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

    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</p> Signup and view all the answers

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

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

    During contraction, actin and myosin filaments shorten in length.

    <p>False</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.</p> Signup and view all the answers

    Myosin heads move actin filaments in the absence of ATP.

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

    Cofilin enhances polymerization of actin filaments at the plus end.

    <p>False</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.</p> Signup and view all the answers

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

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

    Phalloidin is a chemical that destabilizes actin filaments.

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

    Taxol is a chemical that depolymerizes microtubules.

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

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

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

    Signup and view all the answers

    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.</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.</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).</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.</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.</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.</p> Signup and view all the answers

    Which state predominates in microtubules due to nucleotide hydrolysis?

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

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

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

    In which form do subunits generally add to the filament?

    <p>T form</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.</p> Signup and view all the answers

    What two behaviors are associated with nucleoside triphosphate hydrolysis?

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

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

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

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

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

    Which chemical stabilizes actin filaments by binding along their length?

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

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

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

    How does cofilin influence actin dynamics?

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

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

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

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

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

    What role does thymosin play in actin filament dynamics?

    <p>Prevents actin subunits from polymerizing</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.</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.</p> Signup and view all the answers

    What happens to the myosin heads in their relaxed state?

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

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

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

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

    <p>ActA</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.</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.</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.</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.</p> Signup and view all the answers

    Where is actin polymerization focused in bacterial cells?

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

    Which statement about capping protein is correct?

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

    What role does cofilin play in actin filament dynamics?

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

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

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

    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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