Proline transport activity in Escherichia coli membrane vesicles of different buoyant densities.

Cytoplasmic membrane vesicles prepared by lysis of Escherichia coli W 3110 spheroplasts in a French press at 0 degrees C are heterogeneous with respect to density due to membrane protein aggregation as a result of lateral phase separation of membrane phospholipids and to the presence of more or less outer membrane. These different vesicle classes can be separated on isopycnic density gradients. Assays for various membrane-associated functions show that the membranes differ not only with respect to density and structure but also with respect to function. The proline transport system (as detected by uptake experiments with the artificial electron donor ascorbate-phenazine methosulfate) shows maximal activities in membrane fractions that have considerably higher densities than the normal cytoplasmic membrane. This is always the case, whether vesicles are isolated from membranes that exhibit a temperature-induced protein aggregation or not. A correlation between high proline transport activity and the presence of vesicles with double membranes (consisting of outer and inner membrane) has been established. The possibility that the outer membrane protects the transport system in the cytoplasmic membrane during the isolation of vesicles is discussed.