Ca2+ transport in the sarcoplasmic reticulum

Abstract

To identify the functional unit of Ca2+-ATPase in the sarcoplasmic reticulum, we assessed Ca2+-transport activities occurring on sarcoplasmic reticulum membranes with different combinations of active and inactive Ca2+-ATPase molecules. We prepared heterodimers, consisting of a native Ca2+-ATPase molecule and a Ca2+-ATPase molecule inactivated by FITC labelling, by fusing vesicles loaded with each type of Ca2+-ATPase. The heterodimers exhibited neither Ca2+ transport nor ATP hydrolysis, suggesting that Ca2+ transport by the Ca2+-ATPase requires an interaction between functional Ca2+-ATPase monomers. This finding implies that the functional unit of the Ca2+-ATPase is a dimer.