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Biophys. J. BioFAST: First Published May 25, 2007. doi:10.1529/biophysj.106.098038
© 2007 by the Biophysical Society.

A more recent version of this article appeared on August 15, 2007.
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Jacques Prost
Camilla Barbetta
Jean-François Joanny
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BIOPHYSICAL THEORY AND MODELING

Dynamical control of the shape and size of stereocilia and microvilli

Jacques Prost 1*, Camilla Barbetta 1 and Jean-François Joanny 1

1 Institut Curie

* To whom correspondence should be addressed. E-mail: jacques.prost{at}curie.fr.

Submitted on September 28, 2006
Revised on November 22, 2006
Accepted on 19 April 2007


  Abstract
We discuss theoretically the shape of actin based protrusions such as stereocilia or microvilli which have important functions in many biological systems. These linear protrusions are dynamical structures continuously renewed by treadmilling: actin polymerizes at the tip of the cilium and depolymerizes in its bulk. They also often have a well-controlled length such as in the hair bundles of the inner ear cells where they appear in a graded stair-case structure. Recent experimental results by Rzadzinska et al. show that the treadmilling velocity of the hair cell stereocilia is proportional to their length. We use generic arguments to describe the physics of stereocilia taking into account the effect of many individual proteins at a coarse grained level by a few phenomenological parameters. At the tip of the cilium, we find that actin polymerization induces an effective pressure. Below the tip, the shape of the cilium is determined by depolymerization: Agreement with the observed shape requires that depolymerization occurs at least in two steps. Under these conditions, we calculate the cilium shape and provide physical grounds for the proportionality between treadmilling velocity and cilium length. We also calculate the penetration of the stereocilium in the actin cortical layer.

Key Words: actin polymerization, hair cells, membrane, microvilli, stereocilia




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Copyright © 2007 by the Biophysical Society.