Cytoskeleton: Microfilament, Intermediate Filament, and Microtubule

The cytoskeleton is a network of fibers that extends throughout the cytoplasm and aids the structures and activities of a cell. It is responsible for maintaining the shape of a cell, anchoring organelles and vesicles, assisting intracellular transport, and manipulating the plasma membrane to form food vacuoles and phagocytic vesicles. In this article, we will differentiate the three kinds of cytoskeletons- microtubule, microfilament, and intermediate filament- and make a clear comparison between them.

Microtubule

Microtubules are hollow fibers composed of a single type of globular (round) protein, called tubulin. Tubulin is a dimer formed by two closely related polypeptides, α-tubulin and β-tubulin, and it polymerizes (connects together) to form microtubules.

Microtubule is a polar structure, which is important because polarity gives molecules directionality, and microtubule uses this property to direct its movements as it rapidly assembles and disassembles to a certain direction in the cell. The specific mechanism regarding microtubule’s extension and shrinkage is rather complicated, however, and it will be mentioned in another article (Microtubule and Dynamic Instability).

Microtubule plays a very important role in many cellular processes. It forms the structural support of a cell with microfilament and intermediate filament. It also makes up the internal structure of cilia and flagella, in a “9+2” arrangement [click to understand more]. It provides a platform for intracellular transport and is also involved in the formation of spindles and the separation of eukaryotic chromosomes during cell division (mitosis and meiosis).

Microfilament

Microfilaments are thin solid rods composed of globular proteins called actin. Actin subunits form a twisted double chain that results in the shape of a microfilament. Microfilaments, like microtubules, are polar molecules.

Microfilament networks are found just inside the plasma membrane (cortical microfilament) of cells, stabilizing the outer cytoplasmic layer (cortex) of the cell. This is also the reason why cells can form pseudopodia or conduct phagocytic activities. Microfilaments also interact with motor proteins called myosin that bring about the contraction of muscle cells.

Intermediate filament

Intermediate filaments are composed of a variety of proteins, differing from the single-polymer microtubule and microfilament. The type of protein that polymerizes into intermediate filament depends on different cell types, but they share a common structural organization (meaning that they are constructed based on the same principle). Intermediate filaments only provide structural support in a cell.

Intermediate filaments are apolar, meaning that they do not have distinct plus and minus ends like microtubules and microfilaments do. Thus, they are assembled end to end (sort of like DNAs) and both ends are equivalent.

Here are some examples of intermediate filaments to give you a better picture:

• Types I and II intermediate filaments consist of two groups of keratins that are responsible for the production of hair, nails, and horns.
• Type IV intermediate filament proteins include three neurofilament (NF) proteins that support the structures of long, thin axons.
•  Type V intermediate filament proteins are the nuclear lamins, which are components of the nuclear envelope, holding the nucleus in place.

Structural support

Microtubules, along with intermediate filament and microfilament, provide a cell’s structural support. Microtubules, being hollow tubes, act as girders (橫樑) that resist compression and maintain a cell’s dome shape. Microfilaments, which are solid rods, bear tensions exerted on the cell (hold the cell’s shape so the cell would not stretch when being pulled by a force). It is also responsible for the change in cell shape (phagocytosis, etc.). The job of intermediate filaments is to reinforce the shape of a cell and fix the position of certain organelles. Unlike microtubule and microfilament, which assembles and disassembles, the intermediate filament is often fixed in position, so it secures the structure of the cell and keeps organelles such as the nucleus in place.

An overview of the organization of microtubules and microfilaments in an animal cell. Intermediate filaments are not indicated, but keep in mind that they provide structural support for the cell, so their organization should be close to that of microtubules.

The general building of microtubules and microfilaments. Intermediate filaments are not shown, because they varied in composition. However, their structures as solid rods are similar to that of micofilaments. 

Reference:

Campbell, et al. Biology: A Global Approach. 11th ed., Pearson, 2017.

Cooper GM. The Cell: A Molecular Approach. 2nd ed., 2000.

Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th ed., W. H. Freeman, 2000.