Cytoskeleton & Microtubules

What is the cytoskeleton?

A system of filamentous polymers that provide structure, organization, and strength in eukaryotic cells.

What roles does the cytoskeleton play besides providing structure?

Cell division, cell motility, vesicle trafficking, and cell signaling.

What are the three main families of cytoskeletal filaments?

Actin filaments, intermediate filaments, and microtubules.

What are actin filaments made of?

Actin subunits.

What are intermediate filaments made of?

Tough, insoluble protein fibers composed of tetramer fibrils woven like ropes.

What are microtubules made of?

Hollow tubes of spherical protein subunits called tubulins.

How are cytoskeletal filaments formed?

By polymerization of subunits into multiple protofilaments.

Why are cytoskeletal filaments formed from multiple protofilaments rather than single linear arrays?

It makes them more resistant to breakage.

Why are cytoskeletal filaments dynamic?

To facilitate cell movement and cell division.

How do actin filaments contribute to cell movement?

Growth and shrinkage allow cells to explore territory and move around.

How are cytoskeletal filaments reorganized during cell division?

Microtubules quickly rearrange to form a bipolar mitotic spindle for chromosome segregation.

Why can cytoskeletal filaments depolymerize and repolymerize quickly?

Subunits are held by weak non-covalent interactions, allowing rapid disassembly, diffusion, and reassembly.

Why is it easier to transport cytoskeletal subunits than large filaments?

Small soluble subunits diffuse more easily through the cytoplasm.

Can cytoskeletal filaments be non-dynamic?

Yes, depending on regulatory elements, filaments can be extremely stable.

Which cytoskeletal filaments stabilize microvilli in epithelial cells?

Actin bundles.

What is the function of microvilli?

Increase the cell's surface area for absorption or secretion.

Which cytoskeletal structures are stabilized in cilia and flagella?

Microtubule arrays.

What general functions does the cytoskeleton serve?

Provides/supports cell shape, enables migration, serves as a rail system for cargo transport, and generates contractile force.

Which intermediate filaments maintain nuclear structural integrity?

Lamins.

What role do actin homologs play in bacteria?

Determine cell shape.

What happens to B. subtilis cells lacking actin-like proteins MreB and Mbl?

They grow irregularly and die.

How does the cytoskeleton enable cell migration?

Through polymerization and depolymerization of actin filaments (e.g., dendritic cells moving through 3D environments).

Which cytoskeletal element powers sperm swimming?

Microtubule-based flagella.

How does the cytoskeleton provide contractile force in muscle cells?

Motor neurons release acetylcholine → triggers muscle action potential → Ca2+ release from sarcoplasmic reticulum → contraction of filaments.

What are microtubules made of?

Tubulin.

Are microtubules polar or non-polar?

Polar, with distinct minus and plus ends.

What are the main functions of microtubules in cells?

Provide internal skeleton, tracks for vesicle transport, form mitotic spindle, organize organelles, define cilia/flagella structure, drive motility.

How do microtubules organize cilia and flagella?

Highly organized longitudinal MTs, together with dynein, facilitate beating movement.

Why are microtubules important for neurons?

They enable polarized cargo transport, essential for neuronal extension and synaptic signaling.

Which motor protein transports cargo along microtubules?

Kinesin (also dynein for certain directions).

Are microtubules stable or dynamic?

Dynamic; they are remarkably unstable and rapidly reorganize.

What is visualized by comet-like structures in cells?

GTP-labeled microtubule plus-end.

When do microtubules rapidly reorganize?

During stages of the cell cycle.

What are microtubules made of?

13 protofilaments of α- and β-tubulin heterodimers.

Which tubulin hydrolyzes GTP?

β-tubulin.

How is microtubule polarity determined?

By the orientation of α- and β-tubulin subunits.

Which end of a microtubule is built upon?

Plus end (β-tubulin exposed).

Which end is typically destroyed?

Minus end (α-tubulin exposed).

Can microtubules be reconstituted in vitro?

Yes, from tubulin and GTP alone.

What can microtubules do if there is a defect in a protofilament?

Incorporate new dimers internally to heal the damaged site.

What is "dynamic instability" in microtubules?

Cycles of growth and rapid shrinkage (catastrophe).

What drives dynamic instability?

GTP hydrolysis and conformational changes in tubulin.

Which tubulin-bound nucleotide is more stable?

GTP-bound tubulin is more stable than GDP-bound.

What is a GTP "cap"?

A stabilizing structure at the plus end of a microtubule.

What happens if the GTP cap is lost?

The microtubule undergoes catastrophe (rapid depolymerization).

What is the role of the GTP cap besides stabilization?

Provides a platform for end-binding (EB) proteins.

How does GTP hydrolysis occur during polymerization?

The incoming GTP-tubulin dimer promotes hydrolysis of the β-tubulin's GTP.

How is free energy stored in microtubules?

In the microtubule lattice during polymerization.

How is this energy released?

During depolymerization or catastrophe.

What does the release of energy allow microtubules to do?

Perform work, such as moving or organizing cellular components.

How does dynamic instability contribute to microtubule function?

Allows microtubules to explore cellular space.

How do microtubules organize the ER?

Growing MT ends generate forces that can deform ER membranes, positioning it within the cell.

What does GFP-STIM1 co-localization show?

Visualizes the endoplasmic reticulum and its interaction with growing microtubules.