Nucleotide Hydrolysis and Treadmilling in Cytoskeleton

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Questions and Answers

What happens to the concentration of free monomer at the plus end of a polymer compared to the minus end?

  • It is greater than C (correct)
  • It is lower than C
  • It is equal to C
  • It fluctuates constantly

Treadmilling occurs when polymerization and disassembly rates are equal at both ends.

False (B)

What form are the terminal subunits at the plus end of a growing filament?

T form

The critical concentration for polymerization at the T form is __________ than at the D form.

<p>lower</p> Signup and view all the answers

Match the polymer ends with their associated characteristics:

<p>Plus end = Grows faster due to higher ATP bound Minus end = Shrinks due to faster hydrolysis T-form subunits = Polymerize at both ends D-form subunits = Result from hydrolysis being faster</p> Signup and view all the answers

What is the primary function of myofibrils in skeletal muscle cells?

<p>Muscle contraction (B)</p> Signup and view all the answers

Skeletal muscle cells are typically unbranched and have a single nucleus.

<p>False (B)</p> Signup and view all the answers

What structures are located at the ends of a sarcomere?

<p>Z discs</p> Signup and view all the answers

The thick filaments in a sarcomere are primarily composed of __________.

<p>myosin</p> Signup and view all the answers

Match the structure with its description:

<p>T tubules = Relay contraction signals to myofibrils Sarcoplasmic reticulum = Contains Ca2+ release channels Actin = Thin filaments in a sarcomere Myosin = Thick filaments in a sarcomere</p> Signup and view all the answers

Which characteristic is not true about insect flight muscle myofibrils compared to vertebrate myofibrils?

<p>They have an irregular packing of filaments (C)</p> Signup and view all the answers

During contraction, actin and myosin filaments shorten in length.

<p>False (B)</p> Signup and view all the answers

What is the role of T tubules in muscle cells?

<p>To transmit the contraction signal to myofibrils</p> Signup and view all the answers

What happens to the myosin heads when they are in the relaxed state?

<p>They are bent backward and interfere with each other. (A)</p> Signup and view all the answers

Myosin heads move actin filaments in the absence of ATP.

<p>False (B)</p> Signup and view all the answers

What is the primary function of myosin in muscle contraction?

<p>To bind and move actin filaments.</p> Signup and view all the answers

The optical tweezers are used to measure the force exerted on the _______.

<p>bead</p> Signup and view all the answers

In the experiment with purified myosin heads, at what rate did the actin filaments move?

<p>4 µm/sec (A)</p> Signup and view all the answers

Match the following components with their functions:

<p>Myosin = Motor protein that moves along actin Actin = Structural component of muscle ATP = Energy source for muscle contraction Optical tweezers = Device to manipulate and measure forces on particles</p> Signup and view all the answers

Myosin II filaments in non-muscle cells are larger than those in muscle cells.

<p>False (B)</p> Signup and view all the answers

What effect does binding of a single myosin molecule have on the movement of the actin filament?

<p>It decreases thermal motion and results in a displacement of approximately 10 nm.</p> Signup and view all the answers

Which protein is involved in binding actin monomers to facilitate polymerization?

<p>Profilin (B)</p> Signup and view all the answers

Cofilin enhances polymerization of actin filaments at the plus end.

<p>False (B)</p> Signup and view all the answers

What is the role of the Arp2/3 complex in actin filament dynamics?

<p>Nucleates new filament assembly along the sides of existing filaments.</p> Signup and view all the answers

The _____ end of the actin filament is where rapid polymerization occurs.

<p>plus</p> Signup and view all the answers

Match the following proteins with their functions:

<p>Profilin = Binds actin monomers Cofilin = Enhances depolymerization ActA = Activates nucleation Capping protein = Caps filament ends</p> Signup and view all the answers

What happens when the profilin-actin complex binds to the free plus end of an actin filament?

<p>Profilin is released, and the filament grows. (A)</p> Signup and view all the answers

Myosin II thick filaments contain head domains in the central bare zone.

<p>False (B)</p> Signup and view all the answers

Where does polymerization of actin filaments focus in bacteria?

<p>At the rear surface of the bacterium.</p> Signup and view all the answers

What effect does Cytochalasin B have on actin filaments?

