BIOPHYSICAL THEORY AND MODELING |
Flexible Charged Macromolecules on Mixed Fluid Lipid Membranes: Theory and Monte-Carlo Simulations
Shelly Tzlil 1 and Avinoam Ben-Shaul 2*
1 Hebrew University
2 The Hebrew Univ. of Jerusalem
* To whom correspondence should be addressed. E-mail: abs{at}fh.huji.ac.il.
Submitted on June 8, 2005
Revised on July 13, 2005
Accepted on 8 August 2005
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Abstract |
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Fluid membranes containing charged lipids enhance binding of oppositely charged proteins by mobilizing these lipids into the interaction zone, overcoming the concomitant entropic losses due to lipid segregation and lower conformational freedom upon macromolecule adsorption. We study this energetic-entropic interplay using Monte Carlo simulations and theory. Our model system consists of a flexible cationic polyelectrolyte, interacting, via Debye- and short-ranged repulsive potentials, with membranes containing neutral ("PC") lipids, 1% tetravalent ("PIP2") and 10% (or 1%) monovalent ("PS") anionic lipids. Adsorption onto a fluid membrane is invariably stronger than to an equally charged frozen or uniform membrane. While monovalent lipids may suffice for binding rigid macromolecules, polyvalent counter-lipids (PIP2), whose entropy loss upon localization is negligible, are crucial for binding flexible macromolecules, which lose conformational entropy upon adsorption. Extending Rosenbluth's Monte-Carlo scheme we directly simulate polymer adsorption on fluid membranes. Yet, we argue that similar information could be derived from a biased superposition of quenched membrane simulations. Using a simple cell model we account for surface concentration effects, and show that the average adsorption probabilities on annealed and quenched membranes coincide at vanishing surface concentrations. We discuss the relevance of our model to the electrostatic-switch mechanism of, e.g., the MARCKS protein.
Key Words:
Adsorption, Electrostatics, Lipids, Membranes, Polyelectrolyte, Sequestration
