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Originally published as Biophys J. BioFAST on October 5, 2007.
doi:10.1529/biophysj.107.118760
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Biophysical Journal 94:1667-1680 (2008)
© 2008 The Biophysical Society

This is an Open Access article distributed under the terms of the Creative Commons-Attribution Noncommercial License (http://creativecommons.org/licenses/by-nc/2.0/), which permits unrestricted noncommercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

A Quantitative Model for the All-or-None Permeabilization of Phospholipid Vesicles by the Antimicrobial Peptide Cecropin A

Sonia M. Gregory, Allison Cavenaugh, Velvet Journigan, Antje Pokorny and Paulo F. F. Almeida

Department of Chemistry and Biochemistry, University of North Carolina-Wilmington, Wilmington, North Carolina

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

The mechanism of the all-or-none release of the contents of phospholipid vesicles induced by the antimicrobial peptide cecropin A was investigated. A detailed experimental study of the kinetics of dye release showed that the rate of release increases with the ratio of peptide bound per vesicle and, at constant concentration, with the fraction of the anionic lipid phosphatidylglycerol in neutral, phosphatidylcholine membranes. Direct measurement of the kinetics of peptide binding and dissociation from vesicles revealed that the on-rate is almost independent of vesicle composition, whereas the off-rate decreases by orders of magnitude with increasing content of anionic lipid. A simple, exact model fits all experimental kinetic data quantitatively. This is the first time that an exact kinetic model is implemented for a peptide with an all-or-none mechanism. In this model, cecropin A binds reversibly to vesicles, which at a certain point enter an unstable state. In this state, a pore probably opens and all vesicle contents are released, consistent with the all-or-none mechanism. This pore state is a state of the whole vesicle, but does not necessarily involve all peptides on that vesicle. No peptide oligomerization on the vesicle is involved; alternative models that assume oligomerization are inconsistent with the experimental data. Thus, formation of well-defined, peptide-lined pores is unlikely.






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Copyright © 2008 by the Biophysical Society.