Role of lipids in the Neurospora crassa membrane: III. Lipid composition and phase transition properties of the plasma membrane, and its components.

Abstract

TheNeurospora crassa mutantslime, which lacks a cell wall, was used to determine the lipid composition and phase transition properties of the plasma membrane and its components. The lipid composition and phase transition properties of the plasma membrane were then compared to the same parameters of whole cell extracts ofNeurospora slime. Phospholipids extracted from whole cells and plasma membranes have similar fatty acid profiles, but differ somewhat in phospholipid base compositions. These differences in phospholipid base profiles were not sufficient to cause differences in the heat content (ΔH) or mid-point temperature of the phospholipids' gel-liquid crystal phase transitions as detected by scanning calorimetry. Phase transition temperatures and heat contents for plasma membrane and whole cell phospholipid extracts were −23.8 and −22.5°C and 0.3 and 1.7 cal/g, respectively. Commercially available phospholipids when mixed in the ratios of phospholipids found in whole cell and plasma membrane extracts yielded similar results: The “model” system of whole cell extract had a transition temperature of −22°C and ΔH=1.1 cal/g, compared to the “model” system for plasma membrane with a transition temperature of −21°C and ΔH=1.24 cal/g. The neutral lipid fraction of plasma membranes also exhibits endothermic activity in the calorimeter at temperatures below 0°C with a ΔH∼9 cal/g. Total lipid preparations (neutral+phospholipids) and freezedried plasma membranes scanned in excess water do not exhibit any endothermic events in the calorimeter. Neutral lipids probably interact with phospholipids to abolish the calorimetrically detectable phase transition of phospholipids. Whole cell and plasma membrane extracts are sufficiently similar in phospholipid base composition to permit the use of whole cell extracts as a model system for studying the effect of altered lipid composition on the physical properties of the phospholipids in theNeurospora crassa plasma membrane.