A vacuolar-type proton ATPase mediates acidification of plasmalemmal vesicles during potocytosis

Previously we showed that the potocytosis of 5-methyltetrahydrofolate is dependent on the acidification of plasmalemmal vesicles created each time a caveolae closes off from the cell surface. We now report that bafilomycin Al, which is a specific inhibitor of the V-type proton ATPase, inhibits 5-methyltetrahydrofolate uptake into MA104 cells (ED(50) = 150 nM). The inhibitory effect was reversed within 30 min after removal of the drug from the cells. Bafilomycin A(1) had no effect on the binding of folic acid to its receptor, A concentration of up to 200 nM bafilomycin Al did not affect sequestration of folate receptors. Immunoblotting showed that the 70-kDa subunit of the V-type proton pump was localized to caveolae-rich fractions isolated from the plasma membrane of these cells, These results suggest that a V-type proton pump acidifies the lumen of plasmalemmal vesicles during potocytosis. (C) 1996 Academic Press, Inc.