7.3 Microtubules and intracellular transport

Microtubules and Intracellular Transport

Microtubules are hollow tubes built from tubulin proteins that serve as the cell's structural beams and transport highways. They provide mechanical support, organize organelle positioning, and form the mitotic spindle during cell division. What makes them especially interesting is their dynamic instability: they can rapidly switch between growing and shrinking, allowing the cell to reorganize its internal architecture in minutes.

This section covers microtubule structure, how polymerization is regulated, and the motor proteins that haul cargo along these tracks.

Microtubule Structure and Dynamics

Structure and function of microtubules

Each microtubule is a hollow cylinder about 25 nm in diameter, assembled from α-tubulin and β-tubulin heterodimers. These dimers stack head-to-tail into linear chains called protofilaments, and 13 protofilaments associate side-by-side to form the tube wall.

Because the dimers all orient the same way, microtubules have built-in polarity:

• The plus (+) end exposes β-tubulin and is the fast-growing end.
• The minus (−) end exposes α-tubulin and is typically anchored at the centrosome (or another microtubule-organizing center, MTOC).

This polarity matters because motor proteins read it like a one-way sign, determining which direction cargo moves.

Microtubules carry out three broad functions:

• Structural support: they maintain cell shape and resist compressive forces.
• Intracellular transport: they act as tracks for motor proteins carrying vesicles, organelles, and macromolecules.
• Cell division: they form the mitotic spindle that separates chromosomes.
• Organelle positioning: they anchor and organize the Golgi apparatus, ER, and mitochondria.

Microtubule polymerization and regulation

Microtubule growth and shrinkage are governed by GTP hydrolysis. Here's how the cycle works:

1. Free tubulin dimers bind GTP on the β-tubulin subunit.
2. GTP-tubulin adds to the (+) end, extending the protofilament.
3. Shortly after incorporation, GTP is hydrolyzed to GDP. GDP-tubulin has a curved conformation that fits poorly in the straight microtubule lattice.
4. As long as new GTP-tubulin adds faster than hydrolysis catches up, a GTP cap persists at the (+) end. This cap stabilizes the structure.
5. If the GTP cap is lost (hydrolysis outpaces addition), GDP-tubulin is exposed at the tip. The protofilaments splay outward and the microtubule undergoes rapid shrinkage, called catastrophe.
6. A shrinking microtubule can regain a GTP cap and resume growth, an event called rescue.

This stochastic switching between growth and catastrophe is dynamic instability.

Several protein families regulate these dynamics:

• MAPs (microtubule-associated proteins): bind along the microtubule lattice, stabilizing it and promoting polymerization.
• Stathmin: sequesters free tubulin dimers, reducing the pool available for polymerization and promoting depolymerization.
• End-binding proteins (EBs): track the growing (+) end and recruit other regulatory factors.
• XMAP215/CLASP family: accelerate polymerization and stabilize microtubules at specific cellular locations.

Microtubule Functions and Associated Proteins

Roles in cellular processes

Cell division

During mitosis and meiosis, microtubules reorganize into the mitotic spindle, which has three functionally distinct populations:

1. Kinetochore microtubules attach to the kinetochore on each chromosome and generate the pulling forces that segregate sister chromatids.
2. Interpolar microtubules overlap at the spindle midzone and push the two poles apart.
3. Astral microtubules radiate outward from each pole to the cell cortex, helping position and orient the spindle.

Intracellular transport

Motor proteins walk along microtubules carrying cargo. The direction of transport depends on the motor:

• Kinesins (most family members) move toward the (+) end, which typically points toward the cell periphery. This is anterograde transport, carrying newly made vesicles and organelles outward.
• Cytoplasmic dynein moves toward the (−) end, back toward the centrosome and nucleus. This is retrograde transport, returning endosomes, signaling complexes, and recycled material to the cell center.

Motor proteins don't work alone. Adaptor proteins link specific cargoes to specific motors, and regulatory subunits (like dynactin for dynein) are required for processive movement.

Organelle positioning

Microtubules tether organelles in their characteristic locations. For example, the Golgi apparatus clusters near the centrosome because dynein pulls Golgi-derived membranes toward the (−) end. If you depolymerize microtubules with a drug like nocodazole, the Golgi fragments and disperses throughout the cytoplasm.

Key proteins of microtubule dynamics

• Tubulin
  • α-tubulin and β-tubulin form the heterodimers that polymerize into microtubules.
  • γ-tubulin forms ring complexes (γ-TuRC) at the centrosome that serve as templates for microtubule nucleation, establishing the (−) end.
• MAPs
  • MAP1, MAP2 (neurons, dendrites) and tau (neurons, axons) stabilize microtubules and promote polymerization. Hyperphosphorylated tau dissociates from microtubules and aggregates into neurofibrillary tangles, a hallmark of Alzheimer's disease.
  • MAP4 is the major non-neuronal MAP, involved in microtubule bundling and stability.
• Motor proteins
  • Kinesins: large superfamily; most are (+) end-directed. Kinesin-1 is the classic anterograde vesicle transporter.
  • Cytoplasmic dynein: the primary (−) end-directed motor. Requires the dynactin complex for most cargo transport and spindle positioning functions.
• Regulatory proteins
  • EB1/EB3: (+) end-tracking proteins that serve as platforms for recruiting other regulators.
  • Stathmin: promotes catastrophe by sequestering free tubulin dimers, lowering the concentration available for polymerization.
  • XMAP215: a polymerase that accelerates tubulin addition at the (+) end.
  • CLASP: stabilizes microtubules selectively at the cell cortex and Golgi.