<p>Depolymerizes the filaments (B)</p> Signup and view all the answers

Phalloidin is a chemical that destabilizes actin filaments.

<p>False (B)</p> Signup and view all the answers

What does the Arp2/3 complex do?

<p>Nucleates assembly to form a branched network of actin filaments.</p> Signup and view all the answers

Nocodazole binds to ______ subunits to depolymerize microtubules.

<p>tubulin</p> Signup and view all the answers

Match the following chemicals with their effects on actin filaments:

<p>Latrunculin = Depolymerizes actin Phalloidin = Stabilizes actin Cofilin = Accelerates disassembly Tropomyosin = Stabilizes filament</p> Signup and view all the answers

Which accessory protein prevents assembly and disassembly at the minus end of actin filaments?

<p>Tropomodulin (B)</p> Signup and view all the answers

Taxol is a chemical that depolymerizes microtubules.

<p>False (B)</p> Signup and view all the answers

Describe the primary function of gelsolin in the actin cytoskeleton.

<p>Gelsolin severs actin filaments and binds to the plus end.</p> Signup and view all the answers

What is the function of the Arp2/3 complex?

<p>Nucleate new actin filaments (D)</p> Signup and view all the answers

______ binds actin subunits to prevent assembly.

<p>Thymosin</p> Signup and view all the answers

The Arp2/3 complex consists of four proteins.

<p>False (B)</p> Signup and view all the answers

Which of the following is NOT an effect associated with colchicine?

<p>Stabilizes actin filaments (B)</p> Signup and view all the answers

What does 'NPF' stand for in the context of actin-binding proteins?

<p>nucleation-promoting factor</p> Signup and view all the answers

The Arp2/3 complex is involved in the formation of _______ actin filaments.

<p>branching</p> Signup and view all the answers

Which end of the actin filament is known as the plus end?

<p>The end where growth occurs (C)</p> Signup and view all the answers

Most cells contain more than _______ different actin-binding proteins.

<p>a hundred</p> Signup and view all the answers

Match the actin-associated proteins with their functions:

<p>Arp2 = Nucleation of branched filaments Arp3 = Activation of the Arp2/3 complex NPF = Promotion of nucleation Accessory proteins = Regulation of filament dynamics</p> Signup and view all the answers

All actin-associated proteins have been recognized and categorized.

<p>False (B)</p> Signup and view all the answers

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What happens to polymerization rates between the plus and minus ends of a polymer at steady state?

<p>There is net assembly at the plus end and net disassembly at the minus end. (B)</p> Signup and view all the answers

Why do terminal subunits at the plus end always remain in the T form?

<p>Elongation is faster than hydrolysis at the plus end. (C)</p> Signup and view all the answers

What is true about the critical concentrations for polymerization at the T and D forms?

<p>Cc(D) is lower than Cc(T). (C)</p> Signup and view all the answers

What characterizes the process of treadmilling in polymers?

<p>Polymerization at the plus end with a shrinking minus end. (D)</p> Signup and view all the answers

What is the consequence of the actual subunit concentration being between Cc(T) and Cc(D)?

<p>The plus end grows and the minus end shrinks. (A)</p> Signup and view all the answers

What is the consequence of nucleotide hydrolysis during polymer formation?

<p>Decreased binding affinity of the subunit for neighboring subunits. (B)</p> Signup and view all the answers

Which state predominates in microtubules due to nucleotide hydrolysis?

<p>Dynamic instability (B)</p> Signup and view all the answers

What happens after an actin molecule is assembled into a polymer?

<p>It converts ATP into ADP. (B)</p> Signup and view all the answers

In which form do subunits generally add to the filament?

<p>T form (B)</p> Signup and view all the answers

What characterizes the critical concentration difference at the ends of a polymer?

<p>It is lower at the plus end than the minus end. (D)</p> Signup and view all the answers

What two behaviors are associated with nucleoside triphosphate hydrolysis?

<p>Treadmilling and dynamic instability (A)</p> Signup and view all the answers

How is the hydration status of ATP or GTP replenished after hydrolysis?

<p>Through nucleotide exchange reactions. (D)</p> Signup and view all the answers

Which of the following describes 'treadmilling' in cytoskeletal polymers?

<p>Equal rates of polymerization and disassembly. (B)</p> Signup and view all the answers

Which chemical stabilizes actin filaments by binding along their length?

