The phospholipid requirement for activity of the lactose carrier of Escherichia coli.

The transport activity of the lactose carrier of Escherichia coli has been reconstituted in proteoliposomes composed of different phospholipids. The maximal activity was observed with the natural E. coli lipid as well as mixtures containing phosphatidylethanolamine or phosphatidylserine. Phosphatidylcholine or mixtures of phosphatidylcholine with phosphatidylglycerol, phosphatidic acid, or cardiolipin showed low activity. The lactose carrier reconstituted with amino phospholipids of increasing degrees of methylation (dioleoylphosphatidylethanolamine, dioleoylmonomethylphosphatidylethanolamine, dioleoyldimethylphosphatidylethanolamine, and dioleoylphosphatidylcholine) revealed a progressive decrease in both counterflow and proton motive force-driven lactose uptake activities. Trinitrophenylation of phosphatidylethanolamine in the E. coli proteoliposomes resulted in a marked reduction in lactose carrier activity. Partial restitution of transport activity was obtained by detergent extraction of the carrier from these inactive proteoliposomes and reconstitution of the carrier into proteoliposomes containing normal E. coli lipid. These results suggest that the amino group of the amino phospholipids (e.g. phosphatidylethanolamine and phosphatidylserine) is required for the full function of the lactose carrier from E. coli.