Microtubules and Intracellular Transport

Microtubules are stiff, polar tubes of tubulin that organize the cell interior and serve as tracks for the directed transport of organelles and vesicles.

Definition

Microtubules are hollow cylindrical polymers of alpha- and beta-tubulin that provide structural organization and serve as tracks along which motor proteins transport cellular cargo.

Scope

This topic covers the structure and polarity of microtubules, their nucleation from organizing centers such as the centrosome, their dynamic instability, and their role as railways for motor-driven intracellular transport and as the framework for cilia and the spindle.

Core questions

• How are microtubules built from tubulin and why are they polar?
• What is dynamic instability and why is it useful?
• How are microtubules organized from the centrosome?
• How do microtubules support directed intracellular transport?

Key theories

Dynamic instability of microtubules

Individual microtubules alternate between growth and abrupt shrinkage governed by the state of bound GTP at their ends, allowing rapid remodeling and a search-and-capture mode of organizing the cell.

Mechanisms

Microtubules are built from alpha- and beta-tubulin dimers that assemble into a polar tube with a fast-growing plus end and a minus end usually anchored at an organizing center such as the centrosome. GTP hydrolysis after assembly destabilizes the lattice, producing dynamic instability in which microtubules switch between growth and catastrophe. The radial array provides directional tracks: kinesins generally move cargo toward plus ends at the cell periphery and dyneins toward minus ends near the center, organizing organelles and transporting vesicles.

Clinical relevance

Microtubules structure the cell, position organelles, and enable long-range transport, and they form the spindle and the core of cilia, making them central to cell biology. The treatment here is descriptive and non-prescriptive.

History

The 1984 discovery of dynamic instability by Mitchison and Kirschner reshaped understanding of how microtubules organize the cell; the identification of kinesin by Vale, Sheetz, and colleagues revealed the motors that drive transport along these tracks.

Key figures

• Tim Mitchison
• Marc Kirschner
• Ronald Vale
• Michael Sheetz

Seminal works

• mitchison1984
• alberts2014

Frequently asked questions

Why are microtubules polar?

Their tubulin building blocks all point the same way, giving the microtubule two distinct ends, a faster-growing plus end and a minus end, which lets motors move cargo directionally along them.

What is dynamic instability?

It is the behavior in which a microtubule can suddenly switch from steady growth to rapid shrinkage and back, allowing the cell to remodel its microtubule network quickly.