Microscopic “Skeletons” Support Cellular Structure and Movement

Biological Strategy

Dictyostelium discoideum amoeba

AskNature Team

Modify Material Characteristics

The materials found in living systems are variable, yet often made from the same basic building blocks. For example, all insect exoskeletons consist of a material called chitin. Because material resources are limited, each material within or used by a given living system must frequently serve multiple purposes. Therefore, living systems have strategies to modify materials’ softness, flexibility, and other characteristics. To ensure survival, the benefits of these modifications must outweigh the living system’s energy and material expenditure to generate them. For example, spiders store the liquid components of spider silk in a gland, converting them into silk thread when needed. Some threads have different characteristics, such as elasticity and UV reflectance, than others.

Search this Function

Prevent Fracture/Rupture

High force impact or stress can cause materials that comprise living systems to separate into two or more pieces (called fracturing) or to break or burst suddenly (called rupturing). For example, a scallop prevents structural failure from fracture because its shell is comprised of two materials of varying stiffness. When a crack moves from the scallop’s stiff material to the less stiff one, the latter reduces the force at the tip of the crack, thereby stopping it from spreading farther.

Search this Function

Chemically Assemble Polymers

We might think that complex polymers are the result of human industrial ingenuity, but nature cornered the market on polymers billions of years earlier. Examples of biopolymers are proteins, carbohydrates, and genetic material. In contrast to human industrial processes, within a cell, ribosomes covalently bond amino acids together to form proteins.

Search this Function

Regulate Cellular Processes

Cells are the basic building blocks of all living systems, so cellular processes dictate how physiological processes occur within those systems. Cells (whether entire unicellular organisms or parts of multicellular living systems) grow, metabolize nutrients (that is, chemically transform them), produce proteins and enzymes, replicate, and move. Cells as part of multicellular systems rarely act alone, instead having ways to signal to start and complete simple to quite complex interactions. How skin heals is a good example of the role of cellular processes. Blood cells called platelets release clotting factors to stop the bleeding; white blood cells rid the area of foreign materials and release molecules to coordinate healing; cells called fibroblasts start rebuilding using proteins called collagen; new blood vessels form; and skin cells called keratinocytes create the new surface.

Search this Function

Protists

Kingdom Protista (“the very first”): slime molds, giant kelp, red and brown algae

Protists are a numerous, diverse group of organisms that, in the simplest definition, are defined by what they are not. Protists are not plants, animals, fungi, or bacteria. Some protists, like Plasmodium, are harmful parasites that cause malaria. While many are small, obscure, and go unnoticed, others are important foodstuffs. The largest protist in the world, giant kelp (Macrocystis pyrifera), is utilized by humans and marine species for food and shelter.

Search this Living System

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

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 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 s” (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 , 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.

Image: Bruno in Columbus / Public Domain - No restrictions

Dictyostelium aggregation

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).

Last Updated September 14, 2016

References

“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

AskNature