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Originally published as Biophys J. BioFAST on June 23, 2006.
doi:10.1529/biophysj.106.085027
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Biophysical Journal 91:2184-2197 (2006)
© 2006 The Biophysical Society

Temperature and Composition Dependence of the Interaction of {delta}-Lysin with Ternary Mixtures of Sphingomyelin/Cholesterol/POPC

Antje Pokorny *, Lindsay E. Yandek *, Adekunle I. Elegbede {dagger}, Anne Hinderliter {dagger} and Paulo F. F. Almeida *

* Department of Chemistry and Biochemistry, University of North Carolina Wilmington, Wilmington, North Carolina; and {dagger} Department of Pharmaceutical Sciences, North Dakota State University, Fargo, North Dakota

Correspondence: Address reprint requests to P. F. Almeida, Tel.: 910-962-7300; E-mail: almeidap{at}uncw.edu.

The kinetics of carboxyfluorescein efflux induced by the amphipathic peptide {delta}-lysin from vesicles of porcine brain sphingomyelin (BSM), 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC), and cholesterol (Chol) were investigated as a function of temperature and composition. Sphingomyelin (SM)/Chol mixtures form a liquid-ordered (Lo) phase whereas POPC exists in the liquid-disordered (Ld) phase at ambient temperature. {delta}-Lysin binds strongly to Ld and poorly to Lo phase. In BSM/Chol/POPC vesicles the rate of carboxyfluorescein efflux induced by {delta}-lysin increases as the POPC content decreases. This is explained by the increase of {delta}-lysin concentration in Ld domains, which enhances membrane perturbation by the peptide. Phase separations in the micrometer scale have been observed by fluorescence microscopy in SM/Chol/POPC mixtures for some SM, though not for BSM. Thus, {delta}-lysin must detect heterogeneities (domains) in BSM/Chol/POPC on a much smaller scale. Advantage was taken of the inverse variation of the efflux rate with the Ld content of BSM/Chol/POPC vesicles to estimate the Ld fraction in those mixtures. These results were combined with differential scanning calorimetry to obtain the BSM/Chol/POPC phase diagram as a function of temperature.




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