Cytoskeleton 4: Actin Motors
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Questions and Answers

What is the step size taken by a motor that allows it to stay on one side of the actin filament?

  • 54 nm
  • 36 nm
  • 72 nm (correct)
  • 90 nm

Which type of muscle is characterized as non-striated?

  • Cardiac muscle
  • Smooth muscle (correct)
  • Cardiac and skeletal muscle
  • Skeletal muscle

What kind of motor class is muscle myosin categorized under?

  • Myosin IV
  • Myosin I
  • Myosin III
  • Myosin II (correct)

Which structure anchors myo II bipolar filaments in a sarcomere?

<p>M-line (D)</p> Signup and view all the answers

What is the orientation of actin thin filaments in relation to the thick filaments?

<p>Oriented in opposite directions (C)</p> Signup and view all the answers

What primarily fuels muscle contractions by providing ATP?

<p>Mitochondria (D)</p> Signup and view all the answers

What type of muscle function relies predominantly on autonomic control?

<p>Smooth muscle (B), Cardiac muscle (C)</p> Signup and view all the answers

How many sarcomeres make up a myofibril?

<p>Several collinear (D)</p> Signup and view all the answers

What role does ATP play in the mechanochemical cycle of myosin?

<p>It binds to the motor domain and is hydrolyzed. (B)</p> Signup and view all the answers

What determines the step size of a myosin motor?

<p>The length of the neck region and the calmodulin light chains. (D)</p> Signup and view all the answers

How do non-muscle myosin II and muscle myosin II differ?

<p>Non-muscle myosin II can be activated by phosphorylation. (B)</p> Signup and view all the answers

What sequence of positions do the myosin motor heads follow during their mechanochemical cycle?

<p>One head moves from the front to the rear while the other moves forward. (C)</p> Signup and view all the answers

What is the function of the converter region in myosin?

<p>It transmits movement to the lever arm. (A)</p> Signup and view all the answers

Which component is essential for the coordination of the mechanochemical cycle in dimeric myosins?

<p>The IQ motifs in the neck region. (D)</p> Signup and view all the answers

What impact does phosphorylation have on non-muscle myosin II?

<p>It induces the formation of a bipolar filament. (D)</p> Signup and view all the answers

Which statement about the mechanism of myosin is incorrect?

<p>The motor heads do not interact with actin filaments. (C)</p> Signup and view all the answers

What unique feature distinguishes Myo VI from other myosin motors?

<p>It moves towards the actin filament minus end. (B)</p> Signup and view all the answers

Which myosin motors are involved in transporting cargo to the tips of stereocilia?

<p>Myo III and XV (D)</p> Signup and view all the answers

What is the power stroke distance of Myo II towards the barbed end of the actin filament?

<p>5 nm (B)</p> Signup and view all the answers

What does the term 'treadmilling' refer to in actin filaments?

<p>The continuous turnover of actin monomers. (B)</p> Signup and view all the answers

How does Myo V's power stroke distance compare to that of Myo II?

<p>It is longer than Myo II's stroke. (B)</p> Signup and view all the answers

What is the primary function of Myo X in cellular processes?

<p>Moving cargo to the tips of filopodia. (C)</p> Signup and view all the answers

What characteristic of the lever arm in Myo VI is distinct from that of Myo V?

<p>It undergoes a unique conformational change. (D)</p> Signup and view all the answers

What technique is used to examine motor movement along an actin filament?

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

What is the role of titin in muscle contraction?

<p>Provides structural support and elasticity (C)</p> Signup and view all the answers

What is the primary function of myo II filaments during muscle contraction?

<p>To create tension by pulling actin filaments (A)</p> Signup and view all the answers

What happens to the sarcomere during muscle contraction?

<p>The Z-discs move closer to the M-line (A)</p> Signup and view all the answers

What initiates muscle contraction in striated muscles?

<p>Neurotransmitter release from a neuron (A)</p> Signup and view all the answers

Which structure serves as the anchor point for myo II filaments?

<p>M-line (A)</p> Signup and view all the answers

What is the effect of ATP depletion on muscle contraction?

