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

Abstract

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