Effects of ion gradients on H+ transport mediated by human MDR 1 protein

In the previous paper we presented a variety of data consistent with signif icant perturbations in 9.3 yeast plasma membrane ion transport upon overexp ression of the hu MDR 1 protein. Thus, in this paper, we compare formation of Delta pH for inside-out yeast plasma membrane vesicles (ISOV) prepared f rom control 9.3/pVT versus 9.3/hu MDR 1 yeast. Since MDR 1 ATPase activity has a broader, more alkaline pH profile relative to endogenous yeast H+ ATP ase activity, we analyzed Hf pumping at pH I 8.0 in detail in order to sele ctively amplify hu MDR 1 contributions to H+ movement over those of the end ogenous yeast H+ ATPase. We observed: (1) imposition of a Cl- gradient orie nted outside to in enhances acidification for 9.3/pVT ISOV (as expected), b ut decreases acidification for 9.3/hu MDR 1 ISOV; (2) imposition of a Cl- g radient oriented inside to out decreases acidification for 9.3/pVT ISOV (as expected) but enhances acidification for 9.3/hu MDR 1 ISOV; (3) a Na+ grad ient oriented in the same direction as the Cl- gradient amplifies the effec ts due to hu MDR 1 when both gradients are oriented inside to out, but not outside to in. The data are most easily explained by interesting Na+, Cl-, and ATP-dependent Hf transport mediated by hu MDR 1 protein as previously s uggested [Hoffman and Roepe (1997) Biochemistry 36, 11153-11168]. These dat a may help to resolve a variety of conflicting reports in the literature re garding ion transport mediated by hu MDR 1 and have implications for the ph ysiology of a number of polarized epithelia in which hu MDR 1 is endogenous ly expressed.