<p>Phalloidin (A)</p> Signup and view all the answers

What effect does gelsolin have on actin filaments?

<p>Severs and binds to the plus end of filaments (B)</p> Signup and view all the answers

Which of the following chemicals depolymerizes actin structures by capping filament ends?

<p>Cytochalasin B (A)</p> Signup and view all the answers

How does cofilin influence actin dynamics?

<p>Accelerates disassembly of ADP-actin filaments (A)</p> Signup and view all the answers

Which chemical binds to tubulin subunits and is known to depolymerize microtubules?

<p>Nocodazole (C)</p> Signup and view all the answers

What function does the Arp2/3 complex serve in actin filament assembly?

<p>Nucleates assembly to form a branched network (A)</p> Signup and view all the answers

Which of the following statements about profilin is TRUE?

<p>Facilitates the concentration of actin monomers at assembly sites (A)</p> Signup and view all the answers

Which chemical is derived from the Amanita mushroom and functions to stabilize actin filaments?

<p>Phalloidin (A)</p> Signup and view all the answers

What role does thymosin play in actin filament dynamics?

<p>Prevents actin subunits from polymerizing (B)</p> Signup and view all the answers

What occurs to the actin filaments when ATP is added in the experiment with purified myosin heads?

<p>The actin filaments glide along the surface. (D)</p> Signup and view all the answers

What is the effect of a single myosin molecule binding to an actin filament?

<p>It decreases the thermal motion and produces a displacement. (D)</p> Signup and view all the answers

What happens to the myosin heads in their relaxed state?

<p>They are bent backward and nonfunctional. (A)</p> Signup and view all the answers

How does thermal motion of the actin filament behave when myosin is attached?

<p>It decreases abruptly. (C)</p> Signup and view all the answers

What is the role of profilin in actin filament polymerization?

<p>It facilitates binding of actin monomers to the plus end. (B)</p> Signup and view all the answers

What is being measured by optical tweezers in the experiment described?

<p>The force exerted on a bead attached to an actin filament. (C)</p> Signup and view all the answers

Which protein is responsible for nucleating new actin filament assembly along existing filaments?

<p>ActA (B)</p> Signup and view all the answers

What characteristic difference exists between myosin II filaments in muscle and non-muscle cells?

<p>Non-muscle myosin II filaments are smaller in size compared to muscle filaments. (A)</p> Signup and view all the answers

What occurs when myosin binding to actin is prolonged due to low ATP concentration?

<p>Filament position remains constant. (D)</p> Signup and view all the answers

What occurs when the profilin-actin complex binds to a free plus end?

<p>A conformational change in actin reduces its affinity for profilin. (D)</p> Signup and view all the answers

What mechanism is suggested for the gliding motion of actin filaments in the presence of myosin heads?

<p>The many individual steps taken by myosin heads. (B)</p> Signup and view all the answers

Where is actin polymerization focused in bacterial cells?

<p>At the rear surface of the bacterium. (C)</p> Signup and view all the answers

Which statement about capping protein is correct?

<p>It prevents actin filament growth at the plus end. (B)</p> Signup and view all the answers

What role does cofilin play in actin filament dynamics?

<p>It increases depolymerization at the minus ends. (A)</p> Signup and view all the answers

What structural feature is noted in myosin II thick filaments?

<p>They possess a central bare zone devoid of head domains. (B)</p> Signup and view all the answers

What is the primary function of the Arp2/3 complex?

<p>To nucleate new actin filament formation. (A)</p> Signup and view all the answers

What is the significance of Z discs in a sarcomere?

<p>They mark the ends of the sarcomere and anchor actin filaments. (A)</p> Signup and view all the answers

What unique feature distinguishes insect flight muscle myofibrils from vertebrate myofibrils?

<p>Insect myofibrils possess hollow myosin filaments. (B)</p> Signup and view all the answers

During contraction, what happens to the sarcomere?

<p>Actin and myosin filaments slide past each other without shortening. (C)</p> Signup and view all the answers

What role do T tubules play in muscle cells?

<p>They relay signals to initiate contraction throughout the muscle cell. (B)</p> Signup and view all the answers

What is the relative size of a typical adult human muscle cell?

