ENHANCEMENT OF MDR2-MEDIATED PHOSPHATIDYLCHOLINE TRANSLOCATION BY THEBILE-SALT TAUROCHOLATE - IMPLICATIONS FOR HEPATIC BILE FORMATION

Expression of the Mdra-protein in secretory vesicules (SVs) from the y east mutant sec6-4 causes a time- and temperature-dependent enhancemen t of phosphatidylcholine (PC) translocation from the outer to the inne r leaflet of the SV lipid bilayer. We show that this activity is indep endent of changes either in the membrane potential or the pH gradient (inside positive) generated in these SVs by the yeast proton-transloca ting PMA1 ATPase. However, loading of the SVs with the primary bile sa lt taurocholate results in an apparent enhancement of Mdr2-mediated PC translocation activity. Reducing the intravesicular taurocholate (TC) concentration by dissipating the electrochemical potential across the SV membranes eliminates the enhancing effect of TC. Three Lines of ev idence suggest that the enhanced Mdr2-mediated PC translocation activi ty is not caused by a regulatory effect of TC on Mdr2 but rather refle cted the formation of TC/PC aggregates or micelles in the lumen of SVs . First, significantly higher detergent concentrations are required to reveal the fluorescence of (7-niaro-2-1,3-benzoxadiazol-4-yl)amino- m olecules translocated in Mdr2-SV under conditions of TC stimulation th an under control conditions; second, the nonmicelle-forming bile salt taurodehydrocholate does not cause enhancement of PC translocation in Mdr2-SVs; third, enzyme marker studies indicate that TC behaves as a p otent lipid solubilizer directly extracting PC molecules out of the bi layer without causing leakage. This results in the formation of intrav esicular aggregates or mixed micelles, and provokes the apparent stimu lation of Mdr2 activity. These data demonstrate a unique relationship between Mdr2, PC, and TC in the process of bile formation and secretio n.