Change in Target Molecular Size of the Red Beet Plasma Membrane ATPase during Solubilization and Reconstitution

The plasma membrane ATPase from red beet (Beta vulgaris L.) storage tissue associated with either native plasma membrane vesicles, a detergent-solubilized enzyme preparation or reconstituted liposomes was subjected to radiation inactivation analysis to determine if changes in target molecular size occurred with modification of its amphipathic environment. For each preparation of the enzyme, the decline in ATP hydrolytic activity with increasing dose of gamma-ray radiation demonstrated a simple exponential profile indicating the presence of a single target size. Analysis of the radiation inactivation profiles for the plasma membrane associated, solubilized, and reconstituted enzyme revealed target molecular sizes of 225 kilodaltons (kD), 129 kD, and 218 kD, respectively. These results suggest that the plasma membrane associated and reconstituted ATPase preparations consist of enzyme present as a dimer of 100 kD subunits while the solubilized enzyme is present in the monomeric form. These results also indicate that the 100 kD catalytic subunit most likely represents the minimal unit of ATP hydrolytic activity.