The Aeroschmidt Weed Contains an ATP-Driven Ion Pump in Its

Essay

The Aeroschmidt weed contains an ATP-driven ion pump in its vacuolar membrane that pumps potentially toxic heavy metal ions such as Zn²⁺ and Pb²⁺ into the vacuole.The pump protein cycles through a phosphorylated and an unphosphorylated state and works in a similar way to the Na⁺-K⁺ pump of animal cells.To study its action, you incorporate the unphosphorylated form of the protein into phospholipid vesicles containing K⁺ in their interiors.(You ensure that all of the protein molecules are oriented such that their cytosolic domains face the outside of your vesicles.) When you add Zn²⁺ and ATP to the solution outside such vesicles, you find that Zn²⁺ is pumped into the vesicle lumen.You then expose vesicles containing the pump protein to the solutes shown in Table 12-28. $\begin{array}{ccccc}&A&B&C&D&E&F\\\hline \text { Outside }&\mathrm{Zn}^{2+}+\text { ATP }&\mathrm{Zn}^{2+}&\mathrm{Zn}^{2+}+\text { ATP }&\mathrm{Zn}^{2+}&\text { ATP }&\text { ATP }\\\text { Inside }&K^+&&&K^+&&K^+\end{array}$
Table 12-28
You then determine the amount of phosphorylated and unphosphorylated ATP-driven ion pump protein in each sample.Your results are summarized in Table 12-28A, where a minus sign indicates an absence of a type of protein and a plus sign indicates its presence. $\begin{array}{llll}&A&B&C&D&E&F\\\hline \text { Phosphorylated }&+&-&-&-&-&++\\\text { protein present }\\\text { Unphosphorylated }&++&++&++&++&++&-\\\text { protein present }\end{array}$
Table 12-28A
You treat vesicles as in lane F, but before determining the phosphorylation state of the protein, you wash away the outside buffer and replace it with a buffer containing only Zn²⁺.What do you expect to happen in this sample, and what are the results you expect to observe with respect to phosphorylation and movement of K⁺?