The main difference between actin filaments, intermediate filaments and microtubules is that actin filaments determine the shape of the cell and are necessary for locomotion; meanwhile, intermediate filaments provide the mechanical strength and resistance to shear stress, whereas microtubules determine the positions of the intracellular organelles and direct intracellular transport.
Actin filaments, intermediate filaments, and microtubules are the three types of cytoskeletal filaments. They are common to many eukaryotic cells while providing fundamental special arrangements of the cells.
Key Areas Covered
1. What are Actin Filaments
– Definition, Structure, Function
2. What are Intermediate Filaments
– Definition, Structure, Function
2. What are Microtubules
– Definition, Structure, Function
3. Similarities Between Actin Filaments Intermediate Filaments and Microtubules
– Outline of Common Features
4. Difference Between Actin Filaments Intermediate Filaments and Microtubules
– Comparison of Key Differences
Key Terms
Actin Filaments, Intermediate Filaments, Microtubules
What are Actin Filaments
The actin filaments are double-stranded helical polymers of the protein actin. The diameter of the actin filament is 5-9 nm. Also, actin filaments can arrange into linear bundles, 2D networks, and 3D gels. However, actin filaments are dispersed throughout the cell, and they are concentrated in the cortex, just beneath the plasma membrane. On the other hand, the main function of the actin filaments is to maintain the shape of the cell. Also, it is important in cell locomotion. In addition, actin is the most abundant protein in the eukaryotic cell.
Figure 1: Actin Filaments in Cardiac Sarcomere
Furthermore, actin filaments are also known as microfilaments, and they are one of the three components of the cytoskeleton. Apart from that, actin involves in many cellular processes, including muscle contraction, cell division and cytokinesis, cell motility, vesicle and organelle movement, establishment and maintenance of cell shape and cell junctions, and cell signaling.
What are Intermediate Filaments
The intermediate filaments are ropelike fibers of a diameter of around 10 nm. Proteins that make up intermediate filaments are large and heterogeneous. They form a meshwork called nuclear laminar that occurs just beneath the inner nuclear membrane. The main function of the intermediate filaments is to provide mechanical strength to the cell. In the epithelial tissue, intermediate filaments span the cytoplasm from one cell to another through cell junctions.
Figure 2: Intermediate Filament Structure
Moreover, intermediate filaments are a structural component of the cytoskeleton. Also, there are six types of intermediate filaments. Type I and II filaments associate to make keratin proteins. Type III filaments occur in sarcomeres, astrocytes, peripheral neurons, and fibroblasts. Also, type IV filaments occur in axons of nerves in high concentrations. In addition, type V filaments occur in nuclear lamins, while type VI filaments occur in vertebrates.
What are Microtubules
Microtubules are long, hollow, cylindrical protein tubules with a diameter of 25 nm. Therefore, they are more rigid than the actin filaments. Apart from that, they are long and straight. Typically, each microtubule is attached to a centrosome, a single microtubule organizing center.
Figure 3: Microtubule Structure
On the other hand, the main function of microtubules is to determine the position of organelles and direct intracellular transport. Therefore, it is a type of cytoskeletal protein. The alpha and the beta-tubulin are the main structural components of the microtubules.
Similarities Between Actin Filaments Intermediate Filaments and Microtubules
- Actin filaments, intermediate filaments, and microtubules are the three types of cytoskeletal filaments.
- They occur in eukaryotic cells.
- They are important for the fundamental special arrangement of the cells.
- Accessory proteins determine the assembly of cytoskeletal proteins in a particular location.
- Motor proteins are responsible for the movement of filaments and organelles.
- They form helical assemblies of subunits.
- They are held together by weak, non-covalent interactions; their assembly and disassembly occur rapidly.
Difference Between Actin Filaments Intermediate Filaments and Microtubules
Definition
Actin filaments refer to linear polymers of globular actin (G-actin) subunits and occur as microfilaments in the cytoskeleton and as thin filaments, which are part of the contractile apparatus, in muscle and non-muscle cells. Meanwhile, intermediate filaments refer to a primary component of the cytoskeleton, although they are not found in all eukaryotes and are absent in fungi and plants, while microtubules refer to the narrow, hollow tube-like structure found in the cytoplasm (the fluid inside a cell) of plant and animal cells.
Made up of
Actin filaments are made up of compact and globular actin subunits; intermediate filaments are made up of elongated and fibrous subunits, while microtubules are made up of compact and globular tubulin subunits.
Structure
Actin filaments are double-stranded helical polymers of the protein actin; intermediate filaments are rope-like fibers with a diameter of around 10 nm, while microtubules are long, hollow cylinders with a diameter of 25 nm.
Function
Actin filaments determine the shape of the cell and are necessary for locomotion; intermediate filaments provide mechanical strength and resistance to shear stress, while microtubules determine the positions of the intracellular organelles and direct intracellular transport.
Conclusion
In brief, actin filaments, intermediate filaments, and microtubules are the three types of cytoskeletal proteins that occur in eukaryotic cells. They are made up of small subunits attached to non-covalent interactions. Actin filaments are made up of actin subunits that are compact and globular. Also, actin filaments are double-stranded helical polymers of proteins. The main function of actin filaments is to determine the shape of the cell. Also, they are necessary for the locomotion of the cell.
On the other hand, intermediate filaments are made up of elongated, fibrous subunits. However, they are ropelike fibers with a diameter of 10 nm. In addition, the function of intermediate filaments is to provide mechanical strength while providing resistance to shear stress. In comparison, microtubules are made up of compact and globular tubulin subunits. They are long, hollow, cylindrical filaments that determine the positions of intracellular organelles. Also, they direct intracellular transport. Therefore, the main difference between actin filaments, intermediate filaments, and microtubules is their structure and function.
References:
- Alberts B, Johnson A, Lewis J, et al. Molecular Biology of the Cell. 4th edition. New York: Garland Science; 2002. The Self-Assembly and Dynamic Structure of Cytoskeletal Filaments.
Image Courtesy:
- “Cardiac sarcomere structure” By. Mohamed Elshennawy – Own work (CC-BY SA 4.0) via Commons Wikimedia
- “Intermediate filament” By Mikael Häggström – Own Work (Public Domain) via Commons Wikimedia
- “Tubulin Infographic” By PKS615 – Own Work (CC-BY SA 4.0) via Commons Wikimedia
Leave a Reply