Cytoskeletal Motor Proteins Overview

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Questions and Answers

What are the primary types of filament associated with myosin, kinesin, and dynein motor proteins?

Myosin associates with actin filaments, while kinesin and dynein associate with microtubules.

Explain how ATP hydrolysis contributes to the movement of motor proteins.

ATP hydrolysis provides the energy for conformational changes in the motor proteins, enabling them to 'walk' along their filament tracks.

Describe the role of the head and tail regions of motor proteins.

The head region determines the filament and direction of movement, while the tail region determines the cargo being carried.

How do kinesins and dyneins differ in their movement direction along microtubules?

<p>Kinesins typically move towards the plus end of microtubules, while dyneins move towards the minus end.</p> Signup and view all the answers

match the direction the motor protein walks towards

<p>dyneins = - end</p> <ul> <li>= - kinesins = + end myosin = + end of actin filament</li> </ul> Signup and view all the answers

Which part of the myosin motor protein is primarily responsible for binding to the actin filament?

<p>N-terminal head region (C)</p> Signup and view all the answers

all myosin move towards the plus end except myosin 6. (moves towards negative)

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

why myosin is responsible in muscle and non-muscle cell contraction and cytokinesis

<p>myosin 2</p> Signup and view all the answers

which myosin is involved in vesicle and organelle transport

<p>myosin 5</p> Signup and view all the answers

myosin 1 is involved in intracellular organization

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

dicuss what happens during muscle contraction beginning with a signal

<p>when a signal arrives to the myofilament, it results in the calcium voltage gated channels of the T-tubules to release calcium ions into the cytosol which will then bind to the calcium release channels in the sarcoplasmic reticulum membrane. These Ca2+ then bind on to troponin c causing change in conformation which result in tropomyosin exposing myosin binding site on the actin filament allowing for the myosin head to bind. At this stage the myosin head has hydrolyzed ATP and upon release of pi, there will be a conformational change, binding to the actin filament and resulting in a power stroke, upon release of ADP, a new ATP binds and a new cycle begins</p> Signup and view all the answers

Which of the following statements accurately describes the relationship between nucleotide binding and the association of motor proteins with their respective tracks?

<p>Myosin binds tightly to actin when ADP is bound. (A), Kinesin binds tightly to microtubules when ATP is hydrolyzed. (C)</p> Signup and view all the answers

Based on the information provided, which of the following statements best describes the similarity between myosin and dynein?

<p>Both proteins exhibit a tight association with their tracks in the absence of a nucleotide. (C)</p> Signup and view all the answers

how does MLCK regulate myosin activity

<p>when myosin is inactive, its tail is folded on itself and blocking the acting binding site but upon phosphorylation of the light chains by myosin light chain kinase, the tail unfolds and exposes the actin binding site</p> Signup and view all the answers

How does ATP play a role in muscle contraction?

<p>ATP is essential for the sliding motion of actin against myosin filaments during muscle contraction.</p> Signup and view all the answers

What types of muscles are responsible for involuntary movements?

<p>Cardiac and smooth muscles are responsible for involuntary movements.</p> Signup and view all the answers

Describe how skeletal muscle contraction differs from involuntary muscle contraction.

<p>Skeletal muscle contraction is voluntary and relies on conscious control, whereas involuntary muscle contraction occurs automatically.</p> Signup and view all the answers

What is the primary structural unit of skeletal muscle responsible for contraction?

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

The Z disc in a sarcomere is where thick filaments are anchored.

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

Match the following components of the sarcomere with their functions:

<p>Z disc = Anchors thin filaments M line = myosin filaments Actin filaments = Form the light bands Myosin filaments = Form the dark bands</p> Signup and view all the answers

what end of the actin filament is anchored to the z disc

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

what are the subunits that make up the troponin complex

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

why is Ca2+ stored in the SR and not else where

<p>storage in the SR allows for quick contraction and relaxation of the muscle, ensuring that it is always available to cycle</p> Signup and view all the answers

What is the primary function of the protein titin in the sarcomere?

<p>To act as a spring and provide elasticity to muscles (D)</p> Signup and view all the answers

CapZ and tropomodulin are proteins that regulate the length of actin filaments by capping their ends.

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

Why is nebulin referred to as a "ruler" in the context of the sarcomere?

