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ergosterol DPPC phase diagram 2005 BJ

ergosterol DPPC phase diagram 2005 BJ - Biophysical Journal...

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The Effect of Ergosterol on Dipalmitoylphosphatidylcholine Bilayers: A Deuterium NMR and Calorimetric Study Ya-Wei Hsueh,* Kyle Gilbert,* C. Trandum, z § M. Zuckermann,* and Jenifer Thewalt* y *Department of Physics, and y Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia, Canada, V5A 1S6; z MEMPHYS-Center for Biomembrane Physics, University of Southern Denmark, DK-5230 Odense, Denmark; and § Department of Chemistry, Roskilde University Center, 4000 Roskilde, Denmark ABSTRACT We have studied the effect of ergosterol, an important component of fungal plasma membranes, on the physical properties of dipalmitoylphosphatidylcholine (DPPC) multibilayers using deuterium nuclear magnetic resonance ( 2 H NMR) and differential scanning calorimetry (DSC). For the 2 H NMR experiments the sn -1 chain of DPPC was perdeuterated and NMR spectra were taken as a function of temperature and ergosterol concentration. The phase diagram, constructed from the NMR spectra and the DSC thermograms, exhibits both solid-ordered ( so ) 1 liquid-ordered ( lo ) and liquid-disordered ( ld ) 1 lo phase coexistence regions with a clear three-phase line. This is the first demonstration that lo domains exist in liquid crystalline membranes containing ergosterol. The domain sizes in the ld 1 lo phase coexistence region were estimated by analyzing the exchange of labeled DPPC between the two regions, and depend on ergosterol concentration. The DPPC-ergosterol phase diagram is similar to that of the DPPC-cholesterol multibilayer system except that the so 1 lo and ld 1 lo phase coexistence regions are considerably broader. INTRODUCTION Sterols are essential components of eukaryotic cells both as structural membrane components and as initiators and regulators of biological processes. In particular, cholesterol is ubiquitous in mammalian cells and ergosterol is the major sterol in certain fungi and protozoans, where it fulfills several important functions. In the unicellular eukaryote, Saccharo- myces cerevisiae , Rodriguez et al. (1985) have identified four of these functions, which include growth (sparking) and bulk requirements, the latter related to maintaining mem- brane integrity under physiological conditions. Recently Kuebler et al. (1996) found evidence for the presence of Triton X-100 insoluble fractions in S . cerevisiae plasma membrane lipids, which implied the existence of ‘‘rafts’’ in the plasma membrane. Bagnat et al. (2000) showed that the major components of rafts in the plasma membrane of S . cerevisiae are phosphoinositol-based sphingolipids and ergosterol. The same authors found that, in contrast to mammalian rafts, the assembly of rafts in S. cerevisiae starts in the endoplasmic reticulum (ER) and that glycophospha- tidylinositol anchored proteins associate with rafts in the ER.
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