Abstract
Chlortetracycline, a fluorescent probe of its own active transport, has been used to study lateral phase separations of membrane lipid in Bacillus megaterium cells. Arrhenius plots of initial accumulation rates are triphasic, with transitions or characteristic temperatures of 20 degrees and 9.5 degrees . At the higher temperature, the mobility of the chloretracycline, as measured by fluorescence polarization, is markedly altered. Chlortetracycline transport exhibits saturation kinetics, and fluorescence energy transfer from protein to bound antibiotic can be observed. N-Phenyl-1-naphthylamine, a lipophilic fluorescent probe, responds to changes in the hydrophobic regions of the membrane that are distinct from membrane protein. The fluorescent properties of N-phenylnaphthylamine in partitioning and polarization experiments are altered most significantly at the lower characteristic temperature. No fluorescence energy transfer between N-phenylnaphthylamine and membrane protein or bound tetracycline can be detected. In correlative electron spin resonance experiments on the partitioning of a lipid-soluble spin label, the same characteristic temperatures detected in the fluorescence studies were measured. These data suggest that different probes may respond to either or both of the characteristic temperatures describing the lateral phase separation. Between these characteristic temperatures the chlortetracycline transport system is most intimately associated with relatively immobile lipids that are surrounded by a more mobile lipid phase.