Cytoskeleton Notes

Cytoskeleton: Intermediate Filaments, Microtubules, and Actin Filaments

Intermediate Filaments

• Composed of α-helix proteins.
• Provide mechanical strength to cells.
• Keratin reinforces epidermal cells.
• Lamina: Forms a lining supporting the nuclear membrane.
• Two intermediate filament molecules twist to form a coiled-coil dimer.
• Globular domains connect molecules end-to-end.
• Every monomer forms an α-helix.
• Help cells withstand mechanical stress; extend through epithelial cells to prevent rupture.
• Keratins in different cells are connected via pores at tight junctions, forming keratin filaments.
• Connect to other protein fibers like microtubules with plectin.

Nuclear Lamina

• A special type of intermediate filament that supports the nuclear membrane.
• Forms a lining underneath the nuclear membrane.
• Disassemble during mitosis (controlled by phosphorylation) and reassemble after mitosis to reform the nuclear membrane.

Microtubules

• Long, hollow tubes made of tubulin proteins.
• Major component of mitotic fibers.
• Provide transport tracks within the cell.
• Composed of α-tubulin and β-tubulin dimers.
• Polarity: α-tubulin end is the minus end; β-tubulin end is the plus end.
• Tubulins extend in the direction of the plus end (β-tubulin).
• Polarity determines the direction of transport along the microtubule.
• γ-tubulin serves as the base for microtubules to grow from the centrosome.

Assembly and Disassembly

• αβ-tubulin dimer binds to GTP, gaining energy to bind to the elongating tube.
• If GTP is hydrolyzed to GDP before the next dimer binds, the GDP-containing dimer falls off.
• Assembly and disassembly occur constantly, depending on GTP availability.
• Extension and shortening are random.
• Can be stabilized by capping proteins or chromosomes at the plus end, preventing disassembly.
• Organize organelles by constructing a network in the cytosol for anchoring.

Motor Proteins

• Dynein and Kinesin are motor proteins for polar transport along microtubules.
• Each has a motor head and a cargo tail.
• Cargo tails carry cargo, and motor heads slide along microtubules.
• Kinesin moves toward the plus end, and dynein moves toward the minus end.
• Transport directions are crucial in polarized cells.
• Microtubules serve as tracks for motor proteins.
• Example: Transport along axons.

Flagella

• Made of 9+2 bundles of microtubules.
• Each bundle contains two microtubules, with one attached to the dynein cargo tail.
• Dynein arms push neighboring bundles, causing them to pass each other.

Actin Filaments

• Composed of actin molecules (subunits).
• Myosin attaches and walks on actin filaments.
• Reinforce the cell surface via the cell cortex network.
• All actin molecules (subunits) are connected in the same direction.
• Each subunit is identical (unlike microtubules).
• Two strands of actin filaments combine to form a two-stranded helix.

Polymerization

• ATP binds to actin, which incorporates into the growing strand.
• When ATP is hydrolyzed to ADP, stability decreases, and subunits fall apart.
• Capping proteins attach to protect the plus end, stopping polymerization.
• ARP complex attaches to existing actin filaments, providing a site for new filament branching.
• Hydrolyzed ADP attached to actin promotes depolymerization via depolymerizing protein.

Function

• Polymerization elongates filaments, pushing the membrane and causing the cell to crawl.
• Actin cortex performs cellular movement.

Myosin

• Myosin "walks" on actin filaments (Myosin I) for cellular transport.
• Uses binding-releasing actions and conformational changes.
• Walks from the minus end to the plus end.
• One-way walking ensures precise directional movement of myosin-associated organelles.
• Myosin action in muscle (Myosin II): Head contains ATPase, which hydrolyzes ATP for energy to pull actin.

Muscle Contraction

• Head is locked to the actin filament at rest.
• When the head releases ADP, it pulls actin back.
• At the end of the cycle, the head is locked to actin again.
• When ATP binds to the head, the head releases actin.
• When the head hydrolyzes ATP, it moves forward along the actin filament.
• When the head releases phosphate, it binds to a location further ahead.
• Muscular cells are filled with actin and myosin fibers called myofibrils.

Contraction Mechanism

• Muscular cells have sarcomeres with Ca^{2+} channels on their membrane.
• When channels open, Ca^{2+} flows into the cytoplasm to trigger contraction.
• Tropomyosin normally covers myosin-binding sites on actin.
• Ca^{2+} ions bind to troponin, which is associated with tropomyosin.
• This removes tropomyosin from actin, freeing the binding sites for myosin.