<p>Causes rigor mortis (A)</p> Signup and view all the answers

In the sarcomere, what does the lattice-like arrangement of multiple sarcomeres enable?

<p>Uniform contraction across the muscle (C)</p> Signup and view all the answers

How do myosin heads interact with actin filaments during contraction?

<p>They 'walk' toward the Z-discs (B)</p> Signup and view all the answers

What initiates the release of calcium from the sarcoplasmic reticulum during muscle contraction?

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

Which protein complex is responsible for regulating the binding of myosin to F-actin in striated muscle?

<p>Tropomyosin-troponin complex (D)</p> Signup and view all the answers

What happens to tropomyosin when calcium binds to troponin in the sarcomere?

<p>It changes its conformation and positions to unblock myosin binding sites (A)</p> Signup and view all the answers

In smooth muscle, what is activated by calcium-bound calmodulin to trigger contraction?

<p>Myosin light chain kinase (MLCK) (A)</p> Signup and view all the answers

What role does phosphatase play in muscle contraction and relaxation?

<p>It dephosphorylates the myosin light chain, leading to relaxation (D)</p> Signup and view all the answers

When the calcium levels fall, what happens to the tropomyosin in the sarcomere?

<p>It moves to block the myosin binding site (B)</p> Signup and view all the answers

In smooth muscle, how is F-actin anchored throughout the cell?

<p>By integrins and focal adhesions complexes (C)</p> Signup and view all the answers

What does myosin light chain kinase (MLCK) do once activated?

<p>It phosphorylates the myosin light chain, activating the myosin motor (C)</p> Signup and view all the answers

Flashcards

Myosin's mechanochemical cycle

Myosin's movement along actin filaments is driven by cycles of ATP binding, hydrolysis, and release, causing conformational changes that create the power stroke.

Myosin motor domain

The part of myosin responsible for binding to actin and hydrolyzing ATP to drive movement.

Myosin II dimer

A two-headed myosin structure, essential for coordinated movement along actin filaments and processivity.

Myosin lever arm

The part of myosin that transmits the energy from ATP hydrolysis into motion, resulting in the power stroke.

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Non-muscle myosin II activation

Non-muscle myosin II is activated by phosphorylation, leading to bipolar filament formation and contraction.

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Myosin step size

The distance myosin moves along actin in a single step, determined by the length of the myosin neck region and the associated light chains.

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Processivity

The ability of a myosin motor to continuously move along an actin filament without detaching.

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Myosin light chains

Proteins that bind to the neck region of myosin and regulate its activity.

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Myosin VI movement

Myosin VI moves along actin filaments in the opposite direction of other motors, towards the minus end.

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Myosin V movement

Myosin V moves along actin filaments in the plus (barbed) direction, moving cargo.

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Power Stroke (Myosin II)

The movement of Myosin II during a power stroke, approximately 5 nm towards the plus (barbed) end.

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Power Stroke (Myosin V)

A longer lever arm allows Myosin V a power stroke approximately 36 nm towards the plus end.

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Power Stroke (Myosin VI)

Myosin VI's power stroke is 11 nm towards the minus end; the converter region moves in a different way.

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Motor head step size (Myosin V)

The movement of one head of a Myosin V dimer is approximately 73.75 nm. It accounts for the coordinated movement of both motor heads.

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Actin Filament Polarity

Actin filaments have plus (barbed) and minus (pointed) ends; motor proteins move along these ends in specific directions.

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Myosins and Cargo Transport

Different myosins transport different cargo to specific locations within the cell, like filopodia tips or stereocilia.

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Actin Filament Half-Turn

36 nm of actin filament corresponds to half a turn, affecting Myosin step size and binding.

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Myosin Step Size

Optimal Myosin step size is 72nm, allowing Myosin to stay on one actin filament side.

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Muscle Fiber Cell Structure

Multinucleated cells formed by myoblast fusion, containing many myofibrils which contain many sarcomeres.

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Sarcomere Structure

The basic contractile unit of muscle, with bipolar Myosin II filaments anchored at the M-line, and actin filaments (thin filaments) at the Z-disc.

