Role of specific acidic lipids on the reconstitution of sodium-dependent amino acid transport in proteoliposomes derived from Ehrlich cell plasma membranes

Abstract

The effect of acidic phospholipids on the activity of a Na(+)-dependent amino acid transporter (A system) from Ehrlich ascites cell plasma membranes was examined. Plasma membranes were solubilized in cholate/urea and reconstituted with Ba2(+)-precipitated asolectin (soybean phospholipid free of anionic phospholipids) replenished with different acidic phospholipids. In the absence of added acidic phospholipids, transport activity was very low. However, three acidic lipids [cardiolipin greater than phosphatidic acid (PA) greater than phosphatidylinositol] were capable of restoring transport activity (in the order given) to proteoliposomes made from Ba2(+)-precipitated asolectin, while other acidic phospholipids (phosphatidylserine and phosphatidylglycerol) were much less active in this respect. For restoration of optimal activity, PA containing at least one unsaturated fatty acyl moiety, particularly in the beta position, was required. PA containing only saturated fatty acids in the beta and gamma positions was largely inactive. No difference in restoration of function was observed on varying the saturated fatty acyl chain length in PA from 10 carbons to 18 carbons. The specific effects of PA on the A-system transporter were not shared by the Na(+)-independent amino acid exchange system (L system) or the glucose transport system. Treatment with poly(ethylene glycol) 8000 was shown to reduce the nonspecific permeability of the reconstituted proteoliposomes and to enhance Na(+)-dependent amino acid transport.