Lateral mobility of an amphipathic apolipoprotein, ApoC-III, bound to phosphatidylcholine bilayers with and without cholesterol

The technique of fluorescence recovery after photobleaching was used to investigate the lateral mobility of a fluorescein-labeled amphipathic apolipoprotein, ApoC-III, bound to multibilayers prepared from dipalmitoyl phosphatidylcholine, egg phosphatidylcholine, and a 1:1 (molar ratio) mixture of egg phosphatidylcholine and cholesterol. In dipalmitoyl phosphatidylcholine bilayers the lateral diffusion coefficient (D) for the protein is about 2 x 10(-9) cm(2) sec(-1) at 20 degrees C and about 9 x 10(-8) cm(2) sec(-1) at 45 degrees C. Plots of D versus temperature in this system show a transition between about 30 and 35 degrees C. Arrhenius activation energies for the diffusion in this case between 15 and 30 degrees C and between 35 and 45 degrees C are 28.5 and 7.0 kcal mol(-1), respectively (1 calorie = 4.18 joules). In egg phosphatidylcholine bilayers, D is about 3 x 10(-8) cm(2) sec(-1) at 20 degrees C and the Arrhenius activation energy for diffusion is 8.1 kcal mol(-1) between 15 and 35 degrees C in this system. In bilayers prepared from an equimolar mixture of egg phosphatidylcholine and cholesterol D at 20 degrees C is about 1.4 x 10(-9) cm(2) sec(-1) and the Arrhenius activation energy for the diffusion of the protein in this system between 15 and 35 degrees C is 15.1 kcal mol(-1). Light-scattering and fluorescence-polarization results indicate that binding of this protein does not affect the gel-to-liquid crystalline phase transition of bilayer membranes but does mediate a major, reversible aggregation of the vesicles at about 33 degrees C. These results lend support to the view that ApoC-III resides in the head-group region of the bilayer and suggest that its lateral diffusion coefficient represents an upper bound for integral membrane proteins.