Actin filaments are one of the very important cytoskeleton components of a cell. Actin filaments are the most abundant cytoskeleton component making up "5% or more of the total cell protein" (Bruce et al. 2008). These filaments are found throughout the cell but are highly concentrated in the "cortex, just beneath the cell membrane" (Bruce et al. 2008; Lodish et al 2008 P.781). Actin filaments are double stranded helical filaments consisting of the protein, actin (Bruce et al. 2008)
Actin filaments with the right signals and accessory proteins help in cell locomotion and structural support; it is also an important component in muscle and muscle contraction (Bruce et al. 2008). Actin filaments however are highly flexible and thin, only having a diameter of around 5nM to 10nM and therefore are situated in cross-linked bundles to carry out their function in an effective manner making the filaments stronger (Bruce et al. 2008).
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There are three different ways actin is arranged in bundles and they are the Gel-like network arrangement, the Contractile Bundle arrangement and the Tight Parallel bundle arrangement. In the Gel-like Network arrangement, the actin filaments are arranged in a loose manner with many interconnections between the filaments (Bruce et al.2008). This arrangement resembles a mesh work arrangement due to "gel forming accessory proteins" (Bruce et al. 2008). The Contractile Bundle arrangement and the Tight Parallel arrangement are similar as the actin fibres in these arrangements are parallel and therefore the filaments do not interconnect amongst themselves. The neighbouring actin filaments in contractile bundles have opposite polarity whereas in Tight Parallel Bundles the neighbouring actin filaments all have similar polarity. Tight Parallel bundles have much tighter spacing than the contractile bundles (Bruce et al. 2008). Both of these bundle arrangements possess "bundling proteins" (Bruce et al. 2008) which relatively help keep the actin filaments relatively tighter than Gel-like networks.
Actin filaments are held in bundles by specific accessory proteins. These proteins are responsible for keeping actin together and also the arrangement of actin filaments in their respective arrangements. There are different accessory proteins that arrange actin filaments in the gel-like network arrangement, contractile bundle arrangement and tight parallel bundle arrangement. There are many actin bundling proteins or even protein complexes responsible for the arrangement of actin filament bundles (C. Ampe; J. Vanderkerckhove 2005). "Many actin cross-linking proteins belong to the Calponin homology-domain superfamily. Each of these CH-domain proteins has a pair of actin-binding domains whose sequence is homologous to that of calponin, a muscle protein" (Lodish et al 2008 P.782).
Actin filaments are arranged into contractile bundles by the accessory protein called alpha actinin (actinin). Alpha actinin not only plays a pivotal role in the arrangement of actin filaments but also in the major roles of contractile bundles. Alpha actinin is a homodimer protein that contains two actin binding sites like many other actin bundling proteins. This protein consists of two polypeptide chains and it has a total molecular mass of around 102,000Da (Bruce et al. 2008). Alpha actinin is a CH - Domain protein whose actin binding domains are situated at the flexible amino terminals of the two polypeptide chains in this protein dimer. The carboxyl terminals of the polypeptide chains of alpha actinin is a calmodulin shaped domain which possesses an EF-hand which suggests that calcium is required by alpha actinin in its functioning. An important element for dimerization of alpha actinin are the central rods of both the polypeptide chains as they interact with each other to form this complex dimer. The central rod domains consist of repeating spectrin units (the sequences of the polypeptide chains are similar to that of spectrin) (Manual blueprint.com).
Redrawn from: http://manual.blueprint.org/Home/glossary-of-terms/mechano-glossary--a/alpha-actinin
Figure 1. A simplified diagram of the structure of the alpha-actinin accessory protein. CH stands for the CH-domains of alpha actinin at the N-terminus. S stands for the repeating Spectrin units in the central rods acting as the dimerization domain. The EF hands are located at the C-terminusalpha actinin reowrked.jpg
Alpha actinin arranges neighbouring actin filaments in a parallel manner but also in such a way that neighbouring actin filaments have opposite polarities. Alpha actinin also arranges actin filaments in a loose manner (30-60nm apart) which helps the motor protein myosin -II incorporate in the actin bundles (Bruce et al. 2008). The incorporation of myosin II into the bundle is a major role of alpha actinin. During the cytokinesis of eukaryotic cells in mitosis actin and myosin incorporate in a contractile bundle arrangement at the contractile ring. Alpha actinin helps myosin fit between the actin filaments and allows myosin to pull on the actin filaments causing a contraction at the contractile ring allowing the cytoplasm of the two daughter cells to pull inside and split apart. There is also some evidence that alpha actinin helps other proteins fit into the contractile ring to help myosin cause a contraction at the contractile ring (Nigg, E.A. 2001). Alpha actinin is mainly found in stress fibres which consist of contractile bundle arrangement of actin and myosin. These stress fibres start from near the nucleus in the cytoplasm and terminate at the plasma membrane (near the substratum) at points called focal contacts (Bruce et al. 2008). Alpha actinin has a primary role of allowing the myosin II protein to fit into the actin bundles and neighbouring actin filaments to pull along each other and cause contractions that allow these fibres to pull on the substrate and cause some cell movement (Bershadsky, A. D. and Elbaum, M. 2001). It is important to note that alpha actinin helps bring in various other accessory proteins, GTPases etc to allow cell locomotion.
