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Biophys. J. BioFAST: First Published October 14, 2005. doi:10.1529/biophysj.105.066977
© 2005 by the Biophysical Society.

A more recent version of this article appeared on January 1, 2006.
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BIOPHYSICAL THEORY AND MODELING

Extending a Spectrin Repeat Unit II. Rupture Behavior

Sterling Paramore 1, Gary S. Ayton 1 and Gregory A. Voth 1*

1 University of Utah

* To whom correspondence should be addressed. E-mail: voth{at}chem.utah.edu.

Submitted on May 19, 2005
Revised on August 22, 2005
Accepted on 30 September 2005


  Abstract
A spectrin repeat unit was subject to extension using cyclic expansion nonequilibrium molecular dynamics. Periodic boundary conditions were used to examine the effects of the contiguous alpha-helical linker on the force response. The measured force-extension curve shows a linear increase in the force response when the spectrin repeat unit is extended up to about 0.4 nm. After that point, the force response peaks and subsequently declines. The peak in the force response marks the point where the spectrin repeat unit undergoes a change in its material properties from a strongly elastic material to a mostly viscous one, on the time-scales of the simulations. The force peak is also correlated with rupture of the alpha-helical linker, and is likely the event responsible for the peaks in the sawtooth pattern force-extension curves measured by atomic force microscopy experiments. Rupture of the linker involves simultaneously breaking about four hydrogen bonds which maintain the alpha-helical linker. Following this initial rupture, the linker undergoes simple helix-to-coil transitions as the spectrin repeat unit continues to be extended. The implications of linker rupture in the interpretation of unfolding and atomic force microscopy experiments are also discussed.

Key Words: atomic force microscopy, molecular dyanmics, nonequilibrium, protein unfolding, rupture behavior, spectrin




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Copyright © 2005 by the Biophysical Society.