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Actin is one of the key and highly conserved elements of the cytoskeleton. It is indispensable for driving many cellular processes, including cell migration, cytokinesis, vesicle transport, and contractile force generation. To perform diverse functions, actin filaments assemble into higher-order structures such as branched actin networks and actin bundles. This entry describes different types of actin bundles present in cells, their locations, and the bundling proteins involved in their formation.
Actin, one of the key and highly conserved elements of the cytoskeleton, amounts to approximately 5–15% of total cell proteins [1][2]. It is indispensable for driving many cellular processes, including cell migration, cytokinesis, vesicle transport, and contractile force generation [3]. The globular actin monomers (G-actin) polymerize to form semi-flexible double-stranded helical filaments (F-actin), also known as microfilaments. These filaments assemble into higher-order structures, such as branched networks or bundles [5]. It is now recognized that complex actin networks and bundles are essential for many cellular functions. Lately, actin-bundling proteins have been attracting a lot of attention as their malfunction is linked to malignant cancers, muscular dystrophy, bone disease, and immunological disorders [4][5][6][7][8].
More than 150 actin-binding proteins (ABPs) are known to associate with the actin cytoskeleton, and many of them regulate actin functions [9]. These proteins are involved in: (1) regulation of actin assembly and disassembly, (2) actin-driven movements in cells, (3) connecting actin structures to plasma membrane/cell organelles or other cytoskeleton proteins, and (4) organizing actin filaments (by their crosslinking) into higher-order structures, such as branched actin networks or actin bundles [1][10][11]. The proteins forming higher-order structures are among the most represented and diverse functional families of actin-binding proteins.
In the cells, actin is present either in monomeric, filamentous, or higher-order three-dimensional structures, such as bundles and branched networks [10]. The length of filaments varies from dozens of nanometers (e.g., in branched networks) to several dozen micrometers (e.g., in stress fibers, filopodia, and stereocilia) [12][13][14][15][16]. Together, they form a continuum of systems enabling the reception and transduction of mechanical stimuli across the cell and providing mechanical support for the shape and polarity of cells. The necessity of forming higher-order actin structures is dictated by the immense variety of cellular functions supported by the actin cytoskeleton and a broad range of mechanical forces required to carry them out. Forces generated by these higher-order actin networks, ranging from piconewtons to nanonewtons, aid in cell migration and invasion, internal vesicle movements, endocytosis, exocytosis, phagocytosis, and cell division [17][18][19].
Actin bundles are linear arrays of actin filaments crosslinked by one or, more often, several different actin-bundling proteins (Figure 1 A,B). The length and width of such bundles, and the number of filaments present in them are dictated by their unique set of proteins and by kinetic conditions under which these bundles are formed [20][21], giving each bundled complex a specific structure with different mechanical properties [19][22], Figure 2, Table 1. The size of the bundling proteins plays an important role in the topology and the compactness of the bundles (Figure 1 B,C). For example, fascin, plastins/fimbrins, and small espins are small sized actin-bundling proteins that crosslink actin filaments to form compact and tighly packed bundles, whereas α-actinin is a larger-sized crosslinker inducing widely spaced bundles with distorted square lattice (Figure 1 B,C).
There are two main types of actin bundles, with either parallel or mixed polarity filament orientations. In parallel (or uniform polarity) actin bundles, the filaments are ordered with consistent polarity, allowing them to conduct work (e.g., membrane deformation) due to the directional F-actin elongation. Parallel actin bundles are present in chemosensory and mechanosensory cell protrusions (microvilli) of most cell types [22][23], stereocilia of inner ear hair cells [24], bristles in the thorax of Drosophila melanogaster [25], and in ectoplasmic specializations of Sertoli cells (Figure 2, [26]). Filopodia, microspikes, focal adhesions, and distal ends of dorsal stress fibers, found in most cell types, consist also of parallel actin bundles (Figure 2, [7][27]). These bundles create the force for localized membrane protrusions, while helping cells to resist compressive forces from the membrane [28]. They facilitate cell movement in response to extracellular stimuli or intracellular signaling [29]. The length (1 to 100 μm) and number (one to hundreds) of these bundles per cell, their diameter and the number of actin filaments in them (a few to ~1000) vary depending on the cell type and the structures they support (microvilli, stereocilia, bristles, filopodia, (Figure 2, Table 1), and invadosomes (podosomes/invadopodia) [19][25][27][30][31][32][33][34].
More than two actin-bundling proteins are often associated with these actin structures. Filopodia, a thin actin-rich plasma-membrane protrusions, help in chemosensing during cell migration, wound healing, and cell adhesion to the extracellular matrix [33]. They are enriched in fascin, but additional bundling proteins, such as α-actinin, fimbrin/plastin, filamin, and espin are also present in them under certain conditions [33][35]. Podosomes are actin-based dynamic structures near the plasma membrane of various cells (such as monocytic, endothelial, and smooth muscle cells). They contribute to cell migration, matrix invasiveness, bone remodeling, and mechanosensing [34][36][37] and contain fascin, L-plastin (a hematopoietic cell-specific plastin isoform), and α-actinin [34][38][39][40]. Invadopodia are functionally and structurally similar to podosomes of normal cells, but they are present in tumor cells [34]. Additionally, podosomes and invadopodia are unique in their ability to degrade ECM material by locally releasing proteolytic enzymes [34]. Ectoplasmic specializations of Sertoli cells, which are hybrid testis-specific cell–cell contacts (contributing to the blood–testis barrier), contain espin and T-plastin (a most abundant plastin isoform found in cells of most solid tissues) [19][41][42][43]. Microvilli—the finger-like projections on the surface of several types of cells - increase the total cell surface without substantially increasing its volume [19][23]. Most microvilli contain I-plastin (a plastin isoform found in the intestinal and kidney microvilli and stereocilia of the inner ear), small isoforms of espin, and villin. Similarly, stereocilia are the modified microvilli that transduce mechanical signals into stimulus-dependent electrical signals. They predominantly contain I-plastin and fascin, but also have isoforms of espin and other proteins expressed in smaller quantities [44][45].
Location | Microvilli | Stereocilia | Filopodia | Stress fibers | Bristles (drosophila) |
---|---|---|---|---|---|
Cell type | Most cells (frequent in epithelial cells) | Auditory and vestibular sensory cells | Motile cells | Most cells (prominent in fibroblasts, smooth muscle, and endothelial cells) | Sensory organ precursor cells |
Function | Increase apical surface area for absorption | Mechano-electrical signaling | Sensory and guiding | Contraction and adhesions | Mechanosensing |
Length | 100 nm to 2 µm | 1.5–15 µm | ≤10 µm | ≥2 µm | Macrochaetes: 250–300 μm, Microchaetes: 70 μm (non-continuous 1–5 µm units) |
Diameter | 50–100 nm | ~200 nm | 20–200 nm | Varies from cell to cell | Varies |
Number of actin filaments | 30–40 | ~400–3000 | 10–30 | 10–30 | 7–18 bundles with hundreds of filaments |
Actin filament organization | Parallel (unipolar) | Parallel (unipolar) | Parallel (unipolar) | Mixed (bipolar) | Parallel |
Bundling proteins | Espin, plastin, villin | Fascin, espin, plastin | Fascin, α-actinin, plastin, espin | α-Actinin, fascin filamin |
singed (fascin), forked (espin) |