Lateral Phase Separations in Membrane Lipids and the Mechanism of Sugar Transport in Escherichia coli

Changes in slope of Arrhenius plots for transport can, in some instances, be detected at two different temperatures for cells that have a relatively simple fatty-acid composition in the membrane lipids. These characteristic temperatures correlate with the characteristic temperatures that define changes of state in membrane phospholipids as revealed by the paramagnetic resonance of the spin label TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl). The higher of these characteristic temperatures is that at which the formation of solid patches of membrane lipids is first detected. The lower is the end point of the course of lateral phase separations, at which all the membrane lipids are in a solid phase. For cells enriched for elaidic acid, the rate of transport increase by as much as 2-fold as the temperature is decreased by less than 1 degrees , at the higher characteristic temperature. At this characteristic temperature, lateral phase separations begin in the membrane phospholipids. This is also the temperature where one predicts a striking increase in the lateral compressibility of the membrane lipids. These data are thus interpreted to indicate that a component of the transport system vertically penetrates one or both monolayer faces of the membrane during transport, or that some other event involving the lateral compression of the phospholipids is important for transport.