Role of lipids in the Neurospora crassa membrane. I. Influence of fatty acid composition on membrane lipid phase transitions.

The relationship between lipid composition and phase transition was investigated by differential scanning calorimetry for intact and membrane phospholipid extracts of wild-type (w/t) and the cel-(Tw 40) mutant of Neurospora crassa. The cel-(Tw 40) mutant (grown on minimal, sucrose medium supplemented with Tween 40 at approximately 34 degrees C) had approximately twice the saturated fatty acid content of w/t organisms grown at approximately 22 degrees C. The gel-liquid crystal phase transitions of ergosterol-free extracts derived from w/t and cel-(Tw 40) occur at -31 and -11 degrees C, respectively. The heats of transition (delta H) of these extracts were 1 and 13 cal/g, respectively. The addition of ergosterol (the predominant sterol in Neurospora) to the phospholipid extracts decreased the observed heats of transition, but did not alter the transition temperature. Intact Neurospora, whether w/t or cal-(Tw 40) did not manifest similar gel-liquid crystal phase transitions in the differential scanning calorimeter. However, an endothermic peak at approximately 30 degrees C was observed in intact cells and extracted phospholipids of both w/t and cel-(Tw 40) organisms. This peak was insensitive to the addition of ergosterol, had a low heat content (delta H congruent to 1 cal/g), and was reversible.