Unit 5- Cytoskeleton and Cell Movement

What are the two forms of actin

G actin is the globular monomer; F actin is the polymerized filament.

What is the polarity of an actin filament

It has a plus end and a minus end because actin monomers have the same orientation.

Which end of an actin filament grows faster

The plus end grows faster than the minus end.

What is nucleation in actin filament assembly

The formation of an initial small aggregate of three actin monomers.

What happens to ATP-actin after it joins a filament

ATP is hydrolyzed to ADP, making the actin less tightly bound.

What is treadmilling

ATP-actin is added at the plus end while ADP-actin dissociates from the minus end.

Does ATP provide the energy for actin polymerization

No. ATP-actin polymerizes more readily, but ATP does not provide the energy for polymerization.

What is the main role of actin-binding proteins

They regulate actin assembly, disassembly, stability, and organization.

What do formins do to actin filaments

They nucleate and promote long, unbranched actin filaments.

What does Arp2/3 do to actin filaments

It initiates branched actin filaments, especially during cell movement.

What does profilin do

It exchanges ADP for ATP on actin monomers, increasing ATP-actin available for polymerization.

What do capping proteins do

They bind actin filament ends and stabilize them by preventing monomer addition or loss.

What does tropomyosin do to actin filaments

It binds along the filament and helps stabilize it.

What does cofilin do to actin filaments

It severs actin filaments, creating new ends for depolymerization or growth.

How do actin-binding proteins regulate actin filament turnover

They control filament formation, growth, stabilization, severing, and disassembly.

How are actin filaments organized into bundles

Small rigid cross-linking proteins organize closely packed parallel actin filaments.

How are actin filaments organized into networks

Large flexible cross-linking proteins organize filaments into 3D meshworks.

Where are actin filaments especially concentrated in cells

They are highly concentrated beneath the plasma membrane at the cell periphery.

What is the cell cortex

A 3D network of actin filaments and associated proteins located beneath the plasma membrane.

What are the main functions of cortical actin

It helps determine cell shape, provides mechanical support, and enables cell-surface movement.

What is the cell cortex

A 3D network of actin filaments and associated proteins directly beneath the plasma membrane.

How does cortical actin help determine cell shape

Its network beneath the plasma membrane provides structural support and helps maintain cell shape.

What is the main structural protein of the erythrocyte cortical cytoskeleton

Spectrin is the principal structural protein of the erythrocyte cortical cytoskeleton.

How does spectrin organize with actin in red blood cells

Spectrin tetramers connect short actin filaments, forming a spectrin-actin network.

How is the spectrin-actin network connected to the plasma membrane

Ankyrin links spectrin to the cytoplasmic domain of the transmembrane protein band 3.

What is the function of dystrophin

It links actin filaments to transmembrane proteins that connect the cytoskeleton to the extracellular matrix.

What are focal adhesions

Specialized sites where cells attach to the extracellular matrix through integrins.

What are stress fibers

Bundles of actin filaments that anchor at focal adhesions and allow cells to exert tension.

Which proteins help connect actin to integrins at focal adhesions

Talin, vinculin, and α-actinin help link actin to integrins.

What is the role of stress fibers in cells

They anchor the cell and allow it to exert tension against the extracellular environment.

How does actin polymerization help a cell move forward

Actin polymerization at the leading edge pushes the plasma membrane forward.

What are lamellipodia

Broad, sheet-like extensions at the leading edge formed by branched actin networks.

What are filopodia

Thin, finger-like cell extensions containing long, unbranched actin filaments.

What role does Arp2/3 play in cell motility

It creates branched actin networks at the plasma membrane that help push the cell forward.

What role do formins play in cell motility

They produce long, unbranched actin filaments used in structures such as filopodia.

How does cofilin help cell motility

It severs actin filaments, creating new ends that allow continued actin remodeling.

Why is actin remodeling important for cell movement

Actin must continuously assemble and disassemble so the cell can change shape and move.

How do focal adhesions contribute to cell movement

They attach the cell to the extracellular matrix and anchor actin stress fibers.

How do stress fibers help move a cell

They generate tension and help pull the cell forward after attachment to the substratum.

What is the basic sequence of actin-based cell movement

Actin polymerizes at the front, the cell attaches, stress fibers generate tension, and the rear retracts.

What are the two main proteins involved in muscle contraction

Actin and myosin form the contractile machinery.

What is the basic idea behind muscle contraction

Myosin interacts with actin to generate force and cause filament sliding.

What is the role of myosin in muscle contraction

Myosin uses ATP to generate force and move along actin filaments.

What is the role of actin during muscle contraction

Actin forms filaments that interact with myosin during contraction.

Why is actin filament polarity important for myosin

Polarity determines the direction in which myosin moves along actin.

What provides the energy for myosin movement

ATP provides the energy used by myosin for movement and force generation.

How does myosin produce movement along actin

Myosin heads interact with actin and undergo ATP-driven movements.