<p>Typically 50 micrometers in diameter and can be several centimeters long. (A)</p> Signup and view all the answers

What are myofibrils primarily composed of?

<p>A mix of myosin and actin filaments arranged in a specific pattern. (D)</p> Signup and view all the answers

What feature is characteristic of the thick filaments in a sarcomere?

<p>They are primarily made up of myosin II proteins. (B)</p> Signup and view all the answers

Which of the following best describes the arrangement of myofibrils within a skeletal muscle cell?

<p>They are arranged in parallel, showing a regular pattern of cross-striations. (C)</p> Signup and view all the answers

Flashcards

Treadmilling

The rate at which monomers add to the plus end of a polymer is faster than the rate of ATP hydrolysis, while at the minus end, ATP hydrolysis is faster than monomer addition. This results in a net addition of monomers at the plus end and a net removal at the minus end, maintaining a constant polymer length.

Critical Concentration (Cc)

The concentration of free monomers needed for a polymer to start growing at its plus end is lower than the concentration needed for the minus end to grow.

T-form subunit

The form of a subunit with bound ATP, which is more prone to polymerization than the form with hydrolyzed ATP.

D-form subunit

The form of a subunit after ATP hydrolysis, which has decreased affinity for the polymer and is more prone to depolymerization.

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Steady State

The balanced state where the rate of monomer addition at the plus end equals the rate of monomer removal at the minus end, resulting in a stable polymer length.

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Actin-binding proteins

Proteins that bind to actin and regulate its assembly, disassembly, and function. They play a crucial role in various cellular processes including cell motility, shape, and signal transduction.

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Arp2/3 complex

A protein complex that nucleates (initiates) the assembly of new actin filaments.

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Plus end of an actin filament

The end of an actin filament where new actin monomers add, causing the filament to grow.

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Minus end of an actin filament

The end of an actin filament where actin monomers detach, leading to filament disassembly.

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Actin polymerization

The process by which actin monomers assemble into long chains called actin filaments.

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Actin-associated proteins

Specialized molecules called actin-binding proteins that help regulate where and how actin filaments assemble and disassemble within cells.

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Nucleation-promoting factor (NPF)

A protein that binds to actin and promotes the formation of new actin filaments.

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Actin filament branching

A process by which actin filaments can branch out, creating a network of interconnected filaments.

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Inactive Myosin II

The inactive state of a myosin molecule, where its heads are bent backward and sterically interfere with each other, preventing them from binding to actin.

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Cytoplasmic Myosin II

A type of myosin found in non-muscle cells, involved in cell movement and changes in cell shape.

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Myosin Motor Activity

The process where purified myosin heads attached to a glass slide cause actin filaments to glide along the surface due to the repeated binding and movement of myosin heads.

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Optical Trap

A technique used to measure the force exerted by a single myosin molecule on an actin filament by using focused beams of light to trap and manipulate the filament.

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Single Myosin Force

The binding of a single myosin molecule to an actin filament results in a 10-nm displacement of the filament, indicating the force generated by the motor.

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Actin Filament Gliding

The rapid movement of actin filaments along the surface of a glass slide in the presence of ATP, driven by the repeated binding and movement of myosin heads.

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Myosin Head Bending

The backward bending of the myosin head, making it inactive and unable to bind to actin. It is a mechanism for regulating myosin activity.

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Thymosin

A protein that binds to actin monomers and prevents their assembly into filaments.

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Active Myosin

The state of the myosin molecule when it is actively bound to actin and undergoing power stroke, generating force.

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Profilin

A protein that binds to actin monomers and promotes their assembly into filaments.

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Capping protein

A protein that caps the plus end of actin filaments, preventing further polymerization.

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Cofilin

A protein that binds to ADP-actin filaments and promotes their disassembly.

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Gelsolin

A protein that severs actin filaments and binds to the plus end, inhibiting further polymerization.

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Tropomyosin

A protein that stabilizes actin filaments and regulates the binding of other accessory proteins.

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Tropomodulin

A protein that binds to the minus end of actin filaments, preventing both assembly and disassembly.

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Formin

A protein that nucleates the assembly of linear actin filaments, remaining associated with the plus end.

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Fimbrin

A protein that cross-links actin filaments into bundles, providing structural support.

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What is the function of profilin in actin polymerization?