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Myosin II Bipolar Filament

Myosin II dimers arranged in bipolar bundles, forming thick filaments in muscles and engaging with actin filaments.

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Muscle Tissue Types

Skeletal, cardiac, and smooth muscles; Skeletal and cardiac are striated, smooth is not; they differ in their activation pathways.

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Myosin II Filament Orientation

Myosin II filaments are oriented in opposing directions, interacting with actin filaments also oriented in opposite directions, all to ensure coordinated pulling force.

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Myosin II Motor Direction

Myosin II motors are plus end directed, crucial for the pulling motion that shortens the sarcomere during muscle contraction.

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Sarcomere Contraction

Myosin pulls actin filaments, shortening the sarcomere and generating muscle contraction.

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Myosin Movement

Myosin heads move along actin filaments, powered by ATP.

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Myosin-Actin Cycle

ATP binding, hydrolysis, and release drive the myosin head's movement along actin.

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Rigor Mortis

Muscle stiffness after death due to lack of ATP.

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Calcium's Role in Contraction

Calcium triggers muscle contraction by activating proteins.

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T-Tubules

Network of tubules that transmit electrical signals deep into muscle cells.

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Striated Muscle Contraction

Neuron releases acetylcholine to initiate contraction in skeletal and cardiac muscle.

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Sarcomere Structure

Repeating units of muscle fibers, composed of actin and myosin filaments.

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Calcium's role in muscle contraction

Calcium triggers the release of calcium from the sarcoplasmic reticulum, binding to troponin, and causing tropomyosin to shift, enabling myosin to interact with actin, leading to contraction.

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Tropomyosin's function

Tropomyosin is a structural protein wrapped around F-actin, covering myosin binding sites in a relaxed muscle. Its conformation changes when bound to calcium, exposing binding sites to allow muscle contraction.

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Smooth Muscle Contraction

Calcium activates myosin light chain kinase (MLCK), which phosphorylates myosin. Then, the phosphorylated myosin can interact with actin, causing contraction.

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Sarcoplasmic Reticulum

The specialized endoplasmic reticulum for muscle cells that stores and releases calcium, playing a role in muscle contraction.

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Myosin Light Chain Kinase (MLCK)

An enzyme activated by calcium-bound calmodulin to phosphorylate myosin light chains, leading to smooth muscle contraction.

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Troponin's function

Troponin is a protein that, when bound by calcium, modifies tropomyosin, enabling muscle contraction.

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Skeletal Muscle Contraction Mechanism

Calcium release triggers a conformational change in troponin and tropomyosin, exposing myosin-binding sites on actin, allowing myosin to bind and contract the muscle.

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Role of F-actin

F-actin are the thin filaments in muscle fibers that interact with myosin to cause contraction. The filament provides myosin with a place to bind to contract the muscle.

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Study Notes

Cytoskeleton 4: Actin Motors & Regulators

  • Myosin's powerstroke generates force.
  • Myosin "walks" along actin filaments with polarity.
  • Muscle actin and non-muscle myosin perform different biological functions.
  • Muscle contraction mechanisms detailed.

Myosin Motors

  • Myosin contains an actin-binding motor domain that binds ATP.
  • ATP hydrolysis drives the mechanochemical cycle of myosin's movement along F-actin.
  • Two motor heads coordinate stepping on the F-actin filament.
  • The motor and neck region structure determines the step size of each movement.
  • The ATP hydrolysis cycle defines the force generation.

Myosin Motor Movement

  • Movement within the converter region relays the power stroke to the lever arm and neck.
  • The dimerization domain moves the rear head to the front.
  • IQ motifs, including calmodulin (CaM), regulate the myosin light chain, influencing rigidity.
  • CaM light chains and the neck length are critical to determine the step size.
  • Dimerization coiled coil domain is involved.
  • Myosin architecture contrasted with kinesin.

Myosin Motor: Mechanochemical Cycle

  • Myosin motor's nucleotide state monitored during its cycle.
  • Whether the motor head is bound to the filament or not is important.
  • The position of the lever arm relates to power stroke or recovery stroke.
  • Myosins cycle between binding and releasing from F-actin.