What are actin-myosin filaments used for in nonmuscle cells

They generate contractile forces for cell shape changes and movement.

Where can actin-myosin contractile systems function in nonmuscle cells

They function in structures such as stress fibers and the contractile ring.

What is the overall role of actin-myosin contractility in cells

It generates force needed for cell movement, shape changes, and division.

What is a myosin molecule made of

It consists of two heavy chains and associated light chains.

What is the main function of the myosin head

The myosin head binds actin and interacts with ATP to generate movement.

What does the myosin tail region do

The tail helps myosin molecules associate with each other or bind cargo.

How are myosin molecules organized in muscle

Myosin molecules form thick filaments with their heads projecting outward.

What happens when myosin heads interact with actin

They generate force that causes actin filaments to move relative to myosin.

What determines the direction of myosin movement on actin

The polarity of the actin filament determines the direction of movement.

Why is ATP important for myosin function

ATP binding and hydrolysis drive changes in myosin that produce movement.

What is the relationship between actin and myosin during contraction

Myosin moves along actin, producing the force required for contraction.

How does repeated myosin movement produce contraction

Repeated interactions between myosin and actin generate continued filament movement.

What is the sliding-filament basis of muscle contraction

Muscle contracts when myosin-generated force causes actin filaments to slide.

What is the role of actin-myosin bundles in nonmuscle cells

They generate contractile forces that change cell shape and produce movement.

What are stress fibers

Bundles of actin filaments that contain myosin and generate contractile force.

How do stress fibers help cells

They help cells maintain tension and generate forces against the extracellular environment.

How do actin and myosin contribute to cell movement

Their contraction generates forces that help move the cell.

How do actin-myosin filaments contribute to cell shape changes

Their contraction can pull on cellular structures and change cell shape.

How do actin-myosin filaments contribute to cell division

They form a contractile ring that helps separate the two daughter cells.

What is the contractile ring

An actin-myosin structure that contracts during cytokinesis to divide the cell.

Why are actin-myosin systems useful outside muscle cells

They allow cells to generate force without requiring specialized muscle tissue.

What is the relationship between stress fibers and focal adhesions

Stress fibers anchor at focal adhesions and transmit contractile forces.

How do actin-myosin filaments work with focal adhesions

They generate tension while focal adhesions anchor the cell to the extracellular matrix.

What are unconventional myosins

Myosins that perform functions other than the typical muscle contraction role.

Where do unconventional myosins function

They function in many nonmuscle cells and perform specialized transport and movement roles.

What do some unconventional myosins transport

They transport membrane-bound organelles and vesicles along actin filaments.

How do unconventional myosins move cargo

They use ATP to move along actin filaments while carrying specific cargo.

What is an important difference between conventional and unconventional myosins

Unconventional myosins have specialized functions and structures suited to different cellular roles.

How can unconventional myosins affect the plasma membrane

They can move or position membrane-associated structures using actin.

What role can unconventional myosins play in cell polarity

They help position cellular components in specific regions of the cell.

Why are unconventional myosins important for epithelial cells

They help organize and transport cellular components along actin networks.

How do unconventional myosins contribute to intracellular transport

They move specific cargo through the cell along actin filaments.

What is the overall function of unconventional myosins

They use actin-based movement to transport cargo and organize cellular structures.

What are microtubules

Cytoskeletal filaments about 25 nm in diameter.

What are microtubules made of

They are made of α-tubulin and β-tubulin heterodimers.

What is a tubulin heterodimer

One α-tubulin and one β-tubulin joined together.

How do microtubules form

Tubulin heterodimers polymerize to form microtubules.

What are the two ends of a microtubule

A plus end and a minus end.

Where are microtubule minus ends usually located in animal cells

They are anchored at the centrosome.

Where do microtubule plus ends extend

They extend outward toward the cell periphery.

What is dynamic instability

Microtubules rapidly switch between periods of growth and shortening.

What causes dynamic instability in microtubules

GTP hydrolysis after tubulin polymerization causes changes in microtubule stability.

Why are microtubules considered dynamic

They continually assemble and disassemble, allowing cells to reorganize them.

What happens when tubulin is added to a growing microtubule

GTP-bound tubulin is added to the growing end.

What happens to GTP after tubulin is incorporated

GTP is hydrolyzed to GDP, making the microtubule less stable.

What is the GTP cap

A region of GTP-bound tubulin at a growing microtubule end.

What happens when the GTP cap is lost

The microtubule can rapidly begin shortening.

What is microtubule catastrophe

The transition from microtubule growth to rapid shortening.

What is microtubule rescue

The transition from shortening back to microtubule growth.

What is the main function of microtubule motor proteins

They transport cargo and generate movement along microtubules.

What are the two major families of microtubule motor proteins

Kinesins and dyneins.

Which direction do most kinesins move along microtubules

Toward the plus end.

Which direction do dyneins move along microtubules

Toward the minus end.