<p>Nebulin provides information about the length of the sarcomere at any given time.</p> Signup and view all the answers

The ______ protein acts as a spring, giving muscle its elasticity.

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

Match the following accessory proteins with their primary function in the sarcomere:

<p>CapZ = Capping the plus end of actin filaments Nebulin = Acting as a ruler to measure the sarcomere length Tropomodulin = Capping the minus end of actin filaments Titin = Providing elasticity to the sarcomere</p> Signup and view all the answers

Match the following molecular processes with their effects on muscle cells:

<p>cAMP increase = Activates protein kinase A (PKA) PKA activation = Inactivates myosin light chain kinase (MLCK) MLCK inactivation = Reduces muscle contraction strength Point mutations in myosin = Can lead to serious heart disease</p> Signup and view all the answers

Match the following signaling molecules with their roles in smooth muscle activity:

<p>Adrenaline = Regulates contractile activity via cAMP cAMP = Activates PKA to modulate MLCK PKA = Phosphorylates proteins to inhibit contraction Calcium ions (Ca2+) = Trigger muscle contraction via interaction with troponin</p> Signup and view all the answers

mutations in what cause familial hypertrophic cardiomyopathy

<p>cardiac β myosin heavy chain, myosin light chains, troponin, and tropomyosin.</p> Signup and view all the answers

familial hypertrophic cardiomyopathy is a genetically recessive condition

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

mutation in what resulting in dilated cardiomyopathy causes early heart failure

<p>minor missense mutation in cardiac actin gene</p> Signup and view all the answers

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Flashcards

Motor proteins

Proteins that move along cytoskeletal filaments and transport cargo within cells.

Motor domain

A region on a motor protein that binds to and hydrolyzes ATP, powering movement along a filament.

Myosins

A group of motor proteins that move along actin filaments, often involved in muscle contraction.

Kinesins

A group of motor proteins that move along microtubules, often transporting cargo towards the plus end.

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Dyneins

A group of motor proteins that move along microtubules, often transporting cargo towards the minus end.

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Myosin's affinity for Actin

In the absence of any nucleotide (like ATP), the motor protein myosin binds tightly to its actin track.

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Kinesin's ATP-dependent microtubule binding

The motor protein kinesin forms a strong bond with microtubules when it has ATP bound to it.

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Dynein's nucleotide-free microtubule binding

Similar to myosin, dynein strongly binds to microtubules when it's in its nucleotide-free state.

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Motor protein movement mechanism

Myosin, kinesin, and dynein all use ATP hydrolysis to power their movement along cytoskeletal filaments.

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Directionality of motor proteins

Myosin moves along actin filaments, kinesin moves towards the plus end of microtubules, and dynein moves towards the minus end of microtubules.

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Actin-Myosin Sliding Filament Mechanism

The process by which actin and myosin filaments slide past each other, powered by ATP, leading to muscle contraction. This sliding movement is responsible for various bodily functions, including running, walking, heart pumping, and gut peristalsis.

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Actin Filament

A type of protein filament that makes up part of the cytoskeleton and is involved in muscle contraction. It interacts with myosin to generate force and movement.

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Myosin Filament

A type of protein filament that interacts with actin filaments to cause muscle contraction. Myosin acts as a motor protein, converting chemical energy from ATP into mechanical force.

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Skeletal Muscle

A type of muscle that is responsible for voluntary movements, like running or lifting weights. It's attached to bones and contracts in response to signals from the nervous system.

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ATP in Muscle Contraction

The primary energy source for muscle contraction, powering the sliding filament mechanism. ATP is broken down to release energy, which fuels the interaction between actin and myosin.

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Myofibril

A single, long, cylindrical structure within a muscle cell, composed of repeating units called sarcomeres.

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Sarcomere

The basic unit of muscle contraction, consisting of overlapping thin (actin) and thick (myosin) filaments.

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Z Disc

A specialized protein complex that anchors thin filaments (actin) at the edges of the sarcomere.

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Thin Filament

A protein filament composed of actin, responsible for muscle contraction by interacting with myosin.

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Thick Filament

A protein filament composed of myosin, responsible for muscle contraction by interacting with actin.

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Nebulin

A protein that regulates the length and spacing of the sarcomere, acting like a ruler.

