The temperature-pressure phase diagram of a DPPC-ergosterol fungal model membrane -- a SAXS and FT-IR spectroscopy study.

We investigated the phase behaviour of the binary lipid mixture dipalmitoylphosphatidylcholine (DPPC)-ergosterol (78:22mol%) as a function of temperature and pressure using Fourier transform-infrared (FT-IR) spectroscopy and small-angle X-ray scattering (SAXS). A temperature range from 10 to 70 degrees C and a pressure range from 1 to approximately 8500bar was covered to be able to establish a p,T-phase diagram of this model mixture of a fungal plasma membrane. Our results show a liquid-ordered/solid-ordered (l(o)+s(o)) two-phase coexistence region up to 41 degrees C, followed by a liquid-disordered/liquid-ordered (l(d)+l(o)) coexistence region up to approximately 57.5 degrees C, giving way to an all-fluid-like l(d) phase at higher temperatures. With increasing pressure, all phase boundaries shift upwards to higher temperatures. The width of the raft-like l(d)+l(o) two-phase coexistence region is found to be about 2kbar over the temperature range covered. At higher pressures, transitions to a l(o)+s(o) two-phase region and, at still higher pressures and low temperatures, an all-ordered s(o) phase is detected. Finally, the biological (high pressure-induced upregulation of ergosterol synthesis) and biotechnological (inactivation of fungal microorganisms) relevance of these studies is discussed. The results may be particularly interesting for understanding the function of membrane proteins under external stress conditions, such as high pressure. Interestingly, in this pressure range of about 2kbar, ceasing of membrane protein function has been observed in natural membrane environments. They may be inactivated under pressure/temperature conditions when the plasma membrane becomes too rigid, i.e., reaches an overall ordered conformational state.