Where is actin filaments found

A ctin filaments are one type of the cytoskeleton components. In animal cells, actin filaments use to be located close to the plasma membrane Figures 1 and 2 , but their distribution and organization depend much on the cellular type. Actin filaments perform many functions in the cell. Cells need actin filament for dividing, endocytosis, phagocytosis, organelles communication. In animal cells, actin filament provides mechanical support for maintaining or changing the cellular shape.

A ctin filaments are built by polymerization of globular actin proteins Figure 3 , which can be found in two isoforms: alpha- and beta-actin. Alpha-actin is abundant in muscle cells. Beta-actin is the most frequent isoform and is found in most animal cells. In total cytosolic actin protein pool, some are found as part of the actin filaments known as F-actin , and the remaining is free in the cytosol known as G-actin.

A ctin filaments are 7 nm in thickness. This is lower than the thickness of the other cytoskeleton filaments, microtubules and intermediate filaments.

That is why actin filaments are also known as microfilaments. Every actin filament has a minus end and a plus end, which means that they are polarized filaments. At the plus end, the polymerization, addition of new actin proteins, is more frequent than depolymerization, whereas at the minus end depolymerization is more frequent. The increase and decrease in the microfilament length is by polymerization and depolymerization, respectively. In the cell, these changes are happening all the time, as well as nucleation of new microfilaments, but complete depolymerizations as well.

Actin filaments are the most dynamic component of the cytoskeleton. T he cytosol environment and the concentration of free actin proteins in the cytosol prevent the spontaneous assembling of actin filaments. This control of microfilament formation is very useful for the cell because new microfilaments are formed when and where they are needed by precisely placing nucleation protein complexes.

A salient feature of actin filaments is that they are highly adaptable : are formed and removed easily, and are associated between each other in many ways to form 3D scaffolds. This versatility relies on more than different modulator protein types or actin associated proteins Figure 4. These proteins regulate filament polymerization and depolymerization rates, nucleation of new filaments, destruction of existing filaments, as well as 3D organization.

Actually, there is no naked actin filaments or free actin proteins in the cytosol, but they are always linked to some associated protein. P roteins associated to actin filaments can be divided according to their functions. Some proteins, such as profilin, join free actin G-actin and boost actin filament polymerization. Other proteins, such as thymosin, join free actin proteins and hamper the polymerization process by preventing the spontaneous polymerization of actin filaments.

There are proteins, such as fimbrin and alpha-actinin, making cross-bridges between actin filaments to form bundles of actin filaments, whereas other proteins like filamin make possible the arrangement of actin filaments in reticular structures.

In nonmuscle cells, actin filaments are less organized and myosin is much less prominent. Actin filaments are made up of identical actin proteins arranged in a long spiral chain. Like microtubules, actin filaments have plus and minus ends, with more ATP-powered growth occurring at a filament's plus end Figure 2.

In many types of cells, networks of actin filaments are found beneath the cell cortex , which is the meshwork of membrane-associated proteins that supports and strengthens the plasma membrane.

Such networks allow cells to hold — and move — specialized shapes, such as the brush border of microvilli. Actin filaments are also involved in cytokinesis and cell movement Figure 3. Figure 3: Actin filaments support a variety of structures in a cell. The microvilli, which resemble the teeth on a comb, are shown at the top of the cell, and are filled with white lines, which represent the actin filaments.

A second cell that has triangular protrusions in its membrane shows cytoplasmic contractile bundles. The actin filaments lie across the cell and end in the points at the edges of the cell. Lamellipodia, which are sheet-like projections, and filipodia, which are thin, finger-like projections, are shown in a third cell.

The cell has six projections, and inside each projection are actin filaments that run parallel to the projection. The cell division contractile ring is shown in a fourth cell that is undergoing cytokinesis. The contractile ring is lined with actin filaments and is the site where the cell is pinching together in the middle to form two new cells.

