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Theoretical models of viscoelasticity of actin solutions and the actin cortex.

The elastic response of plant and animal cells depends on a network of protein filaments that form the cytoskeleton. This is a complex and highly dynamic composite of filamentous proteins, together with a range of accessory proteins for initiating and terminating polymerization, introducing cross-links, and forming lateral arrays or bundles of filaments (Alberts, 1994). A principal component of this is the actin cortex, which is itself an entangled and cross-linked network of F-actin. This cortex appears to be responsible for the mechanical stability and resistance of eukaryotic cells to external stresses. Coordinated assembly and disassembly of this network in response to cellular signals also appears to play a crucial role in cell locomotion (Stossel, 1994). The actin cortex, consisting of entangled or crosslinked actin filaments (F-actin), resembles solutions and gels of common synthetic polymers. However, the actin cortex in vivo--as well as in vitro models of the actin cortex, consisting of solutions of reconstituted F-actinexhibit unique properties that cannot be accounted for by the well-established models of synthetic polymer gels and solutions (DeGennes, 1979; Doi, 1988). For instance, the elastic moduli (specifically, the shear modulus) of actin networks can be several orders of magnitude larger than for comparable synthetic polymer systems (e.g., at the same concentration). This is a key property of actin networks, as many types of cells must withstand shear stresses as large as 1000 Pa, or even more. Furthermore,