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 gamma-rays from a Co-60 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.