Microtubles + Intermediate filaments

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Microtubule structure

Hollow tube of 13 protofilaments made of αβ-tubulin dimers; plus end = β-tubulin, minus end = α-tubulin.

Tubulin polarity

All dimers oriented the same way → polarity, just like actin.

GTP hydrolysis role

β-tubulin hydrolyzes GTP → GDP; GDP-tubulin favors depolymerization.

GTP cap

Stabilizes microtubule; loss = catastrophe (rapid shrinkage).

Dynamic instability

Microtubules switch between growth + shrinkage independently; HIGHLY TESTED.

Microtubule polymerization rule

Growth occurs ONLY if polymerization is faster than GTP hydrolysis.

Microtubule depolymerization rule

Shrinkage occurs when hydrolysis > polymerization, exposing GDP-tubulin.

MTOC

Microtubule-organizing center where minus ends are anchored; plus ends grow outward.

Centrosome definition

Animal-cell MTOC containing two perpendicular centrioles + γ-tubulin.

γ-tubulin function

Nucleates microtubules by binding minus end; plus end grows outward.

Centriole structure

Nine microtubule triplets arranged in a cylinder. HIGH-YIELD.

gamma-tubulin

Nucleates microtubules at MTOC; essential for minus-end anchoring.

Tau protein

Stabilizes microtubules; misregulation → neurodegenerative disease (he emphasized this).

Katanin

Severs microtubules → induces rapid disassembly.

Augmin

Creates branching microtubules; he compares it to Arp2/3 for actin — likely to be asked.

Kinesin direction

Walks toward plus end (anterograde); uses ATP.

Dynein direction

Walks toward minus end (retrograde); much larger than kinesin.

Dynactin

Dynein adaptor complex containing actin-like filament + CapZ.

Kinesin walking cycle

ATP binding → neck linker swings → new leading head → ATP hydrolysis.

Inactive kinesin

Folds in half, hiding ATP-binding site & microtubule-binding site until cargo binds.

Kinesin organelle positioning

Pulls ER outward; shapes ER network along microtubules.

Dynein organelle positioning

Pulls Golgi inward (toward nucleus).

Axoneme structure

9 doublets + 2 singlets (9+2 arrangement). He repeatedly emphasizes this.

Basal body

Modified centriole (9 triplets) that nucleates cilia/flagella.

Dynein in axoneme

Causes sliding between doublets → bending due to nexin links.

Ciliary beating

Alternating dynein activation → whip-like bending motion.

Eukaryotic vs prokaryotic flagella

Eukaryotic = microtubule sliding; prokaryotic = rotary motor.

IF function

Bear tensile stress; provide mechanical stability; NOT dynamic like actin/MTs.

IF monomer

Long α-helix with N- and C-terminal domains; monomer = polar.

Coiled-coil dimer

Two monomers aligned; still polar.

Tetramer

Two dimers arranged antiparallel → tetramer is nonpolar (he emphasizes this contrast with actin/MTs).

IF filament formation

5–8 tetramers form rope-like cable → highly resistant to stretching.

IF stretching

α-helix → β-sheet transition under stress → can double length.

IF stability

Very stable, not undergoing rapid remodeling; found in cells under mechanical stress.

Nuclear lamina

IF network of lamins under inner nuclear membrane; gives nucleus shape.

Plectin

Links IFs to microtubules + actin; integrates the cytoskeleton.

MT vs Actin polymerization

MT uses GTP; actin uses ATP.

MT vs Actin dynamics

MT = dynamic instability; actin = treadmilling.

MT polarity vs IF polarity

MT + actin are polar; IFs are nonpolar (after tetramer formation).

Centrosome vs Basal body

Centrosome = 9 triplets (centrioles) but no 9+2 structure. Basal body seeds cilia/flagella