The types of actin filament networks produced depend on the concentration of the binding proteins that cross-link filaments together. Figure 1 shows a protein cross-linking actin filaments. Figures 2a-d show filament networks produced with increasing protein concentration, respectively: weakly cross linked (2a), composite (2b), bundles (2c), and bundle cluster (2d). Artist: Emily Harrington. Copyright: All rights reserved. See gallery for details.
“The actin cytoskeleton, a network of protein-polymers, is responsible for the mechanical stability of cells. This biopolymer network is also crucial for processes that require spatial and temporal variations in the network structure such as cell migration, division and intracellular transport. The cytoskeleton therefore has to combine structural integrity and mechanical stability with the possibility of fast and efficient network reorganization and restructuring. Cells meet this challenge by using proteins to link filamentous actin (F-actin) and construct complex networks. The molecular properties of the cross- linking proteins determine to a large extent the (micro)structure, viscoelastic properties and dynamics of the resulting networks…To construct dynamic F-actin assemblies with specific morphologies and mechanical properties cells make use of actin binding proteins (ABPs).” (Lieleg 2010:218)
“[T]wo generic types of ABP-induced F-actin assemblies: networks of individual cross-linked actin filaments and actin bundles. In particular regions of the cytoskeleton either one of these assembly types may dominate or they may coexist forming a rather complicated composite phase.” (Lieleg et al. 2010:219)
“[S]mall cross-linking proteins…tend to tightly pack actin filaments into parallel bundles. Larger cross-linking molecules…tend to induce a more complex phase behavior: while at low concentrations they cross-link actin filaments into networks or gels, at higher concentrations purely bundled phases or composite networks with a rather diverse geometry occur.” (Lieleg et al. 2010:220)
“The binding affinity of cross-linking proteins is often also sensitive to specific chemical stimuli. Such stimuli may make it possible to switch between different network architectures…The high histidine content of Dictyostelium discoideum hisactophilin causes the binding of this protein to F-actin to be pH sensitive…An increase in ABP concentration not only alters the structure of an F-actin network but can also enhance its elastic response up to 1000 fold.” (Lieleg et al. 2010:221)
“[I]n contrast to flexible polymers—semi-flexible biopolymers such as F-actin are anisotropic and show a different response to forces perpendicular (bending) or parallel (stretching/compression) to the mean contour.” (Lieleg et al. 2010:222)