Transfer of phosphatidylcholine between different membranes in Tetrahymena as studied by spin labeling

Transfer of phosphatidylcholine molecules between different membrane fractions of Tetrahymena pyriformis cells grown at 15, 27 and 39.5°C was studied by electron spin resonance (ESR). Microsomes were labeled densely with a phosphatidylcholine spin label and the spin-labeled microsomes were incubated with non-labeled cilia, pellicles or microsomes. The transfer of the phosphatidylcholine spin labels was measured by decrease in the exchange broadening of the electron spin resonance spectrum. In one experiment, the lipid transfer was measured between ³²P-labeled microsomes and non-labeled pellicles by use of their radioactivity. The result was in good agreement with that by ESR. The fluidity of the membrane was estimated using a fatty-acid spin label incorporated into the membranes. Transfer between lipid vesicles was also studied. The results obtained were as follows: (1) The transfer between sonicated vesicles of egg- or dipalmitoyl phosphatidylcholine occurred rapidly in the liquid crystalline phase, with an activation energy of 20 kcal/mol, whereas it hardly occurred in the solid crystalline phase. (2) The transfer rate between microsomal membranes increased with temperature, and an activation energy of the reaction was 17.8 kcal/mol. (3) The transfer from the spin-labeled microsomes to subcellular membranes of the cells grown at 15°C was larger than that to the membranes of the cells grown at 39.5°C. The membrane fluidity was larger for the cells grown at lower temperature. (4) Similar tendency was observed for the transfer between microsomal lipid vesicles prepared from the cells grown at 15°C and at 39.5°C. (5) The transfer from microsomes to various membrane fractions increased in the order, cilia < pellicles < microsomes. The order of increase in the membrane fluidity was cilia < microsomes < pellicles, although the difference between microsomes and pellicles was slight. These results indicate a crucial role of the membrane fluidity in the transfer reaction. (6) Some evidence supported the idea that the lipid transfer between these organelles occurred through the lipid exchange rather than through the fusion.