Unit 4, Part 1: Cytoskeleton

What are the three major components of the cytoskeleton?

Actin filaments, microtubules, and intermediate filaments.

What are the major functions of the cytoskeleton?

Structural support, positioning of organelles, intracellular transport, cell movement, and cell division.

What is the basic building block of actin filaments?

G-actin monomers, which polymerize into F-actin.

What gives actin filaments polarity?

Their + (barbed) end grows faster than the - (pointed) end.

What is the critical concentration in actin assembly?

The minimum concentration of ATP-actin monomers needed for polymerization at each filament end.

What is actin treadmilling?

When monomers add at the + end and are lost at the - end at equal rates.

What is the ARP2/3 complex?

A branched actin nucleator that creates 70° actin branches.

What do formins do?

Nucleate long, straight, unbranched actin filaments.

What is myosin?

An ATP-powered actin motor protein.

What direction do most myosins move?

Toward the actin + (barbed) end.

What is myosin II responsible for?

Muscle contraction and tension generation in cells.

What triggers the myosin power stroke?

Phosphate release triggers a conformational change that pulls actin.

How does Listeria move inside cells?

It hijacks host actin to form comet tails that propel it through the cytoplasm.

What are microtubules made of?

α/β-tubulin heterodimers forming 13 protofilaments.

What gives microtubules polarity?

The + end (β-tubulin exposed) grows faster; the - end (α-tubulin exposed) grows slower.

What nucleotide does tubulin bind?

GTP (β-tubulin hydrolyzes it; α-tubulin holds nonhydrolyzable GTP).

What is dynamic instability?

Microtubules switch randomly between growth and rapid shrinkage (catastrophe and rescue).

What is an MTOC?

A microtubule-organizing center where microtubules nucleate.

What is the centrosome?

A major MTOC with two centrioles and pericentriolar material containing γ-tubulin rings.

What do +TIP proteins do?

Bind and track the growing microtubule + ends to regulate dynamics.

What is MAP4?

A structural MAP that bundles microtubules and 'zippers' anti-parallel microtubules.

What do kinesins do?

Microtubule-based motors that mostly move toward the + end.

What does dynein do?

A -end directed motor that transports cargo inward and positions organelles.

What is the structure of a skeletal muscle fiber?

A long multinucleated cell filled with myofibrils made of repeating sarcomeres.

What is a sarcomere?

Contractile unit from Z-line to Z-line containing thin and thick filaments.

What happens to sarcomere bands during contraction?

I-band and H-zone shrink; A-band stays the same.

Describe the sliding filament model.

Myosin pulls actin filaments toward the center of the sarcomere, shortening the muscle.

What is the role of ATP in myosin function?

ATP binding releases myosin from actin; hydrolysis resets the head for another stroke.