Microfilaments

What are Microfilaments ?

Fibers found in the cytoplasm of cells support cell shape and mobility as well as likely serve as anchoring sites for other cellular structures. These fibers are referred to as the cytoskeleton when grouped. Microfilaments, intermediate filaments, and microtubules are the 3 kinds of fibers that make up the cytoskeleton, with microfilaments being the smallest. Actin, the utmost prevalent protein in the majority of animal cells, is a component of microfilaments also called actin filaments. Actin protein subunits that make up the microfilaments are tiny (7 nm) molecules that polymerize to create elongated actin filaments which are part of the cytoskeleton. The plasma membrane is covered by these long, slender cylindrical filaments, which are present in eukaryotic plant and animal cells and are essential for muscle contraction, cell division, and cellular mobility.

Microfilament Structure

• Actin proteins are used to make solid rods known as microfilaments. They are the smallest filaments, with a typical diameter of 7 nm.

• Actin is initially generated by the cell in a globular form (G-actin).

• A microfilament is made up of individual subunits of actin that are all linked in the same direction.

• However, in microfilaments, they take the shape of lengthy series of molecules polymerized and are twisted around into a spiral to produce a cylinder-like structure, i.e. F-actin.

• Actin filaments have an extending + end to which more actin molecules are added, as well as a dormant minus end. It is known that microfilaments go through a process known as "treadmilling," in which monomers are constantly added to the + end and removed from the -ve end while maintaining the filament's gross length.

• Because of this, each microfilament displays polarity, with the two ends of the filament being distinguishable from one another.

• The growth rate of microfilaments is impacted by their polarity; typically, the plus end assembles and disassembles more quickly than the minus end.

• Although these polymers are hard, their framework is flexible. The surface of the cell is shaped and moved by microfilaments.

• Microfilaments are frequently nucleated there at plasma membranes. Therefore, the concentration of microfilaments is often the largest at the border (edges) of a cell.

• Microfilament structures are controlled by a group of actin-binding proteins particularly the cross-linking proteins which control filament orientation and spacing.

• Numerous other kinds of actin-binding proteins, namely motor, branching, capping, and severing proteins additionally the polymerization promoters, also control these structures.

Microfilament Formation/Self-assembly

A microfilament starts to form when 3 G-actin proteins combine to develop into a trimer. Then, more actin bonds to the barb-like end. Autoclampin proteins assist in the self-assembly process by acting as motors to help build the long strands that make up microfilaments. Actin polymerizes into two long strands that are arranged in a spiral to generate microfilaments.

Functions of Microfilaments

• Microfilaments make up the active part of the cytoskeleton.

• Its primary function is to give the cell support and structure.

• Helps in cytoplasmic streaming (cyclosis). A cell organelle may become attached to a microfilament, which may then contract and drag the organelle to a different location within the cell during cyclosis.

• Microfilaments are flexible and work in conjunction with myosin to provide the force necessary for cellular contraction and the fundamental movements of cells. Myosin functions as a motor to cause one end of a microfilament to elongate while the other end must shorten for cells to move.

• Microfilaments help spindle fibers and cleavage furrows develop during cytokinesis.

• The eukaryotic cells can withstand environmental stress because of the integrity of the actin filaments.

• In some animals, the microfilaments are also crucial for amoeboid motility.

• Organelles are held in place within cells by microfilaments and give cells their form and stiffness.

Location of Microfilaments

• The microfilaments are bundled and form an intracellular three-dimensional (3D) web-like structure.

• With other intracellular proteins including myosin, lamin, and spectrin, there is significant intracellular attachment and inter-linking.

• The filaments are primarily seen at the outermost position of the cell, where they connect to the plasma membrane and develop into microvilli.

• Typically, at the plasma membrane, the microfilaments are highly nucleated.

• Therefore, the concentration of microfilaments is often largest at the exterior (corners) of a cell.

• Microfilaments can occasionally be seen floating freely and joined to other filaments and tubules.

Conclusion

The rigid microfilaments, which have a width of 7 nm, are the thinnest and are primarily constructed of two tangled strands of the globular protein actin. Microfilaments are also referred to as actin filaments because of the actin proteins which make up microfilaments. The microfilaments are found in bundles, creating an intracellular meshwork that is three-dimensional (3D). They are essential for muscle contraction, cell division, and cellular mobility. The surface of the cell is shaped and moved by microfilaments.

FAQs

Q1. Describe actomyosin.

Ans. Actin, myosin, and related proteins make up the protein complex known as actomyosin, which is found in skeletal muscle. It is present in muscle fibers and shortens in response to stimulation, which results in muscular contraction.

Q2. What exactly is "cytoplasmic streaming"?

Ans. In different organisms such as bacteria, higher plants, and mammals, cyclosis, also known as cytoplasmic streaming, is characterized by the fast motion of organelles and other cellular elements across the cell. Cytoplasmic streaming aids in the distribution of supplies to all cell parts, including organelles, metabolites, and nutrients.

Q3. Relationship between microvilli and actin filaments.

Ans. Microfilaments/actin filaments can extrude the plasma membrane, resulting in the formation of structures that protrude from the cell body, such as flat, broad lamellipodia, or finger-like filopodia and microvilli. Each microvillus contains a dense bundle of cross-linked actin filaments as its structural core.

Q4. Difference between microfilament and microtubule?

Ans. Microtubules have a larger diameter than microfilaments. Tubulin proteins polymerize to produce microtubules in cells. They aid in intracellular trafficking and give the cell mechanical support. Actin protein monomers are polymerized to create microfilaments. They assist in the mobility of the cell on a surface. The major distinction between microfilaments and microtubules is that the former are double-stranded helical polymers formed of actin proteins, while the latter are long, hollow cylinders built of tubulin protein units.

Q5. How does microfilament aid in cell division?

Ans. Microfilaments play a crucial role in mitosis by assisting in cell division. The cytokinesis process, in which the cell physically divides into two daughter cells, is made easier by microfilaments. A ring of actin surrounds the cell that is separating during cytokinesis, and myosin proteins tug on the actin, causing it to contract. Up until it splits into two cells, the ring around the cell gets progressively smaller while dragging the cell membrane along with it. When the ring is no longer required, the microfilaments depolymerize, or split into actin molecules, as a result of which the ring disassembles.