Module 1: CELL STRUCTURE AND FUNCTION

Lecture 4: Cytoskeletal Elements and Architecture

c. Microtubules

Microtubules were fisrt of all observed in the axoplasm of the myelinated nerve fibres by Robertis and Franchi (1953). In the plant cells they were first described in detail by Ledbetter and Porter (1963). A microtubule is a polymer of globular tubulin subunits, which are arranged in a cylindrical tube measuring 24 nm in diameter which is more than twice the width of an intermediate filament and three times the width of a microfilament (Figure 6). Microtubules are also much stiffer than either microfilaments or intermediate filaments because of their tubelike construction. The building block of a microtubule is the tubulin subunit, a heterodimer of α- and β-tubulin. Both of these 55,000-MW monomers are found in all eukaryotes, and their sequences are highly conserved. Although a third tubulin, γ-tubulin, is not part of the tubulin subunit, it probably nucleates the polymerization of subunits to form αβ-microtubules. The interactions holding α-tubulin and β-tubulin in a heterodimeric complex and are strong enough ensuring rare dissociation of a tubulin subunit under normal conditions. Each tubulin subunit binds two molecules of GTP. One GTP-binding site is located in α-tubuli and  binds GTP irreversibly and does not hydrolyze it, whereas the second site, located on β-tubulin, binds GTP reversibly and hydrolyzes it to GDP.

In a microtubule, lateral and longitudinal interactions between the tubulin subunits are responsible for maintaining the tubular form. Longitudinal contacts between the ends of adjacent subunits link the subunits head to tail into a linear protofilament. Within each protofilament, the dimeric subunits repeat every 8 nm. Polarity of microtubule arises from the head-to-tail arrangement of the α- and β-tubulin dimers in a protofilament. Because all protofilaments in a microtubule have the same orientation, one end of a microtubule is ringed by α-tubulin, while the opposite end is ringed by β-tubulin. Microtubule-assembly experiments discussed later show that microtubules, like actin microfilaments, have a (+) and a (−) end, which differ in their rates of assembly.

Figure 6: In cross section, a typical microtubule, a singlet, is a simple tube built from 13 protofilaments. In a doublet microtubule, an additional set of 10 protofilaments forms a second tubule (B) by fusing to the wall of a singlet (A) microtubule. Attachment of another 10 protofilaments to the B tubule of a doublet microtubule creates a C tubule and a triplet structure.

Every microtubule in a cell is a simple tube or a singlet microtubule, built from 13 protofilaments. In addition to the simple singlet structure, doublet or triplet microtubules are found in specialized structures such as cilia and flagella (doublet microtubules) and centrioles and basal bodies (triplet microtubules). Each of these contains one complete 13-protofilament microtubule (the A tubule) and one or two additional tubules (B and C) consisting of 10 protofilaments.

Functions

1. Mechanical function: The shape of the cell (red blood cells of non-mammalian vertebrates) and cells such as axons and dendrites of neurons, microvilli, etc., have been correlated to the orientation and distribution of microtubules.
2. Morphogenesis: During cell differentiation, the mechanical function of microtubules is used to
determine the shape of the developing cells. The enormous elongation in the nucleus of the spermatid during spermiogenesis is accompanied by the production of an orderly array of microtubules that are wrapped around the nucleus in a double helical arrangement. Similarly, the elongation of the cells during induction of the lens placode in the eye is also accompanied by the appearance of numerous microtubules.
3. Cellular polarity and motility: The determination of the intrinsic polarity of certain cells is governed by the microtubules. Directional gliding of cultured cells is depended on the microtubules.
4. Contraction: 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 macromol-ecules, granules and vesicles within the cell. The protozoan Actinosphaerium (Heliozoa) sends out long, thin pseudopodia within which cytoplasmic particles migrate back and forth. These pseudopodia contain as many as 500 microtubules disposed in a helical configuration.
6. The Microtubule Organizing Centre (MTOC) is the major organizing structure in a cell and helps determine the organization of microtubule-associated structures and organelles (e.g., mitochondria, the Golgi complex, and the endoplasmic reticulum). In a nonpolarized animal cell such as a fibroblast, an MTOC is perinuclear and strikingly at the center of the cell. Because microtubules assemble from the MTOC, microtubule polarity becomes fixed in a characteristic orientation. In most animal cells, for instance, the (−) ends of microtubules are closest to the MTOC or basal body (Figure 7). During mitosis, the centrosome duplicates and migrates to new positions flanking the nucleus. There the centrosome becomes the organizing center for microtubules forming the mitotic apparatus, which will separate the chromosomes into the daughter cells during mitosis.