Main Difference – Microtubules vs Microfilaments
Microtubules and microfilaments are two components of the cytoskeleton of a cell. The cytoskeleton is formed by microtubules, microfilaments, and intermediate filaments. Microtubules are formed by the polymerization of tubulin proteins. They provide mechanical support to the cell and contribute to the intracellular transport. Microfilaments are formed by the polymerization of actin protein monomers. They contribute to the cell’s movement on a surface. The main difference between microtubules and microfilaments is that microtubules are long, hollow cylinders, made up of tubulin protein units whereas microfilaments are double-stranded helical polymers, made up of actin proteins.
1. What are Microtubules
– Structure, Function, Characteristics
2. What are Microfilaments
– Structure, Function, Characteristics
3. What is the difference between Microtubules and Microfilaments
What are Microtubules
Microtubules are polymers of tubulin protein found everywhere in the cytoplasm. Microtubules are one of the components of the cytoplasm. They are formed by the polymerization of the dimer alpha and beta tubulin. The polymer of tubulin can grow up to 50 micrometers in a highly dynamic nature. The outer diameter of the tube is around 24 nm, and the inner diameter is around 12 nm. Microtubules can be found in eukaryotes and bacteria.
Structure of Microtubules
Eukaryotic microtubules are long and hollow cylindrical structures. Inner space of the cylinder is referred to as the lumen. The monomer of the tubulin polymer is α/β-tubulin dimer. This dimer associates with their end-to-end to form a linear protofilament which is then laterally associated to form a single microtubule. Usually, around thirteen protofilaments are associated in a single microtubule. Thus, the amino acid level is 50% in each α and β – tubulins in the polymer. The molecular weight of the polymer is around 50 kDa. The microtubule polymer bears a polarity between two ends, one end contains a α-subunit, and the other end contains a β-subunit. Thus, the two ends are designated as (-) and (+) ends, respectively.
Figure 1: Structure of a Microtubule
Intracellular Organization of Microtubules
Organization of microtubules in a cell varies according to the cell type. In epithelial cells, (-) ends are organized along the apical-basal axis. This organization facilitates the transport of organelles, vesicles, and proteins along the apical-basal axis of the cell. In mesenchymal cell types like fibroblasts, microtubules anchor to the centrosome, radiating their (+) end to the cell periphery. This organization supports the fibroblast movements. Microtubules, along with the assistant of motor proteins, organize Golgi apparatus and the endoplasmic reticulum. A fibroblast cell, containing the microtubules is shown in figure 2.
Figure 2: Microtubules in a fibroblast cell
Microtubules are fluorescent labeled in green color and actin in red color.
Function of Microtubules
Microtubules contribute to form cytoskeleton, the structural network of the cell. The cytoskeleton provides the mechanical support, transport, motility, chromosomal segregation and the organization of the cytoplasm. Microtubules are capable of generating forces by contracting, and they allow cellular transport along with motor proteins. Microtubules and the actin filaments provide an inner framework to the cytoskeleton and enable it to change its shape while moving. Components of the eukaryotic cytoskeleton are shown in figure 3. Microtubules are stained with green color. Actin filaments are stained in red color and nuclei are stained in blue color.
Figure 3: Cytoskeleton
Microtubules involved in the chromosomal segregation during mitosis and meiosis, form the spindle apparatus. They are nucleated in the centromere, which is the microtubule organizing centers (MTOCs), in order to form the spindle apparatus. They are also organized in the basal bodies of cilia and flagella like internal structures.
Microtubules allow gene regulation through the specific expression of transcription factors, which maintain the differential expression of genes, with the aid of dynamic nature of microtubules.
Associated Proteins with Microtubules
Various dynamics of microtubules such as the rates of polymerization, depolymerization, and catastrophe are regulated by microtubule-associated proteins (MAPs). Tau proteins, MAP-1, MAP-2, MAP-3, MAP-4, katanin, and fidgeting are considered as MAPs. Plus-end tracking proteins (+TIPs) like CLIP170 are another class of MAPs. Microtubules are the substrates for the motor proteins, which are the last class of MAPs. Dynein, which moves towards the (-) end of the microtubule and kinesin, which moves towards the (+) end of the microtubule, are the two types of motor proteins found in cells. Motor proteins play a major role in cell division and vesicle trafficking. Motor proteins hydrolyze ATP in order to generate mechanical energy for the transportation.
What are Microfilaments
The filaments which are made up of actin filaments are known as microfilaments. Microfilaments are a component of the cytoskeleton. They are formed by the polymerization of actin protein monomers. A microfilament is around 7 nm in diameter and composed of two strands in a helical nature.
