Cytoskeletons of an amoeba change properties quickly by varying cross-links of actin polymer filaments in response to changing environmental cues.

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Imagine if our skeletal structure could change in response to immediate circumstance--bones thicken and solidify when supporting heavy loads or become lighter, more airy and springy when jogging. While we can't do that, single-celled organisms such as amoeba can. Actin polymer filaments, the basis of cellular skeletons (cytoskeleton), cross-link to each other in different ways to form a variety of network archictectures. Key players in this system are "actin binding proteins" (ABP) that cross-link actin filaments together. The amoeba, Dictyostelium discoideum, uses actin filament and ABPs to form structural materials with different shapes and properties for diverse functions such as locomotion, internal transport of nutrients, and reproduction. To play these various functional roles, actin fiber networks need to be quickly and repeatedly broken down and reformed. One way to control these changes is by varying pH levels. D. discoideum's ABPs contain a high content of the amino acid, histidine, which makes the actin fiber networks susceptible to structural regulation by pH adjustment. The adjustment conditions that effect ABP positioning and concentration allow for the cell to change its cytoskeletal shape and properties in relatively short order.

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.

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“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)

Journal article
Structure and dynamics of cross-linked actin networks

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