The cytoskeleton forms the internal scaffolding that gives a cell its shape, organizes its internal components, and facilitates movement. Microtubules are a major component of this cellular framework, constantly undergoing rapid changes in length. This inherent characteristic of switching between growth and shrinkage is known as dynamic instability. This behavior is necessary for many aspects of cellular life, from cell division to internal organization.
Microtubules: The Structural Foundation
Microtubules are stiff, hollow, cylindrical polymers composed of the protein tubulin. They are built from globular subunits called alpha-beta-tubulin dimers. These dimers link end-to-end to form linear strands called protofilaments. Thirteen protofilaments typically associate laterally to form the wall of the hollow tube. Microtubules measure approximately 25 nanometers in diameter and can extend for many micrometers in length within the cell.
The arrangement of the tubulin dimers gives the microtubule a distinct structural polarity. This means the two ends are chemically and kinetically different. One end is the plus (+) end, where the beta-tubulin subunit is exposed, and this is typically the site of fast growth. The opposite end is the minus (-) end, which grows much slower or remains anchored to an organizing center. This polarity guides motor proteins, such as kinesin and dynein, which transport vesicles and organelles along these tracks to specific locations.
The Mechanism of Dynamic Instability
Dynamic instability describes the rapid switching of an individual microtubule between periods of growth and fast shrinkage. This behavior is powered by the hydrolysis of Guanosine Triphosphate (GTP), a molecule bound to the beta-tubulin subunit. The process is a continuous cycle involving polymerization, nucleotide hydrolysis, catastrophe, and rescue.
Polymerization (Growth)
The cycle begins as alpha-beta-tubulin dimers bound to GTP are added rapidly to the plus end. This rapid addition creates a protective structure at the tip called the “GTP Cap,” consisting of GTP-bound tubulin subunits. The presence of this cap stabilizes the microtubule structure by promoting strong lateral bonds between the protofilaments.
GTP Hydrolysis
Once incorporated into the main body, the GTP bound to the beta-tubulin subunit is converted to Guanosine Diphosphate (GDP). This reaction releases energy and causes a conformational change in the tubulin subunit, which weakens the lateral bonds within the microtubule lattice. The speed of hydrolysis is slower than the rate of new GTP-tubulin addition, which maintains the stabilizing GTP cap at the tip.
Catastrophe
Catastrophe is the abrupt transition from the growth phase to the rapid shrinkage phase. This occurs when the rate of GTP hydrolysis overtakes the rate of new GTP-tubulin addition, causing the loss of the stabilizing cap. The newly exposed GDP-tubulin lattice is structurally unstable, causing the protofilaments to curl outward and rapidly disassemble. The swiftness of this depolymerization is a distinguishing feature.
Rescue
The process of shrinkage can suddenly stop and reverse in a phenomenon known as Rescue. This switch back to the growth phase occurs if the shrinking microtubule manages to regain a GTP cap. This happens if enough GTP-bound tubulin dimers are added to re-establish the protective structure. The alternating nature of catastrophe and rescue ensures that microtubules are constantly exploring the cell’s interior.
Cellular Roles and Biological Importance
The ability of microtubules to rapidly extend and retract enables the cell to perform complex tasks. This constant restructuring is exploited by the cell to quickly organize its interior and execute precise movements.
Mitosis and Meiosis
A primary function relying on dynamic instability is the segregation of chromosomes during cell division. Dynamic instability allows the cell to quickly assemble the mitotic spindle, a temporary structure made of microtubules that attaches to chromosomes. The controlled growth and shrinkage of these spindle microtubules push and pull the chromosomes to opposite ends of the dividing cell, ensuring each daughter cell receives a complete set of genetic material.
Intracellular Organization
The rapid growth and retraction allow for exploration of the cytoplasm. By continuously extending and shrinking, microtubules efficiently search for and find specific docking sites on organelles or at the cell periphery. Once a target is found, the microtubule stabilizes its connection, establishing the precise location of organelles like the Golgi apparatus or the endoplasmic reticulum.
Cell Shape and Migration
Dynamic instability is fundamental to cell shape and migration. Cells that need to move, such as immune cells or those involved in tissue repair, must constantly change their shape to navigate complex environments. Targeted growth of microtubules at the leading edge pushes the membrane forward. Disassembly of microtubules elsewhere permits the cell body to follow, facilitating swift, directed movement.