Microfilaments (cytoskeleton) :
Define the term microfilaments? Describe the structure and function of microfilaments in the cell?
In this article we will discuss about microfilaments:-(1)Definition,(2) Distribution, (3)Chemical composition, (4)function.
Microfilaments are fine, contractile protenaceous filaments of about 7 u in diameter. These are chiefly composed of actin but small amounts of myosin, tropomyosin, troponin and actinin are also present in some cells. The actin and myosin filaments are the most important contractile proteins of muscle cells. The sliding of actin and myosin filaments results in muscle contraction.Microfilaments are located just under the surface of the cell membrane but they are more concentrated in those parts of cytoplasm involved in streaming movement.
Microfilaments are generally distributed in the cortical regions of the cell just beneath the plasma membrane. In contrast, intermediate filaments and microtubules are found in subcortical and deeper regions of the cell. Microfilaments also extend into cell processes, especially where there is move ment. Thus, they are found in the microvilli of the brush border of intestinal epithelium and in cell types where amoeboid movement and cytoplasmic streaming are prominent.
Actin is the main structural protein of microfilaments. The concentration of actin in non-muscle cells is surprisingly high; it may account up to 10 per cent of total cell protein. It can be extracted and in vitro settings will undergo polymerization reactions from G actin monomer state to F-actin. In fact, the globular (=Gactin) – fibrillar (=F-actin) transition is the basis of the classical sol-gel transition in the cytoplasm of moving cells. Further, there are glob present three types of actins -alfa, beta and gama. The alfa-form of actin is found in fully mature muscle tissue. The other two forms are more characteristic of non-muscle cells.
In non-muscle cells, microfilaments, being of actin composition, can bind myosin (a contractile protein). In vitro and in situ mircrofilaments can be coated or “decorated” with heavy myosin (HMM) or S¹ heads. This binding results in an arrow-head pattern to the microfilaments in which the arrowheads all point in the same direction . This pattern indicates that microfilaments possess a polarity, a property that is probably crucial to their role in mediating cell movements.
The HMM binding method has become a very useful method for identifying and localizing microfilaments in any type of cell. Intermediate filaments are not decorated by HMM.
- Microfilaments provide mechanical support to the cell surface.
- They are known to be associated with exocytosis and endocytosis.
- They are involved in motility as seen during muscle contraction and amoeboid movement.
- Microfilaments are found to be involved in movement with furrow formation in cell division, cytoplasmic streaming in plant cells (e.g., Nitella and Chora) and cell migration during embronic development.