Effect of different phospholipids on the reconstitution of two functions of the lactose carrier of Escherichia coli.

The lactose carrier was extracted from membranes of Escherichia coli and transport activity reconstituted in proteoliposomes containing different phospholipids. Two different assays for carrier activity were utilized: counterflow and membrane potential-driven uptake. Proteoliposomes composed of E. coli lipid or of 50% phosphatidylethanolamine--50% phosphatidylcholine showed very high transport activity with both assays. On the other hand, proteoliposomes containing asolectin, phosphatidylcholine or 25% cholesterol/75% phosphatidylcholine showed good counterflow activity but poor membrane potential-driven uptake. The discrepancy between the two types of transport activity in the latter group of three lipids is not due to leakiness to protons, size of proteoliposomes, or carrier protein content per proteoliposome. Apparently one function of the carrier molecule shows a broad tolerance for various phospholipids, while a second facet of the membrane protein activity requires very restricted lipid environment.