Microfilaments- Definition, Structure, Functions and Diagram


Microfilaments are one of the three main types of protein filaments that make up the cytoskeleton, the network of protein structures that gives shape, support and movement to eukaryotic cells. The other two types are microtubules and intermediate filaments. Microfilaments are the thinnest and most flexible of the cytoskeletal filaments, with a diameter of about 6 to 7 nanometers.

Microfilaments are composed of two intertwined strands of a globular protein called actin. Each strand consists of many actin subunits, also known as globular actin (G-actin), that polymerize to form a long chain called filamentous actin (F-actin). The two strands of F-actin are twisted around each other in a helical orientation, creating a microfilament .

Microfilaments are polar structures, meaning they have a plus end and a minus end that differ in their rates of growth and shrinkage. The plus end is also called the barbed end, and the minus end is also called the pointed end. The plus end grows faster than the minus end because it has a higher affinity for G-actin subunits. The process of adding or removing subunits from the ends of microfilaments is called polymerization or depolymerization, respectively .

Microfilaments have various functions in cells, depending on their location, orientation and interactions with other proteins. Some of their main functions are:

  • Muscle contraction: Microfilaments work together with another protein called myosin to generate force and movement in muscle cells. Myosin is a motor protein that can bind to actin and slide along it, causing the microfilaments to shorten and pull on each other. This process is called actomyosin contraction and it is responsible for muscle contraction and relaxation .
  • Cell movement: Microfilaments also enable non-muscle cells to move by changing their shape and extending protrusions called pseudopodia (false feet), lamellipodia (flat extensions) or filopodia (thin extensions). These protrusions are driven by the polymerization and depolymerization of actin at the leading edge of the cell, creating pushing and pulling forces that propel the cell forward. This type of movement is called amoeboid movement and it is important for many biological processes such as wound healing, immune response and development .
  • Cell division: Microfilaments play a crucial role in cytokinesis, the final stage of cell division when the cytoplasm of a parent cell splits into two daughter cells. Microfilaments form a ring-like structure called the contractile ring at the equator of the cell, where they contract with the help of myosin and pinch off the cell membrane, creating a cleavage furrow that separates the two cells .
  • Cell shape and stability: Microfilaments provide mechanical support and rigidity to cells by resisting compression and bending forces. They also help maintain the shape and structure of cell surface projections such as microvilli, which are finger-like extensions that increase the surface area for absorption in some cells. Microfilaments are often found just beneath the plasma membrane, where they form a meshwork called the cell cortex that regulates the shape and movement of the cell surface .
  • Cytoplasmic streaming: Microfilaments can also facilitate the flow of cytoplasm within cells, which is called cytoplasmic streaming. This process allows for the transport and distribution of nutrients, organelles and other molecules within the cell. Cytoplasmic streaming is especially important for plant cells, which have large vacuoles that occupy most of their volume .

Microfilaments are dynamic structures that can rapidly assemble and disassemble in response to various signals and stimuli. They are regulated by many proteins that influence their nucleation, polymerization, depolymerization, branching, cross-linking, capping, severing and binding to other molecules. Some examples of these proteins are profilin, cofilin, Arp2/3 complex, formin, fimbrin, villin, gelsolin and tropomyosin . By interacting with these proteins, microfilaments can adapt to different cellular needs and functions.

In summary, microfilaments are thin protein filaments composed of actin subunits that form part of the cytoskeleton. They have various roles in cell structure, movement and division. They are highly dynamic and regulated by many proteins that modulate their behavior.