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Biophys. J. BioFAST: First Published August 5, 2005. doi:10.1529/biophysj.105.061721
© 2005 by the Biophysical Society.

A more recent version of this article appeared on November 1, 2005.
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MEMBRANES

Unfolding and extraction of a transmembrane {alpha}-helical peptide: Dynamic Force Spectroscopy and Molecular Dynamics Simulations

Sonia Antoranz Contera 1*, Vincent Lemaitre 1, Maurits R R de Planque 1, Anthony Watts 1 and John F Ryan 1

1 University of Oxford

* To whom correspondence should be addressed. E-mail: s.antoranzcontera{at}physics.ox.ac.uk.

Submitted on February 24, 2005
Revised on April 24, 2005
Accepted on 16 May 2005


  Abstract
An atomic force microscope (AFM) was used to visualize CWALP1923 peptides (+H3N-ACAGAWWLALALALALALALWWA-COO-) inserted in gel-phase DPPC and DSPC bilayers. The peptides assemble in stable linear structures and domains. A model for the organization of the peptides is given from AFM images and a 20 ns molecular dynamics (MD) simulation. Gold coated AFM cantilevers were used to extract single peptides from the bilayer through covalent bonding to the cystein residue. Experimental and simulated force curves show two distinct force maxima. In the simulations these two maxima correspond to the extraction of the two pairs of tryptophan residues from the membrane. Unfolding of the peptide precedes extraction of the second distal set of tryptophans. To probe the energies involved, AFM force curves were obtained from 10 to 104nm/s while MD force curves were simulated with 108 to 1011 nm/s pulling velocities (V). The velocity relationship with the force F was fitted to two fluctuation adhesive potential models. The first assumes the pulling produces a constant bias in the potential and predicts a F~ ln(V) relationship. The second takes into account the ramped bias the linker feels as it is being driven out of the adhesion complex and scales as F ~ (lnV)2/3.

Key Words: atomic force microscopy, dynamic force spectroscopy, membrane protein unfolding, molecular dynamics simulations




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