Non-Muscle Myosin II

  • Non-muscle myosin II is regulated by phosphorylation.
  • Phosphorylation leads to bipolar filament formation enabling interactions with F-actin for force generation.
  • Non-muscle myosin II functions include cell adhesion, cell migration, membrane trafficking, cytokinesis, and tissue remodeling.

Many Non-Muscle Myosins

  • Many non-muscle myosins exist beyond myosin II.
  • These myosins have various structures and walk mechanisms along F-actin.
  • Unique myosin VI walks towards the F-actin minus end.
  • Non-muscle myosins function in diverse cellular processes, including localization and cargo transport, shown in examples like Myo X and Myo III/XV during stereocilia tip transport.

Myosin Motor Polarized Motility

  • Kymography analysis examines motor movement and determines polarity along F-actin.
  • In vitro studies using fluorophore-labeled single motor dimers reveal different movement directions (Myosin V moves towards the plus end, while Myosin VI moves towards the minus end).

Myosin Power Stroke

  • Myosin II power stroke's direction is toward the barbed end (plus end) of F-actin.
  • The power stroke's distance is approximately 5 nm.
  • Myosin V's longer lever arm allows for a larger approximately 36 nm power stroke also toward the plus end.
  • Myosin VI's distinct power stroke mechanism, starting with the lever arm in the forward position and then moves it to the rear position, generates a 11 nm power stroke.

Myosin's Step Size

  • Myosin VI has a longer lever arm compared to Myosin II, allowing for a larger step size.
  • The average step size of Myosin V is found to be 73.75 nm, despite its 36 nm movement per motor head. This difference is due to the sequential movements of two heads, resulting in a larger macroscopic step.

Myosin's ATP Hydrolysis' Cycle

  • The myosin mechanochemical cycle is critical for bipolar filaments to work on F-actin generating large force.
  • Myosin lever arm movement location depends on the nucleotide-bound states (ATP or ADP) of the motor.
  • When myosin is bound to the red actin at the left, its net movement will be in the plus end direction.

Muscle Contraction

  • Myosin II movement, the bipolar heads walking along opposing F-actin filaments towards the plus end anchored at the Z-discs.
  • The myosin can't physically move, it pulls on the actin, moving the Z-discs closer to the M-line during muscle contraction.
  • The myosin heads release actin when calcium levels fall, causing muscle relaxation.

Molecular Signal for Muscle Contraction

  • Calcium (Ca2+) is the crucial molecular signal triggering coordinated actomyosin contraction in muscle.

Striated Muscle Contraction (Skeletal and Cardiac)

  • A neuron activates skeletal muscle via acetylcholine release, initiating an electrical impulse.
  • The electrical impulse travels deep into the muscle cells via T-tubules, triggering calcium release from the sarcoplasmic reticulum (a part of the endoplasmic reticulum).
  • This released calcium binds to troponin, causing tropomyosin to shift and expose myosin-binding sites on F-actin, enabling myosin to utilize its ATP hydrolysis mechanism for contraction.

Contraction in Smooth Muscle

  • Calcium is also an activator of muscle contraction, but it acts differently in smooth muscle compared to skeletal or cardiac muscle.
  • Smooth muscle involves activation of the myosin light chain kinase (MLCK) by calcium-bound calmodulin (CaM).
  • MLCK's action triggers myosin motor activation and subsequent muscle contraction.
  • Relaxation is initiated by a phosphatase dephosphorylating the myosin light chain.

Summary

  • Myosin's common structural framework is a motor domain with ATPase activity interacts with the actin filament.
  • Diverse non-muscle myosins perform various functions, including those involved in cellular localization, cargo transport, and other cellular processes.
  • Muscle cell architecture includes sarcomeres, T-tubules, and sarcoplasmic reticulum, enabling them to respond to a neuron's trigger for contraction followed by relaxation.

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Lecture 21 Actin Motors & Regulators PDF

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Explore the intricate dynamics of myosin motors and their role in muscle contraction in this quiz on Actin Motors and Regulators. Understand the mechanisms behind myosin's powerstroke, ATP hydrolysis, and the structural features that dictate movement along actin filaments.

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