Lecture 18-20: Cytoskeletal Proteins and Microtubule Dynamics

Microfilaments

One of the three types of cytoskeletal proteins.

Microtubules

One of the three types of cytoskeletal proteins.

Intermediate Filaments

One of the three types of cytoskeletal proteins.

Structure & support

One of the four main functions of the cytoskeleton.

Intracellular transport

One of the four main functions of the cytoskeleton.

Contractility & motility

One of the four main functions of the cytoskeleton.

Spatial organization

One of the four main functions of the cytoskeleton.

Tubulin dimers

Composed of alpha-tubulin and beta-tubulin (heterodimer).

Alpha-tubulin

Binds GTP permanently.

Beta-tubulin

Binds GTP but hydrolyzes it to GDP.

Role of beta-tubulin

Acts as a GTPase; helps regulate microtubule dynamics.

Protofilaments

What dimers assemble into; 13 protofilaments form a microtubule.

Microtubule interactions

Stabilized by non-covalent end-to-end and lateral interactions.

Microtubule growth/shrinkage

The (+) end (beta-tubulin) grows/shrinks faster than the (-) end (alpha-tubulin).

Microtubule polarity

Determines directionality of growth, shrinkage, and motor movement.

GTP-bound tubulin dimer

What happens when a tubulin dimer adds to a microtubule; GTP is hydrolyzed to GDP after incorporation.

Stability of middle segments

They have both lateral and longitudinal interactions.

Growth vs. shrinkage

Determined by GTP-tubulin concentration compared to critical concentration.

Critical concentration

Threshold where MTs switch from growing to shrinking or vice versa.

MTOC

Microtubule Organizing Center; controls MT number, polarity, and location.

Centrosome

Made of 2 centrioles and pericentriolar material.

Gamma-tubulin ring complex

Nucleates MTs at the centrosome.

Dynamic instability

Rapid switching between growth and shrinkage.

Catastrophe

Triggered by exposure of GDP-tubulin at the (+) end.

Rescue

Addition of GTP-tubulin to stabilize the end and stop shrinkage.

Benefits of dynamic instability

Allows efficient space exploration and reorganization.

AMP-PNP and GMP-PNP

Non-hydrolyzable ATP/GTP analogs for studying MT dynamics.

MAPs

Regulate growth, shrinkage, turnover; bind MTs and other molecules.

Examples of MAP functions

Remove GTP cap, act as cutters, sequester dimers.

Motor proteins

Powered by ATP hydrolysis.

Types of motor proteins

Myosin (actin), Kinesin (MT+), Dynein (MT-).

Motor protein structure

Head (binds MT), tail (binds cargo), mechanical/catalytic cycle.

Kinesin movement direction

Toward the plus end (anterograde transport).

Kinesin structure

2 heavy chains, 2 light chains; smallest motor protein.

Kinesin walking mechanism (step 1)

Leading head binds beta-tubulin, releases ADP -> tight binding.

Kinesin walking mechanism (step 2)

ATP binds leading head -> conformational change.

Kinesin walking mechanism (step 3)

Trailing head swings forward.

Kinesin walking mechanism (step 4)

New leading head binds MT, releases ADP -> tight binding.

Kinesin walking mechanism (step 5)

Trailing head hydrolyzes ATP, detaches. Cycle restarts.

Dynein movement direction

Toward the minus end (retrograde transport).

Unique aspect of dynein

Larger, complex structure; moves like a wheel.

Microtubule functions

1) Mechanical support, 2) Internal organization, 3) Golgi positioning, 4) Cell wall patterning, 5) Cell division.