The functional unit of the Ca(2+)-ATPase in the sarcoplasmic reticulum is a dimer, as interaction between functional monomers is required for Ca(2+) transport.
Caution warranted in modeling SR calcium transport; leaves open in vivo dimer requirement in human cardiomyocytes.
To identify the functional unit of Ca(2+)-ATPase in the sarcoplasmic reticulum, we assessed Ca(2+)-transport activities occurring on sarcoplasmic reticulum membranes with different combinations of active and inactive Ca(2+)-ATPase molecules. We prepared heterodimers, consisting of a native Ca(2+)-ATPase molecule and a Ca(2+)-ATPase molecule inactivated by FITC labelling, by fusing vesicles loaded with each type of Ca(2+)-ATPase. The heterodimers exhibited neither Ca(2+) transport nor ATP hydrolysis, suggesting that Ca(2+) transport by the Ca(2+)-ATPase requires an interaction between functional Ca(2+)-ATPase monomers. This finding implies that the functional unit of the Ca(2+)-ATPase is a dimer.
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Ushimaru et al. (2008) studied this question.
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