Profilin is a protein that binds to actin monomers, preventing them from attaching to the minus end of an actin filament. This allows the actin monomer to bind to the plus end of the filament, promoting polymerization in the forward direction.

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What is the role of the Arp2/3 complex in actin polymerization?

The Arp2/3 complex is a protein complex that nucleates new actin filament growth, branching off from existing filaments.

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How are actin filaments stopped from growing?

Capping proteins bind to the plus end of actin filaments, preventing further addition of monomers and stopping elongation.

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What is the role of cofilin in actin dynamics?

Cofilin is a protein that breaks down actin filaments by promoting depolymerization at the minus end.

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Describe the process of actin polymerization.

The process of actin filament assembly, where monomers are added to the plus end of the filament, promoting polymerization.

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Describe the structure of a myosin II thick filament.

A myosin II thick filament is composed of many myosin II molecules assembled tail-to-tail, with their heads projecting outward. This creates a central bare zone with no head domains.

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What is the central bare zone in a myosin II thick filament made of?

The central bare zone of a myosin II thick filament is composed entirely of myosin II tails and lacks heads, giving it a characteristic gap in the center.

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What is the purpose of the myosin II heads in a thick filament?

The head domains of myosin II molecules project outward from the thick filament, allowing them to interact with actin filaments and generate force.

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Skeletal Muscle Cell

A huge, multinucleated cell formed by the fusion of many muscle cell precursors called myoblasts.

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Myofibrils

The individual, cylindrical units within a muscle cell, responsible for muscle contraction.

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Sarcomere

The basic unit of contraction in a skeletal muscle cell, consisting of repeating units of myosin and actin filaments.

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Z Disc

The site of attachment for the plus ends of actin filaments in a sarcomere.

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M Line

The location of proteins that link adjacent myosin II filaments in a sarcomere, acting as the center of the sarcomere.

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Sarcoplasmic Reticulum (SR)

A network of interconnected tubules that surround myofibrils in muscle cells, serving as a reservoir and release point for calcium ions.

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T Tubules

Invaginations of the plasma membrane that run perpendicular to the muscle cell surface, allowing for the rapid propagation of action potentials.

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Calcium-Release Channel

A specialized protein complex located at the junction of the T-tubule and SR membranes, responsible for releasing calcium ions from the SR in response to muscle cell depolarization.

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Study Notes

Nucleotide Hydrolysis

  • Actin molecules carry a tightly bound ATP molecule
  • ATP is hydrolyzed to ADP soon after assembly into the polymer
  • Tubulin molecules carry a tightly bound GTP
  • GTP is converted to GDP after assembly into the polymer
  • Hydrolysis of the nucleotide reduces the binding affinity of the subunit for neighboring subunits
  • This makes the subunit more likely to dissociate from the filament
  • The T form (with ATP or GTP) usually adds to the filament
  • The D form (with ADP or GDP) usually leaves the filament
  • Hydrolysis of ATP or GTP must be replenished by a nucleotide exchange reaction of the free subunit.

ATP Caps and GTP Caps

  • The rate of addition of subunits to a growing actin or microtubule filament can be faster than the rate at which the bound nucleotide is hydrolyzed
  • Under such conditions, the end of the filament has a "cap" of subunits containing the nucleotide triphosphate
  • This forms an ATP cap on an actin filament
  • This forms a GTP cap on a microtubule

Treadmilling

  • Treadmilling is a dynamic instability behavior of cytoskeletal polymers, which is observed in actin filaments
  • This phenomenon accompanies polymer formation
  • Hydrolysis changes the critical concentration at each end of the filament
  • Polymerization can proceed until the concentration of free monomer reaches a value that exceeds the critical concentration for the plus end (C+) but is below the critical concentration for the minus end (C-)
  • This results in constant polymer length with a net flux of subunits through the polymer

ATP Hydrolysis Within Actin Filaments

  • Subunits with bound ATP (T-form subunits) polymerize at both ends of a growing filament
  • ATP is hydrolyzed within the filament, and the subunits then become D-form subunits
  • Elongation is faster than hydrolysis at the plus end, thus the terminal subunits are always in the T form
  • Elongation is slower than hydrolysis at the minus end as the terminal subunits are therefore always in the D form
  • The rate of addition of the subunits should be faster than the rate of ATP hydrolysis
  • The subunits with bound ATP usually add to the filament
  • The subunits with bound ADP or GDP usually leave the filament

