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...

Cytoplasmic membranes of an unsaturated fatty acid auxotroph of Escherichia coli have been studied using spin labeled hydrocarbon probes. These studies reveal that the membrane lipids undergo changes of state at critical temperatures which reflect the physical properties of the fatty acid supplement supplied to the cells during growth. The critical temperatures observed in spin labeled membranes correlate with characteristic temperatures in membrane functions. Lipid analysis reveals that fatty acid composition and distribution in membrane phospholipids are primary determinants of the temperatures at which changes of state are observed in membrane lipids. Fatty acid composition and distribution can also produce unique interactions between certain spin label probes and their lipid environment.

Membranes compartmentalize biological functions and separate the cell from its environment. Membranes from various sources display a wide variety of specialized functions, but all contain proteins and lipids as basic constituents.With rare exceptions, phospholipids constitute the bulk of membrane lipids, and the "fluid" lipid membrane is essential for maintenance of the living state.The thermotropic (melting) behavior of phospholipids is dictated by the constituent fatty acid side chains, and also by head-group composition and degree of hydration. In general, the melting point of a phospholipid is inversely related to the extent of unsaturation and directly related to the chain length of its fatty acid chains. Microbial fatty acid composition can be modified by growth conditions. For example, most microorganisms contain more unsaturated fatty acids in membrane lipids when grown at lower temperatures.1-8 This observation stimulated interest in the relationship between the physical state of membrane lipids and membrane function. The first concrete evaluation of the effects of fatty acid composition on the physical state of membrane lipids in the intact cell and also in the membranes of these cells or in the lipids extracted from them was performed by Steim and coworkers who used differential scanning calorimetry to study a thermotropic change in state of Mycoplasma membrane lipids.9Auxotrophs (mutants with a nutritional requirement) of Escherichia coli, which require an unsaturated fatty acid supplement for growth, provide another system in which the membrane lipid fatty acid composition can be manipulated experimentally.

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'The rate of sugar transport as a function of temperature has been compared in t w o unsaturated fatty acid auxotrophs. O n e of these, the parent strain 30E, can 0-oxidize the unsaturated fatty acid supplements, whereas the p-oxidation defective progeny strain 30Epox-cannot. In a previous study, Arrhenius plots for transport of 0-glucosides and 8-galactosides by strain 30Eoox-~ revealed striking departures from linearity at both a lower and an upper characteristic temperatures. By electron spin resonance (esr) these temperatures were shown t o correlate with the temperatures where the membrane lipids undergo a transition from a totally solid state to a solidliquid equilibrium and from a solid-liquid equilibrium t o a totally liquid state, respectively ( 1 ) . In the present study with strain 30E we have made the following observations:I . Arrhenius plots for transport rate are usually more complex, often revealing three characteristic temperatures. T w o of these correlate with the upper and lower characteristic temperatures observed in strain 30Epox -. The third characteristic temperature falls between the previously described upper and lower ones.2. In cells supplemented during growth with elaidate, the third characteristic temperature was identical within experimental limits for both 0-glucoside and p-galactoside transport. indicating that it is likely t o arise from some interaction in the bulk lipid phase. This conclusion is supported by the fact that the boundary of a change in physical state is also observed at this temperature by electron spin resonance.3. In cells supplemented during growth with oleate, two o r three characteristic temperatures were observed depending upon the transport system studied. Although glucoside and galactoside transport had the same lower characteristic temperature, these systems had n o common upper characteristic temperature.acid, three characteristic temperatures were observed for P-glucoside transport in 120th strains 30E and 30Epox-.4. In cells supplemented during growth with the lipid density label, bromostearic