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

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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 × 10-9 cm2 sec-1 at 20°C and about 9 × 10-8 cm2 sec-1 at 45°C. Plots of D versus temperature in this system show a transition between about 30 and 35°C. Arrhenius activation energies for the diffusion in this case between 15 and 30°C and between 35 and 45°C are 28.5 and 7.0 kcal mol-1, respectively (1 calorie = 4.18 joules). In egg phosphatidylcholine bilayers, D is about 3 × 10-8 cm2 sec-1 at 20°C and the Arrhenius activation energy for diffusion is 8.1 kcal mol-1 between 15 and 35°C in this system. In bilayers prepared from an equimolar mixture of egg phosphatidylcholine and cholesterol D at 20°C is about 1.4 × 10-9 cm2 sec-1 and the Arrhenius activation energy for the diffusion of the protein in this system between 15 and 35°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°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.

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