Treadmilling Range

  • Treadmilling occurs at intermediate concentrations of free subunits
  • The critical concentration for polymerization on a filament end in the T form (Cc(T)) is lower than that for a filament end in the D form (Cc(D))
  • If the actual subunit concentration is between Cc(T) and Cc(D), the plus end grows while the minus end shrinks

Chemical Inhibitors of Actin and Microtubules

  • Actin:
  • Latrunculin: Depolymerizes actin filaments by binding to actin subunits
  • Cytochalasin B: Depolymerizes actin filaments by capping the plus ends
  • Phalloidin: Stabilizes actin filaments by binding along the filaments
  • Microtubules:
  • Taxol (paclitaxel): Stabilizes microtubules by binding along the filaments
  • Nocodazole: Depolymerizes microtubules by binding to tubulin subunits
  • Colchicine: Depolymerizes microtubules by capping both filament ends

Actin Filaments

  • Formins: Nucleate the growth of unbranched filaments
  • They can be cross-linked to form parallel bundles
  • They have binding sites for monomeric actin
  • The dimeric structure nucleates actin polymerization and remains associated with the rapidly growing plus end
  • Arp2/3 complex: Nucleates branched networks and remains associated with the minus end of the growing filaments
  • Profilin: Binds monomers and concentrates them at sites of filament assembly
  • Thymosin: Binds to subunits and prevents assembly in the cells
  • Tropomodulin: prevents assembly and disassembly at minus ends
  • Cofilin: binds ADP-actin filaments, accelerates disassembly
  • Gelsolin: severs filaments and binds to plus end
  • Fimbrin: filament bundling, cross-linking, and attachment to membranes
  • a-actinin: filament bundling, cross-linking, and attachment to membranes
  • Spectrin: filament bundling, cross-linking, and attachment to membranes
  • Plasma membrane: interaction sites with actin filaments
  • ERM: actin-associated proteins linked to the plasma membrane

Actin Filament Elongation

  • Polymerization of actin depends on further addition of actin monomers at the plus end of each filament
  • Profilin blocks binding sites on the minus side of the actin monomer, forcing association with the plus end
  • Profilin falls off after a cycle, which allows a new monomer to be added to the plus end

Actin at the Cell Cortex

  • Actin is organized into arrays in the cell cortex
  • This includes branched networks, parallel bundles, and combinations that are initiated by proteins like Arp2/3 complex (branched networks) and formins (bundles)
  • These structures and dynamics generate various cell shapes and properties, such as filopodia and lamellipodia

Myosin and Actin

  • The actin cytoskeleton can form contractile structures
  • Myosin motor proteins are involved in cross-linking and sliding, allowing for movement
  • Contractile actin structures are involved in functions such as cell migration
  • Myosin drive muscle contraction

Actin-based Motor Proteins

  • Myosin motor proteins generate force for muscle contraction
  • Myosin II is formed from two heavy chains, and two copies of each light chain
  • Their structure includes a globular head domain that generates force
  • They have an extended coiled-coil tail region that mediates dimerization

Sliding of Myosin II Along Actin

  • Muscle contraction involves ATP-driven sliding of actin filaments against myosin II filaments
  • This creates a shortening movement in muscles, including skeletal, cardiac, and smooth muscle

A Sudden Rise in Cytosolic Ca2+

  • A sudden increase in cytosolic Ca2+ triggers muscle contraction
  • Ca2+ floods the muscle cell cytosol - from the sarcoplasmic reticulum
  • The rise is transient, with Ca2+ pumped back into the sarcoplasmic reticulum quickly
  • This allows relaxation (The Ca2+ pumps are ATP-driven)

Other Actin Functions

  • Severing proteins, like gelsolin, regulate filament depolymerization by breaking filaments
  • Filament severing changes the physical properties of the cytoplasm
  • Actin-severing proteins, like cofilin, can promote the rapid disassembly of actin filaments

Bacteria and Actin Cytoskeleton

  • Pathogens like Listeria monocytogenes activate and use the Arp2/3 complex to build actin filaments
  • Filaments push the bacteria through the cytoplasm
  • This allows them to move and invade other cells

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