Temperature-induced physical changes in fungal plasma membranes

Abstract

Ordered structures suggesting phase separation of lipids were observed in freeze-fracture replicas of plasma membranes of Fusarium sulphureum after free protoplasts had been fixed at 0 °C. Areas representing up to 50% of the protoplast plasma membrane inner fractured face assumed a lateral, hexagonally ordered structure from which the normally randomly distributed 25-nm membrane protein complexes had been excluded. Intact macroconidia only rarely showed small regions of lateral ordering of this type when fixed at the same, nonlethal temperature.The normal impermeability of the macroconidial plasma membrane to divalent cations such as Ni2+ declined rapidly when the cells were incubated at temperatures 5 °C or more above the physiological growth temperature range of the organism. Admission of divalent cations and death of the cells was associated with an irreversible thermotropic change in the fluidity in the membrane lipids as indicated by lipophilic spin probes.Partitioning spin probe and freeze-fracture techniques were best suited to the observation of thermotropic physical changes in the fungal plasma membrane which were induced at supra- and sub-physiological temperatures, respectively.