The following points highlight the three main classes of protein fibers in cytoskeleton. The classes are: 1. Microtubules 2. Actin Filaments 3. Intermediate Filaments.
Protein Fibers in Cytoskeleton: Class # 1. Microtubules:
Microtubules are long thread-like structures measuring about 25 nm in diameter; their length varies up to several millimeters.
Microtubules have the following two main properties:
(i) Long and rigid shape, and
(ii) Capacity to generate movement.
Cytoplasmic tubules are similar to microtubules which form the backbone of centrioles, cilia, flagella and mitotic spindle.
The microtubules contain a hollow core of 15 nm diameter; their outside diameter is 25 nm. The wall of a microtubule contains 13 filaments which are made up of the protein tubulin. There are two types of tubulins: α-tubulin and β-tubulin. The tubulins make a “dimer” of about 8 nm length and 110,000 Daltons M.W.
The tubulin dimers are arranged length-wise to produce proto-filament. A total of 13 protofilaments construct a microtubule. These protofilaments are arranged in a helical way with respect to the tubulin dimers (Fig. 2.25).
The regions from which the microtubules extend are called microtubule organization centres which are of different types, such as, basal bodies, kinetochores and centrosome. Polymerization of tubulin into microtubule requires the accessory proteins called MAP-1 and MAP-2 (microtubule associated proteins). These proteins (M.W. 300,000 Daltons) are related with the assembly and disassembly of the microtubules.
Protein Fibers in Cytoskeleton: Class # 2. Actin Filaments:
Actin filaments are composed of proteins and are related to the thin filaments of muscles. The monomeric protein is 43,000 Daltons in molecular weight. These actin molecules get polymerized into long filaments. Two such filaments are twisted around each other in a helical manner.
These filaments occur like individual organized filaments, regularly cross-linked filament bundles and less regularly cross-linked filaments. These filaments exert force to move a cell on surface or to change the shape of the cell internally. They interact with ‘myosin’ to generate the force.
Protein Fibers in Cytoskeleton: Class # 3. Intermediate Filaments:
Intermediate filaments are made up of proteins. They are of the following five types; each type of filament is found in a particular type of cell:
(i) Neuro-filament, found in neuronal cell.
(ii) Keratin, found in epithelial or skin cells.
(iii) Vimentin, found in mesenchymal cells.
(iv) Desmin, found in myogenic (muscle) cells,
(v) GFAP, found in astroglial (brain) cells.
Each filament has a central region of more than 300 amino acids. It forms a rod with a- helical organisation. The N-terminal and C-terminal regions differ in the particular type of microfilament. Most of these filaments provide rigidity to cell shape.