Microfilaments are fine, thread-like protein fibers, nm in diameter. They are composed predominantly of a contractile protein called actin, which is the most abundant cellular protein. Microfilaments' association with the protein myosin is responsible for muscle contraction. Microfilaments can also carry out cellular movements including gliding, contraction, and cytokinesis. Microtubules are cylindrical tubes, nm in diameter.
They are composed of subunits of the protein tubulin--these subunits are termed alpha and beta. They function in cellular movement, have a diameter of about 7 nm, and are made of two intertwined strands of a globular protein called actin. For this reason, microfilaments are also known as actin filaments. Microfilaments are the thinnest component of the cytoskeleton. Actin is powered by ATP to assemble its filamentous form, which serves as a track for the movement of a motor protein called myosin.
This enables actin to engage in cellular events requiring motion such as cell division in animal cells and cytoplasmic streaming, which is the circular movement of the cell cytoplasm in plant cells. Actin and myosin are plentiful in muscle cells. When your actin and myosin filaments slide past each other, your muscles contract. Microfilaments also provide some rigidity and shape to the cell. They can depolymerize disassemble and reform quickly, thus enabling a cell to change its shape and move.
They can move to the site of an infection and engulf the pathogen. Privacy Policy. Skip to main content. Organization at the Cellular Level. Search for:. The Cytoskeleton. Key Takeaways Key Points Microtubules help the cell resist compression, provide a track along which vesicles can move throughout the cell, and are the components of cilia and flagella. Cilia and flagella are hair-like structures that assist with locomotion in some cells, as well as line various structures to trap particles.
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. These protrusive structures contain actin filaments, with elongating barbed ends orientated toward the plasma membrane. B During cellular arm extension, the plasma membrane sticks to the surface at the leading edge. C Next, the nucleus and the cell body are pushed forward through intracellular contraction forces mediated by stress fibers.
D Then, retraction fibers pull the rear of the cell forward. Filopodia: molecular architecture and cellular functions. Nature Reviews Molecular Cell Biology 9, All rights reserved. Figure Detail. This page appears in the following eBook. Aa Aa Aa. Microtubules and Filaments. What Is the Cytoskeleton Made Of? The cytoskeleton of eukaryotic cells is made of filamentous proteins, and it provides mechanical support to the cell and its cytoplasmic constituents.
All cytoskeletons consist of three major classes of elements that differ in size and in protein composition. Microtubules are the largest type of filament, with a diameter of about 25 nanometers nm , and they are composed of a protein called tubulin.
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