Chapter 17 part 1

What are the main functions of the cytoskeleton?

Shape, strength, movement, transport, communication, and cell division.

What are the 3 types of cytoskeletal filaments?

• Intermediate filaments (~10 nm)

• Microtubules (~25 nm)

• Actin filaments (~7 nm)

Which filament is most rigid?

Microtubules

Which filaments are most flexible?

Actin filaments

What is the main function of intermediate filaments?

Provide mechanical strength and resistance to stress.

Where are intermediate filaments found?

Cytoplasm and nucleus (nuclear lamina)

hat structures anchor intermediate filaments?

Desmosomes

What everyday structures rely on IFs?

Hair, nails, skin

What is the structure of intermediate filaments like?

Rope-like (twisted strands → high tensile strength)

Do intermediate filaments have polarity?

No (unlike microtubules & actin)

Do IFs bind nucleotides (ATP/GTP)?

No

Are there motor proteins for IFs?

No

Are IFs dynamic or stable?

More stable (slow subunit exchange)

What is the structure of IF subunits?

Fibrous proteins with α-helical rod domain

How do IFs assemble?

• Dimers (coiled-coil)

• Tetramers (antiparallel, staggered)

• Lateral association into filaments

Are interactions covalent or non-covalent?

Non-covalent

Where are keratins found?

Epithelial cells

What are hard keratins?

Hair & nails (rich in disulfide bonds)

What are soft keratins?

Cytokeratins in epithelial cells

What happens if keratin is mutated?

Skin blistering (epidermolysis bullosa simplex)

What proteins are included in intermediate filaments?

Desmin, vimentin, GFAP

What does desmin do?

Maintains muscle structure

What are neurofilament subunits?

NF-L, NF-M, NF-H

Function of neurofilaments?

Maintain axon structure and diameter

Where are lamins located?

Inner nuclear membrane

Function of lamins?

Support nucleus and form nuclear lamina

What happens during mitosis?

Lamina disassembles (via phosphorylation)

Disease associated with lamin defects?

Progeria (premature aging)

What is plectin?

Cross-linker connecting IFs to other cytoskeleton elements

What do intermediate filaments NOT do?

Transport vesicles (that’s microtubules)

Main functions of microtubules?

• Tracks for transport

• Vesicle/organelle movement

• Cell division (mitotic spindle)

• Cilia & flagella structure

What are microtubules made of?

α-tubulin + β-tubulin dimers

How are dimers arranged?

End-to-end → protofilaments

How many protofilaments form a microtubule?

13

Do microtubules have polarity?

Yes (+) and (−) ends

Where do microtubules grow from?

Centrosome (MTOC)

What complex nucleates MTs?

γ-tubulin ring complex (γ-TURC)

Orientation of MTs?

• (−) end at centrosome

• (+) end grows outward

Do centrioles nucleate MTs?

No

What is dynamic instability?

Alternating growth and shrinkage of MTs

What causes catastrophe (rapid shrinkage)?

Loss of GTP cap

What stabilizes growth?

GTP-β-tubulin cap

Which tubulin hydrolyzes GTP?

β-tubulin

What about α-tubulin?

GTP is NOT hydrolyzed

What happens if depolymerization is inhibited?

Cell division stops

What happens if depolymerization is promoted?

Cell division stops

Example drugs?

• Taxol → stabilizes MTs

• Colchicine/vinblastine → prevent polymerization

How do microtubules help cell polarity?

Directional transport along aligned MTs

What stabilizes MT ends?

• (−) end → centrosome

• (+) end → binding proteins

What do motor proteins do?

Transport cargo along microtubules

What powers motor proteins?

ATP

What direction do kinesins move?

Toward (+) end

What direction do dyneins move?

Toward (−) end

What part binds cargo?

Tail

What part moves?

Head (motor domain)

What powers movement in cilia/flagella?

Dynein

Do MTs in cilia show dynamic instability?

No (they are stabilized)

What is the structure of cilia/flagella?

“9 + 2” arrangement

What does dynein cause?

Bending motion

How are intermediate filament proteins expressed in the body?

They are tissue-specific and heterogeneous.

In what types of cells are intermediate filaments especially prominent?

Cells under mechanical stress:

• Neurons (axons)

• Muscle cells

• Epithelial cells (skin)

Why are intermediate filaments more stable than other filaments?

• No nucleotide binding

• Stronger interactions

• Slower subunit exchange

Why do intermediate filaments lack polarity?

Because they assemble in an antiparallel manner.

Are intermediate filament subunits soluble before assembly?

Yes (dimers and tetramers are soluble).

What gives hard keratin (hair/nails) its strength?

Disulfide bonds between cysteine residues.

What is the difference between desmosomes and hemidesmosomes?

• Desmosomes → cell-to-cell attachment

• Hemidesmosomes → cell-to-extracellular matrix

What is the difference between centrosomes and centrioles?

• Centrosome = microtubule organizing center (MTOC)

• Centrioles = part of centrosome but do NOT nucleate microtubules

Why is the (+) end of microtubules more dynamic?

It is where the GTP cap is added or lost, controlling growth/shrinkage.

What determines whether a microtubule grows or shrinks?

Presence or absence of a GTP cap on β-tubulin.

How do motor proteins determine what cargo they transport?

The tail region determines cargo specificity.

Besides transport, what else do microtubules help do in cells?

Position organelles in the cytoplasm.

Why don’t microtubules in cilia/flagella show dynamic instability?

They are stabilized by associated proteins.