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Coexistence of Domains with Distinct Order and Polarity in Fluid Bacterial Membranes ¶
Author(s) -
Vanounou Sharon,
Pines Dina,
Pines Ehud,
Parola Abraham H.,
Fishov Itzhak
Publication year - 2002
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1562/0031-8655(2002)0760001codwdo2.0.co2
Subject(s) - membrane , polarity (international relations) , order (exchange) , biophysics , chemistry , chemical physics , biology , biochemistry , economics , cell , finance
In this study we sought the detection and characterization of bacterial membrane domains. Fluorescence generalized polarization (GP) spectra of laurdan‐labeled Escherichia coli and temperature dependencies of both laurdan's GP and fluorescence anisotropy of 1,3‐diphenyl‐1,3,5‐hexatriene (DPH) (r DPH ) affirmed that at physiological temperatures, the E. coli membrane is in a liquid‐crystalline phase. However, the strong excitation wavelength dependence of r laurdan at 37°C reflects membrane heterogeneity. Time‐resolved fluorescence emission spectra, which display distinct biphasic redshift kinetics, verified the coexistence of two subpopulations of laurdan. In the initial phase, <50 ps, the redshift in the spectral mass center is much faster for laurdan excited at the blue edge (350 nm), whereas at longer time intervals, similar kinetics is observed upon excitation at either blue or red edge (400 nm). Excitation in the blue region selects laurdan molecules presumably located in a lipid domain in which fast intramolecular relaxation and low anisotropy characterize laurdan's emission. In the proteo‐lipid domain, laurdan motion and conformation are restricted as exhibited by a slower relaxation rate, higher anisotropy and a lower GP value. Triple‐Gaussian decomposition of laurdan emission spectra showed a sharp phase transition in the temperature dependence of individual components when excited in the blue but not in the red region. At least two kinds of domains of distinct polarity and order are suggested to coexist in the liquid‐crystalline bacterial membrane: a lipid‐enriched and a proteo‐lipid domain. In bacteria with chloramphenicol (Cam)–inhibited protein synthesis, laurdan showed reduced polarity and restoration of an isoemissive point in the temperature‐dependent spectra. These results suggest a decrease in membrane heterogeneity caused by Cam‐induced domain dissipation.