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.