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Gel-like networks of actin filaments are held together by the actin bundling protein, filamin. Filamin has a major role in not only spacing out actin filaments in a loose manner but also cross linking actin filaments which helps form gel. "Filamin molecules are naturally found in cells as elongated, V-shaped dimers" (Manuel Blueprint) in cell cytoplasm. Filamin has a total molecular weight of 280,000Da and it is also a CH-domain protein whose actin binding sites are found at the amino terminal ends of the proteins in this dimer, similar to that of a typical immunoglobulin molecule (Manuel Blueprint). The carboxyl end of the filamin protein is the dimerization domain where bonds are formed between the polypeptide chains to keep this dimer protein together (Manuel Blueprint).
Figure 2. A simplified diagram of the structure of the filamin accessory protein. CH stands for the CH-domain located at the N-terminus. Notice the proteins ends Y-shaped due to the dimerization domain located at the C-terminusfilamin figure.jpg
Filamin has flexible arms which can hold actin filaments at numerous angles (Lodish et al 2008 P.782, Manuel Blueprint). Filamin is important in gel like networks as it helps hold actin filaments orthogonally and also in a loose manner due to the positioning of the two actin binding domains in the filamin structure (Manuel Blueprint). The orthogonal and loose arrangement in gel like networks due to filamin gives this bundle a gel-like and elastic character (Lodish et al 2008 P.783). Lamellipodia of a cell contains actin filaments in the gel-like network arrangement held together by Filamin (Insall, R. and Machesky, L. 2001). Migrating cells while moving extend "a broad, flat, leading edge or lamellipodium" (Stebbings, H. 2005) from the cytoplasm which contains an abundant meshwork of actin filaments arranged by filamin. The loose arrangements of actin filaments by filamin push out the cell membrane forming this protrusion that adheres to the substratum and form focal contacts which help in cell motility (Bruce et al.2008, Bershadsky, A. D. and Elbaum, M. 2001).
Actin filaments are held together in tight parallel bundles by the accessory proteins, fimbrin and villin. Both these accessory proteins arrange actin tight parallel bundles in a similar way but differ in location and protein structure. Fimbrin is a common accessory protein found in most tight parallel bundle arrangements in a cell but villin is mainly highly abundant in microvilli (Bruce et al. 2008). Fimbrin is also found in microvilli and was actually first discovered in microvilli (Manuel Blueprint). "Fimbrin represents one of the most basic structures of an actin cross linking protein" and it is also a single protein (Manuel Blueprint). Fimbrin has a molecular weight of just 68,000KDa (Lodish et al 2008 P.783). Fimbrin is a CH-domain protein which contains two calmodulin like actin binding domains at the N-terminal of the protein. The C-terminal of fimbrin is composed of two calcium binding sites in the form of EF hands (Manuel Blueprint). Villin has a total molecular weight of 92,000Da (Lodish et al 2008 P.783). Villin however is not a CH-domain protein but contains two actin binding sites. The C-terminal consists of two actin binding domains however there is a lot of debate as the affinity of this domain for actin varies greatly in vivo (Vardar et al. 2002). This domain is also rather larger than the other domains in the protein. The N-terminus of the protein has a calcium binding region suggesting that calcium can have a role in non CH-domain proteins also.
Villin and Fimbrin play an important role in the actin tight bundle arrangement in microvilli. Villin and Fimbrin also arrange the actin filaments tightly (only "10-20Nm apart") in the bundle and also in such a way that all actin filaments have the same polarity (Bruce et al. 2008). The tight arrangement results in greater structural support in microvilli which is important as microvilli are always subject to abrasion. Tight parallel arrangements of actin filaments result in no contraction as the actin filaments are so close together that they don't allow other accessory proteins to enter the bundle to cause a contraction or other responses. Therefore this arrangement is really for structural support in the microvillus. There is evidence that villin also helps elongation of microvilli during development (Bruce et al. 2008). So Villin and Fimbrin play an important role of arranging actin filaments for structural support of the microvilli structure.
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Figure 3. A simplified diagram of the structure of the filamin accessory protein. CH- stands for the CH-domain located at the N-terminus. Notice that Filamin is just a single protein. The C-terminus contains two EF hands.
Fimbrin is also important in Filopodia or micorspikes which are protrusions made by a cell to explore the external environment. Filopodia are much finer and rigid than Lamellipodia (Bruce et al. 2008). Fimbrin also arranges the actin bundles within the Filopodia to make all the actin filaments growing ends face the plasma membrane of the growing protrusion which shows that the extension of Filopodia is generated by actin polymerisation (Bruce et al. 2008). Filopodia like Lamellipodia is an actin rich protrusion made by a cell. Fimbrin help arrange actin filaments tightly to form this thin protrusion. Filopodia generally extend out from cell membrane and bind to the substrate "the cell then moves forward as a result of traction within the cytoplasm" (Stebbings, H. 2005) due to adherence to substratum (Stebbings, H. 2005).
filopodia and lamellipodia image.gif
Ampe, C. and Vandekerckhove, J. 2005. Actin and Actin Filaments. Encyclopedia of Life Sciences
Figure 4. Two pictures showing the difference in actin filament arrangement in Lamellipodia and Filopodia. The above picture shows the gel-like network arrangement of actin filaments in Lamellipodia. The bottom picture shows the tight parallel bundle arrangement in Filopodia.
So actin bundling proteins are indeed important to actin arrangements. Actin bundling proteins may differ in structure but they all arrange actin filaments into either contractile bundles, gel-like networks or tight parallel bundles. Actin bundling proteins may also not only have a role in only actin arrangement but also assisting actin filaments and various other accessory proteins and complexes to carry out other functions such as contractions, cell motility and even structural support.