The effect of osmotic pressure on the membrane fluidity of Saccharomyces cerevisiae at different physiological temperatures

Membrane fluidity in whole cells of Saccharomyces cerevisiaeW303-1A was estimated from fluorescence polarization measurements using the membrane probe, 1,6-diphenyl-1,3,5-hexatriene, over a wide range of temperatures (6–35 °C) and at seven levels of osmotic pressure between 1.38 MPa and 133.1 MPa. An increase in phase transition temperatures was observed with increasing osmotic pressure. At 1.38 MPa, a phase transition temperature of 12±2 °C was observed, which increased to 17±4 °C at 43.7 MPa, 21±7 °C at 61.8 MPa, and 24±9 °C at an osmotic pressure of 133.1 MPa. From these results we infer that, with increases in osmotic pressure, the change in phospholipid conformation occurs over a larger temperature range. These results allow the representation of membrane fluidity as a function of temperature and osmotic pressure. Osmotic shocks were applied at two levels of osmotic pressure and at nine temperatures, in order to relate membrane conformation to cell viability.