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 eac h 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 AT Pase, inhibits 5-methyltetrahydrofolate uptake into MA104 cells (ED(50 ) = 150 nM). The inhibitory effect was reversed within 30 min after re moval of the drug from the cells. Bafilomycin A(1) had no effect on th e binding of folic acid to its receptor, A concentration of up to 200 nM bafilomycin Al did not affect sequestration of folate receptors. Im munoblotting showed that the 70-kDa subunit of the V-type proton pump was localized to caveolae-rich fractions isolated from the plasma memb rane of these cells, These results suggest that a V-type proton pump a cidifies the lumen of plasmalemmal vesicles during potocytosis. (C) 19 96 Academic Press, Inc.