Intermediate filaments come in several types, but they are generally strong and ropelike. Their functions are primarily mechanical and, as a class, intermediate filaments are less dynamic than actin filaments or microtubules. Intermediate filaments commonly work in tandem with microtubules, providing strength and support for the fragile tubulin structures. All cells have intermediate filaments, but the protein subunits of these structures vary.

Some cells have multiple types of intermediate filaments, and some intermediate filaments are associated with specific cell types. For example, neurofilaments are found specifically in neurons most prominently in the long axons of these cells , desmin filaments are found specifically in muscle cells, and keratins are found specifically in epithelial cells.

Other intermediate filaments are distributed more widely. For example, vimentin filaments are found in a broad range of cell types and frequently colocalize with microtubules. Similarly, lamins are found in all cell types, where they form a meshwork that reinforces the inside of the nuclear membrane. Note that intermediate filaments are not polar in the way that actin or tubulin are Figure 4.

Figure 4: The structure of intermediate filaments Intermediate filaments are composed of smaller strands in the shape of rods. Eight rods are aligned in a staggered array with another eight rods, and these components all twist together to form the rope-like conformation of an intermediate filament. Cytoskeletal filaments provide the basis for cell movement. For instance, cilia and eukaryotic flagella move as a result of microtubules sliding along each other.

In fact, cross sections of these tail-like cellular extensions show organized arrays of microtubules. Other cell movements, such as the pinching off of the cell membrane in the final step of cell division also known as cytokinesis are produced by the contractile capacity of actin filament networks.

Actin filaments are extremely dynamic and can rapidly form and disassemble. In fact, this dynamic action underlies the crawling behavior of cells such as amoebae. At the leading edge of a moving cell, actin filaments are rapidly polymerizing; at its rear edge, they are quickly depolymerizing Figure 5. A large number of other proteins participate in actin assembly and disassembly as well. Figure 5: Cell migration is dependent on different actin filament structures.

Figure 7. Examples of multilocalizing proteins in the actin filament and focal adhesion proteome. The first two examples show common or overrepresented combinations for multilocalizing proteins in the actin filament and focal adhesion proteome while the last shows an example of the underrepresented overlap between this proteome and vesicles.

LIMA1 is another member of the LIM family of proteins and can be found at the actin filaments, focal adhesion sites, plasma membrane and cytoplasm.

It inhibits actin filament depolymerization and stabilizes filaments via crosslinking of filament bundles shown in U-2 OS cells. Transcriptome analysis and classification of genes into tissue distribution categories Figure 8 shows that genes encoding proteins that localize to actin filaments and focal adhesion sites are more likely to be expressed in many tissues, but less likely to be detected in all tissues, compared to all genes presented in the Cell Atlas.

Thus, these genes tend to show a somewhat more restricted pattern of tissue expression. Figure 8. Bar plot showing the percentage of genes in different tissue distribution categories for actin filaments-associated protein-coding genes compared to all genes in the Cell Atlas. Parikh K et al. Cell Res. Nat Methods. J Proteomics. J Proteome Res. Alberts B et al, Molecular Biology of the Cell. New York: Garland Science. Actin Filament Assembly.

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Reliability Supported Approved. Validation Supported Approved Uncertain. Annotation Intracellular and membrane Secreted - unknown location Secreted in brain Secreted in female reproductive system Secreted in male reproductive system Secreted in other tissues Secreted to blood Secreted to digestive system Secreted to extracellular matrix. Searches Enhanced Supported Approved Uncertain Intensity variation Spatial variation Cell cycle intensity correlation Cell cycle spatial correlation Cell cycle biologically Custom data cell cycle dependant Cell cycle dependent protein Cell cycle independent protein Cell cycle dependent transcript Cell cycle independent transcript Multilocalizing Localizing 1 Localizing 2 Localizing 3 Localizing 4 Localizing 5 Localizing 6 Main location Additional location.

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