- Microtubules are found in the cytoplasm of all types of eukaryotic cells with rare absence, such as in human erythrocytes.
- They are tiny, hollow, bead-like tubular structures that help cells maintain their shape.
- They are microscopic hollow tubes found inside cells that also provide motor functions for the cell.
Figure: Diagram of Microtubules
Structure of Microtubules
- They are long fibers (of indefinite length) about 24 nm in diameter.
- In cross-section, each microtubule appears to have a dense wall of 6 nm thickness and light or hollow center. In cross-section, the wall of a microtubule is made up of 13 globular subunits, called protofilaments, about 4 to 5 nm in diameter.
- Chemically, they are composed of two kinds of protein subunits: α-tubulin (tubulin A) and β-tubulin (tubulin B), each of M.W. 55,000 daltons.
- The wall of a microtubule is made up of a helical array of repeating α and β tubulin subunits.
- Assembly studies have indicated that the structural unit is an αβ dimer of 8 nm length.
- Thus, in each microtubule, there are 13 protofilaments, each composed of αβ dimers that run parallel to the long axis of the tubule. The repeating unit is an αβ heterodimer which is arranged ‘head to tail’ within the microtubule, that is αβ→ αβ→αβ.
- Thus, all microtubules have a defined polarity: their two ends are not structurally equivalent.
- Microtubules undergo reversible assembly- disassembly (i.e., polymerization– depolymerization), depending on the need of the cell or organelles.
- Their polymerization is regulated by certain microtubule-associated proteins (MAPs).
- The assembly of microtubules involves preferential addition of subunits (αβ dimers) to one end of the tubule, called A end (or net assembly end); the other end of the tubule is called D end (or net disassembly end). Such an assembly involves the hydrolysis of GTP to GDP. Thus, the assembly of tubulin in the formation of microtubules is a specifically oriented and programmed process.
- Centrioles, basal bodies, and centromeres of chromosomes are the sites of orientation for this assembly. Calcium and calmodulin (an acidic protein having four Ca2+ binding sites) are some other regulating factors in the in vivo polymerization of tubulin.
Microtubules may work alone or join with other proteins to form more complex structures. Cell organelles derived from special assemblies of microtubules include:
- Cilia and flagella.
- Basal bodies and Centrioles
Functions of Microtubules
- They move vesicles, granules, organelles like mitochondria, and chromosomes via special attachment proteins.
- Along with microfilaments and intermediate filaments, they form the cytoskeleton of the cell, as well as participate in a variety of motor functions for the cell.
As part of the cytoskeleton of the cell, microtubules contribute to:
- Giving shape to cells and cellular membranes.
- Cell movement, which includes a contraction in muscle cells and more.
- Transportation of specific organelles within the cell via microtubule “roadways” or “conveyor belts.”
- Mitosis and meiosis: movement of chromosomes during cell division and creation of the mitotic spindle.
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