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CapZ

A protein that caps the plus end of actin filaments, increasing their stability.

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Tropomodulin

A protein that caps the minus end of actin filaments, increasing their stability.

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Titin

A protein that acts like a spring, giving muscle elasticity and helping to maintain sarcomere structure.

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Half-life of actin filaments

The time it takes for a protein to break down and be replaced. Capping proteins like CapZ and Tropomodulin increase the half-life of actin filaments in muscle cells.

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Protein Kinase A (PKA)

A protein kinase that is activated by cAMP, leading to phosphorylation and inactivation of MLCK, which ultimately relaxes smooth muscle.

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Myosin Light Chain Kinase (MLCK)

The enzyme responsible for phosphorylating myosin light chains (MLC), leading to smooth muscle contraction.

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Familial Hypertrophic Cardiomyopathy

A dominant genetic condition caused by mutations in genes encoding cardiac proteins, resulting in thickening of the heart muscle (hypertrophy).

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Dilated Cardiomyopathy

A genetic condition caused by missense mutations in the cardiac actin gene, leading to a weakened and dilated heart muscle, often resulting in heart failure.

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Study Notes

Motor Protein Differences

  • Motor proteins associate with different filaments (e.g., actin, microtubules).
  • They move in various directions and carry diverse cargo.
  • Some transport organelles within the cell.
  • Others generate force for muscle contraction by causing cytoskeletal filaments to interact. This includes the contraction of skeletal, cardiac, and smooth muscles.
  • Myosin has a tight binding to actin when no nucleotide is present.
  • Kinesin binds tightly to microtubules when ATP is bound.
  • Dynein, similar to myosin, also has tight binding to microtubules in the nucleotide-free state.
  • Muscle contraction, including running, walking, and swimming, relies on organized actin and myosin filaments.
  • Skeletal muscle fibers are large, single cells formed from the fusion of multiple cells.
  • Myofibrils are the contractile elements within these muscle cells.
  • Myofibrils are composed of repeating units, called sarcomeres.
  • Sarcomeres consist of overlapping thin (actin) and thick (myosin) filaments.
  • External signaling molecules like adrenaline regulate smooth muscle contraction.

Cytoskeletal Motor Protein Action

  • Motor proteins move along filaments through a "head" region (motor domain).
  • The head binds and hydrolyzes ATP, driving conformational changes for movement.
  • The head domain determines the filament type and the direction of movement.
  • The tail domain of the motor protein determines the cargo carried.
  • Muscle contraction involves the shortening of sarcomeres.
  • Adrenaline increases cAMP, activates PKA, which phosphorylates and inactivates MLCK.

Types of Cytoskeletal Motor Proteins

  • Myosins associate with actin filaments. Myosin II is critical for muscle contraction.
  • Kinesins and dyneins associate with microtubules (MTs).
  • Thin filaments are composed of actin and associated proteins, are anchored by plus ends to the Z-disk.
  • Thin filaments minus ends overlap thick filaments centrally in the sarcomere.
  • The light area in a sarcomere is primarily composed of actin filaments.
  • The dark area, the Z-disk, consists of capping proteins.
  • The dark band in the middle of the sarcomere is formed by the myosin filaments (M line).
  • The M-line is where individual myosin fibers join.
  • The "bare zone", a portion of the sarcomere, is where myosin filaments overlap.
  • The shrinking sarcomere gives rise to muscle contraction.
  • Heart cells express specific isoforms of cardiac muscle myosin and actin.

Sarcomere Regulation

  • Accessory proteins CapZ (plus end cap), nebulin (ruler), tropomodulin (minus end cap), and titin (spring) regulate organization, length, and spacing in the sarcomere.
  • Titin gives muscle elasticity, and is a protein 1 μm in length.
  • The capping proteins extend the half-life of the actin subunits (several days in muscle, several minutes in most other cell types).
  • Nebulin is called a ruler because it provides information about the size of a sarcomere at any given time.
  • Familial hypertrophic cardiomyopathy results from mutations in genes encoding cardiac β myosin heavy chain, myosin light chains, troponin, and tropomyosin.
  • Minor missense mutations in cardiac actin cause dilated cardiomyopathy, often leading to early heart failure.

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