Abstract
The energy requirements for the rapid transmembrane movement of phosphatidylethanolamine in membranes of Bacillus megaterium KM have been investigated by means of pulse label experiments. The transmembrane movement continues at a high rate in cells blocked in the production of metabolic energy by treatment with a combination of inhibitors. The movement is shown to be completely independent of the synthesis of lipid and of protein and, more generally, independent of sources of metabolic energy. The rate constant ki, defined as the fraction of the internal phosphatidylethanolamine that exchanges with the external layer of the membrane per unit time, has been found to have a value of about 0.1 per min. The compositional asymmetry of phosphatidylethanolamine in membranes of B. megaterium persisted, and indeed was somewhat enhance, in energy-poisoned cells under conditions in which rapid mixing of inner and outer layers was taking place. Therefore, the compositional asymmetry is not maintained by kinetic barriers to transbilayer exchange or by expenditure of metabolic energy. It must be an equilibrium condition, and presumably reflects the differential binding of phospholipids by proteins and other ligands on the two sides of the membrane.