Individual filament structure and function

INTRODUCTION
Cytoskeleton is an intricate network of protein filaments that extends throughout the cytoplasm. It helps support the large volume of cytoplasm in a eukaryotic cell. It can be said that the cytoskeleton is not only the bones of a cell but also the muscles. The cytoskeleton takes credit for cell shape, motility (movement) of the cell as a whole, and motility of organelles within the cell. The cytoskeleton is composed by three types of protein filaments which are :


 * Intermediate filaments
 * Microtubules
 * Actin filaments

Each of these are formed from a different protein subunit and have distinct mechanical properties. You can see further in the article the structure and function of these three filaments in more detail.

STRUCTURE
A family of fibrous proteins form the intermediate filaments. The intermediate filaments are ropelike fibres with a diameter of about 10nm. The strands of the rope are elongated fibrous protiens. Each of these elongated fibrous proteins are composed of an N-terminal globular head, a C-terminal globular tall and a central elongated rod domain. The rod domain has an extended alpha helical region that enables pairs of intermediate filament proteins to form stable dimers by wrapping around each other. The central rod domains of different intermediate filament protiens have similar amino acid sequences and sizes. This ensures the formation of of similar diameter and internal structure when packed together. The globular domains vary greatly in the amino acid sequence and size from one filament protien to another. The filaments are given the name immediate because their diameter 10nm is between that of the thin actin-containing filaments and the thicker myosin filaments of smooth muscle cells. Intermediate filaments are the toughest and most durable amongst the three cytoskeletal filaments. While cytoplasmic intermediate filaments form rope like structures the intermediate filaments lining the inside of the nuclear membrane are organised as a two-dimensional mesh. These type of intermediate filaments are composed from a class of intermediate filament protiens called lamins. Intermediate filaments can be classed into four groups. Each group is formed by polymerization of their corresponding protien subunits.

FUNCTION
Intermediate filaments have great tensile strenght and their main function is to withstand mechanical stress occuringg from the stretching of the cells. Due to this function intermediate filaments are normally more prominent in the cytoplasm of cells more prone to mechanical stress. One of the places they are present in large numbers are along the length of nerve cell axons, where they provide essential internal reinforcement. They are also largely present in muscle cells and in epithelial cells of the skin. Intermediate filaments keep these cells and their membranes from breaking in response to mechanical shear, by stretching and distributing the effect of locally apliedd forces.

Medical signifiance of the functions of intermediate filaments
The rare human genetic disease epidermolysis bullosa simplex illustrates the importance of the intermediate filaments. In this disease mutations in the keratin genes interfere with the formation of keratin filaments in the epidermis, due to which the skin is highly vulnerable to mechanical injury, and even a gentle pressure can rupture its cells, causing the skin to blister. Mutations in the gene plectin cause a devastating human disease that combines epidermolysis bullosa simplex (caused by disruption of skin keratin), muscular dystrophy (caused by disruption of intermediate filaments in muscle), and neurodegenaration (caused by disruption of neurofilaments).

STRUCTURE
Microtubules are long and relatively stiff hollow tubes of protien that can rapidly disassemble in one location and reassemble in another. They are built from molecules of tubulin, each one of whick is itself a dimer composed of two very similar globular proteins called alpha tubulin and beta tubulin, and the are bound tightly together by noncovalentt bonding forming the wall of the hollow cylindrical microtubule. This tubelike structure is made of 13 parallel protofilaments,eacg a linear chain of tubulin dimers with alpha and beta tubulin alternating along its length. Each protofilament has a structural polarity, with alpha and beta tubulin exposed at two different ends, adn this directional arrow embodied in the structree is the same for all protofilaments, giving a structural polarity to the microtubule as a whole. The beta tubulin end of the microtubule is called its plus end and the alpha tubulin end is called the minus end. A microtubule grows from an initial ring of 13 tubulin molecules. The tubulin dimers are added individually slowly building up the structure of the hollow tube. It is crucial for the structure of the microtubule to have a definite direction, with the two ends being chemically different and behaving differently. This is responsible for the assembly of microtubules and for their role once the microtubules are formed. If there was no polarity the microtubules could not serve their function in defining a direction for intracellular transport.