Microtubules (Structure, Chemical composition, Function)
What are the microtubules? Describe their structure, chemical composition?
In this article we will discuss about the Microtubules:-(1) Definition, (2) structure, (3) Chemical composition, (4) Function
- Chemical Composition
All eukaryotic cells possess fine, cylindrical fibrils called microtubules. The hollow, central core of microtubule is about 150 Aº in diameter and the thickness of its wall varies from 50 A to 60 Aº. Chemically, these are made up of a globular protein called tubulin. They are rigid and stable as seen in the cilia, flagella and in the cytoplasm or they may be labile as seen in the pseudopodia of Amoeba. Microtubules are essential components of centrioles, basal bodies, mitotic spindle, cilia and flagella. However, their number, arrangement and activity shows considerable variation.
Microtubules are found in all eukaryotic cells, either free in the cytoplasm or forming part of centrioles, cilia and flagella. The most abundant source of microtubules for the biochemical studies is vertebrate brain-high densities of microtubules exist in axons and dendrites of nerve cells. In the cytoplasm of animal and plant cells, microtubules occur at following seven sites :- 1. cilia and flagella, 2. centrioles and basal bodies, 3. nerve processes, 4. the mitotic apparatus, 5. the cortex of meristematic plant cells, 6. elongating cells such as during the formation of the lens or during spermatogenesis of certain insects. 7. selected structures in Protozoa such as the axostyle of parasitic flagellates, the axoneme of Echinosphaerium, the fibre systems of Stentor, and the cytopharyngeal basket of Nassula.The stability of different microtubules varies. Cytoplasmic and spindle microtubules are rather labile structures, whereas, those of cilia and flagella are more resistant to various treatments.
Microtubules constitute a class of morphologically and chemically related filamentous rods which are common to both plant and animal cells. A microtubule consists of a long, unbranched, hollow tubules 24-25 nm in diameter, several micrometers long and with 6 nm thick wall having 13 subunits or protofilaments. Thus, the wall of the microtubule consists of 13 individual linear or spiralling filamentous structures about 5 nm in diameter, which in turn, are composed of tubulin. These protofilaments have a centre-to-centre spacing of 4.5 nm. Application of negative staining techniques has shown that microtubules have a lumen 14 nm wide and a protofilament or subunit structure in the wall.
Biochemically, a protofilament of microtubule is made of a protein called tubulin. Tubulin is an acidic protein with a molecular weight of 55,000 and a sedimentation constant of 6S. It occurs in two different forms, called alfa-tubulin and ß-tubulin, each containing about 450 amino acids. Both of these proteins have a distinct, though closely related, amino acid sequences and are thought to have evolved from a single ancestral protein. The two proteins show very little divergence from the lowest to the highest eukaryotes; for example, the beta-tubulins of sea urchin flagella and chick brain cells differ only in one amino acid. Similarities such as this suggest that most mutations disrupt the functions of microtubules and are thus lethal and are eliminated by selection.
Microtubules have several functions in the eukaryotic cells such as follows:
1. Mechanical function:
The shape of the cell (e.g., red blood cells of non-mammalian vertebrates) and some cell processes or protuberances such as axons and den drites of neurons, microvilli, etc., have been correlated to the orientation and distribution of microtubules.
During cell differentiation, the mechanical function of microtubules is used to determine the shape of the develop ing cells. For example, the enormous elongation in the nucleus of the spermatid during spermiogenesis is accompanied by the produc tion of an orderly array of microtubules that are wrapped around the nucleus in a double helical arrangements.The elongation of the cells during induction of the lens placode in the eye is also accompanied by the appearance of numerous mi crotubules.
3. Cellular polarity and motility:
The determination of the intrinsic polarity of certain cells is also related to the microtubules. Directional gliding of cultured cells is found to depend on the microtubules.
Microtubules play a role in the contraction of the spindle and movement of chromosomes and centrioles as well as in ciliary and flagellar motion.
5. Circulation and transport:
Microtubules are involved in the transport of macromolecules , granules and vesicles within the cell.