Structure of Microfilaments
The thinnest fibers in the cytoskeleton are microfilaments. The monomer, which forms the microfilament is called globular actin subunit (G-actin). One filament of the double-helix is called filamentous actin (F-actin). The polarity of the microfilaments is determined by the binding pattern of myosin S1 fragments in the actin filaments. Therefore, the pointed end is called the (-) end and the barbed end is called the (+) end. The structure of the microfilament is shown in figure 3.
Figure 3: A microfilament
Organization of Microfilaments
Three of the G-actin monomers are self-associated to form a trimer. Actin, which is ATP-bound, binds with the barbed end, hydrolyzing the ATP. The binding capacity of the actin with the neighboring subunits is reduced by autocatalyzed events until the former ATP is being hydrolyzed. Actin polymerization is catalyzed by actoclampins, a class of molecular motors. Actin microfilaments in cardiomyocytes are shown, stained with green color in figure 4. The blue color shows the nucleus.
Figure 4: Microfilaments in Cardiomyocytes
Function of Microfilaments
Microfilaments are involved in cytokinesis and cell motility like amoeboid movement. Generally, they play a role in cell shape, cell contractility, mechanical stability, exocytosis, and endocytosis. Microfilaments are strong and relatively flexible. They are resistant to fractures by tensile forces and buckling by multi-piconewton compressive forces. The motility of the cell is achieved by the elongation of one end and contraction of the other end. Microfilaments also act as the actomyosin- driven contractile molecular motors, along with the myosin II proteins.
Associated Proteins with Microfilaments
The formation of the actin filaments are regulated by the associated proteins with microtubules like,
- Actin monomer-binding proteins (thymosin beta-4 and profilin)
- Filament cross-linkers (fascin, fimbrin and alpha-actinin)
- Filament-nucleator or actin-related protein 2/3 (Arp2/3) complex
- Filament-severing proteins (gelsolin)
- Filament-end tracking protein (formins, N-WASP and VASP)
- Filament barbed-end cappers like CapG.
- Actin depolymerizing proteins (ADF/cofilin)
Difference Between Microtubules and Microfilaments
Structure
Microtubules: Microtubule is a helical lattice.
Microfilaments: Microfilament is a double-helix.
Diameter
Microtubules: Microtubule is 7 nm in diameter.
Microfilaments: Microfilament is 20-25 nm in diameter.
Composition
Microtubules: Microtubules are composed of alpha and beta subunits of protein tubulin.
Microfilaments: Microfilaments are predominantly composed of contractile protein called actin.
Strength
Microtubules: Microtubules are stiff and resist bending forces.
Microfilaments: Microfilaments are flexible and relatively strong. They resist buckling due to compressive forces and filament fracture by tensile forces.
Function
Microtubules: Microtubules help cell functions such as mitosis and various cell transport functions.
Microfilaments: Microfilaments help cells to move.
Associated Proteins
Microtubules: MAPs, +TIPs and motor proteins are the associated proteins regulating the dynamics of microtubules.
Microfilaments: Actin monomer-binding proteins, filament cross-linkers, actin-related protein 2/3 (Arp2/3) complex and filament-severing proteins are involved in the regulation of the dynamics of microfilaments.
Conclusion
Microtubules and microfilaments are two components in the cytoskeleton. The main difference between microtubules and microfilaments is in their structure and function. Microtubules have a long, hollow cylindrical structure. They are formed by the polymerization of tubulin proteins. The major role of microtubules is to provide mechanical support to the cell, involve in chromosomal segregation and maintain the transport of components inside the cell. On the other hand, microfilaments are helical structures, more strong and flexible compared to microtubules. They are involved in the movement of the cell on a surface. Both microtubules and microfilaments are dynamic structures. Their dynamic nature is regulated by associated proteins with the polymers.
Reference:
1. “Microtubule.” Wikipedia. Wikimedia Foundation, 14 Mar. 2017. Web. 14 Mar. 2017.
2. “Microfilament.” Wikipedia. Wikimedia Foundation, 08 Mar. 2017. Web. 14 Mar. 2017.
Image Courtesy:
1. “Microtubule structure” By Thomas Splettstoesser (www.scistyle.com) – Own work (rendered with Maxon Cinema 4D) (CC BY-SA 4.0) via Commons Wikimedia
2. “Fluorescent image fibroblast” By James J. Faust and David G. Capco – NIGMS Open Source Image and Video Gallery (Public Domain) via Commons Wikimedia
3. “Fluorescent Cells” By (Public Domain) via Commons Wikimedia
4. “Figure 04 05 02″By CNX OpenStax – (CC BY 4.0) via Commons Wikimedia
5. “File:F-actin filaments in cardiomyocytes” By Ps1415 – Own work (CC BY-SA 4.0) via Commons Wikimedia
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