The following points highlight the top three filamentous structures of eukaryotic cell. The filamentous structures are: 1. Actin Filaments 2. Microtubules 3. Intermediate Filaments.
Filamentous Structure # 1. Actin Filaments:
a. The G-actin protein of non-muscle cells has a molecular weight of about 43,000 and contains N-methylhistidyl residues.
b. This actin spontaneously polymerizes in the presence of magnesium and potassium chloride to form the double helical F-actin filaments like those in muscle.
c. There are two types of actin in non-muscle cells: β-actin and γ-actin. Both types can coexist in the same cell and even copolymerize in the same filament.
d. Actin forms microfilaments of 7-9.5 nm in the cellular cytoplasm. The bundles of filaments are prominent just underlying the plasma membrane of resting cells and are there referred to as stress fibres. These stress fibers disappear as cell motility increases.
e. Actin microfilaments are associated with other muscle-like proteins in non-muscle cells. α-Actinin is present at the plasma membrane sites to which microfilaments are attached.
f. Myosin is found in association with actin microfilaments at the bases of microvilli.
g. Tropomyosin promotes the formation of bundles of actin stress fibres. α-Actinin promotes the attachment of actin microfilaments to membranes, substratum and other cell organelles.
Filamentous Structure # 2. Microtubules:
a. Microtubules are the integral component of the cellular cytoskeleton. They consist of cytoplasmic tubes 25 nm in diameter and of indefinite length. They are also necessary for the formation and function of the mitotic spindle.
b. They perform a number of other cellular functions and are responsible for the intracellular movement of endocytotic and exocytotic vesicles.
c. They form the major structural component of cilia and flagella.
d. They are major protein components of axons and dendrites, where they maintain the structure and participate in the axoplasmic flow of material along this neuronal processes.
e. They are the cylinders of 13 longitudinally arranged protofi laments, each consisting of dimers of a-tubulin and P- tubulin which are closely related protein molecules.
f. Many important alkaloids can present microtubule assembly.
g. The movement of chromosomes during anaphase of mitosis is dependent upon microtubules.
Filamentous Structure # 3. Intermediate Filaments:
a. They are distinct from microfilaments and microtubules. There are five major classes of these filaments.
b. Each filament consists of distinct subunits —biochemically and immunologically.