Target molecular size suggesting a dimeric structure for the red beet plasma membrane Ca²⁺-ATPase

The target molecular size of the red beet (Beta vulgaris L. var. Detroit Dark Red) plasma membrane Ca²⁺-ATPase was determined by radiation inactivation analysis involving γ-rays from a ⁶⁰Co source. The Ca²⁺ transport activity associated with the Ca²⁺-ATPase in native plasma membrane vesicles declined in an exponential manner with radiation dose and the target molecular size was estimated to be about 245 000 ± 5000. As the catalytic subunit (phosphorylated intermediate) of the red beet plasma membrane Ca²⁺-ATPase has been shown to be about 100 000 to 120 000, these results suggest that the enzyme exists as a dimer in the native membrane.