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

Abstract

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-l-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 increases by as much as 2-fold as the temperature is decreased by less than 10, 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.Essential fatty-acid auxotrophs of Escherichia coli are powerful tools that have been exploited to study the influence of lipid physical properties on the function and assembly of cellular membranes (1-7). Arrhenius plots for transport have been shown to have a biphasic shape, and the characteristic temperature that defines the change in slope in these plots is exquisitely responsive to the physical characteristics of the essential fatty-acid supplement present during cellular growth. The characteristic temperatures reflect the melting characteristics of the essential fatty-acid supplement, e.g., the characteristic temperatures are in decreasing order when determined in cells grown in media supplemented with fatty acids that are trans-monoenoic, cis-monoenoic, and cis, cisdienoic (or cis, cis, cis-trienoic), (2-4, 6). The first strong indications that the characteristic temperatures correspond to a change of state in the membrane lipids came from two independent studies: (1) The characteristic temperatures for two unrelated transport systems were identical for cells grown with a single essential fatty acid (2, 4). (2) A change in state detected in a monolayer of phosphatidylethaAbbreviations: T is the absolute temperature (degrees Kelvin), th and tI refer to "higher" and "lower" characteristic temperatures (0C) as revealed by spin labeling (or other physical techniques), and th* and tt* refer to the higher and lower temperatures (0C) as revealed by transport rate. The spin label TEMPO is 2,2,6,6-tetramethylpiperidine-1-oxyl. MATERIALS AND METHODSGrowth and Properties of Bacterial Strains. Strain 30E,3ox-was used exclusively in the studies reporte...

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