Cell & Molecular Biology

Microtubules

• Microtubules are part of a structural network (the cytoskeleton) within the cell's cytoplasm.

• Microtubules are rigid hollow rods approx. 25 nm in diameter, composed of single type of globular protein called Tubulin.

• α and β-tubulin dimers polymerize end-to-end into 13 linear protofilaments that associate laterally to form a single microtubule.

• In a microtubule, there is one end, the (+) end, with only β-subunits exposed, while the other end, the (−) end, has only α-subunits exposed.

STRUCTURE

• Microtubules are organized by Microtubule Organizing Centers (MTOCs) such as the centrosome found in the center of many animal cells or the basal body found in cilia and flagella or the spindle pole bodies found in fungi.

• The γ tubulin combines with several other associated proteins to form a Lock-Washer like structure known as the γ Tubulin Ring Complex (γ TURC). This complex acts as a template for tubulin dimers to begin Polymerization ; it acts as a cap of the (-) end while microtubule growth continues away from the MTOC in the (+) direction.

DYNAMICS

• Both α and β tubulin bind GTP, which regulate polymerization.• The GTP bound to β tubulin is hydrolyzed to GDP during polymerization which weakens

the binding affinity of tubulin for adjacent molecules, thereby favouring depolymerisation.• Tubulin molecules bound to GDP are continually lost from the (–)end and replaced by the

addition of tubulin molecules bound to GTP to the (+) end of the same microtubule.• This alternation between cycles of growth and shrinkage is Dynamic instability.

• As long as new GTP bound tubulin molecules are added more rapidly than GTP is hydrolyzed , the microtubules retains a GTP cap at its (+) end and growth continues.

• If the rate of polymerization slows, the GTP bound to tubulin at the (+) end will be hydrolyzed to GDP resulting in shrinkage of the microtubule.

FUNCTIONS • A notable structure composed largely of microtubules is the mitotic spindle, used

by eukaryotic cells to segregate their chromosomes during cell division. • Cilia and flagella are microtubule based projections of the plasma membrane that are

responsible for movement of a variety of eukaryotic cells.• The cytoskeleton formed by microtubules is essential to the morphogenetic process of an

organism’s development e.g. a network of polarized microtubules is required within the oocyte of Drosophila melanogaster during its embryogenesis in order to establish the axis of the egg.

• They are involved in maintaining the structure of the cell and, together with microfilaments and intermediate filaments, they form the cytoskeleton. They provide platforms for intracellular transport and are involved in a variety of cellular processes, including the movement of secretory vesicles, organelles, and intracellular macromolecular assemblies

REFERENCES-THE CELL, 2ND EDITIONA MOLECULAR APPROACHGEOFFREY M COOPER.

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