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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?
What is the step size taken by a motor that allows it to stay on one side of the actin filament?
Which type of muscle is characterized as non-striated?
Which type of muscle is characterized as non-striated?
What kind of motor class is muscle myosin categorized under?
What kind of motor class is muscle myosin categorized under?
Which structure anchors myo II bipolar filaments in a sarcomere?
Which structure anchors myo II bipolar filaments in a sarcomere?
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What is the orientation of actin thin filaments in relation to the thick filaments?
What is the orientation of actin thin filaments in relation to the thick filaments?
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What primarily fuels muscle contractions by providing ATP?
What primarily fuels muscle contractions by providing ATP?
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What type of muscle function relies predominantly on autonomic control?
What type of muscle function relies predominantly on autonomic control?
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How many sarcomeres make up a myofibril?
How many sarcomeres make up a myofibril?
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What role does ATP play in the mechanochemical cycle of myosin?
What role does ATP play in the mechanochemical cycle of myosin?
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What determines the step size of a myosin motor?
What determines the step size of a myosin motor?
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How do non-muscle myosin II and muscle myosin II differ?
How do non-muscle myosin II and muscle myosin II differ?
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What sequence of positions do the myosin motor heads follow during their mechanochemical cycle?
What sequence of positions do the myosin motor heads follow during their mechanochemical cycle?
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What is the function of the converter region in myosin?
What is the function of the converter region in myosin?
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Which component is essential for the coordination of the mechanochemical cycle in dimeric myosins?
Which component is essential for the coordination of the mechanochemical cycle in dimeric myosins?
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What impact does phosphorylation have on non-muscle myosin II?
What impact does phosphorylation have on non-muscle myosin II?
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Which statement about the mechanism of myosin is incorrect?
Which statement about the mechanism of myosin is incorrect?
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What unique feature distinguishes Myo VI from other myosin motors?
What unique feature distinguishes Myo VI from other myosin motors?
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Which myosin motors are involved in transporting cargo to the tips of stereocilia?
Which myosin motors are involved in transporting cargo to the tips of stereocilia?
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What is the power stroke distance of Myo II towards the barbed end of the actin filament?
What is the power stroke distance of Myo II towards the barbed end of the actin filament?
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What does the term 'treadmilling' refer to in actin filaments?
What does the term 'treadmilling' refer to in actin filaments?
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How does Myo V's power stroke distance compare to that of Myo II?
How does Myo V's power stroke distance compare to that of Myo II?
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What is the primary function of Myo X in cellular processes?
What is the primary function of Myo X in cellular processes?
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What characteristic of the lever arm in Myo VI is distinct from that of Myo V?
What characteristic of the lever arm in Myo VI is distinct from that of Myo V?
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What technique is used to examine motor movement along an actin filament?
What technique is used to examine motor movement along an actin filament?
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What is the role of titin in muscle contraction?
What is the role of titin in muscle contraction?
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What is the primary function of myo II filaments during muscle contraction?
What is the primary function of myo II filaments during muscle contraction?
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What happens to the sarcomere during muscle contraction?
What happens to the sarcomere during muscle contraction?
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What initiates muscle contraction in striated muscles?
What initiates muscle contraction in striated muscles?
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Which structure serves as the anchor point for myo II filaments?
Which structure serves as the anchor point for myo II filaments?
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What is the effect of ATP depletion on muscle contraction?
What is the effect of ATP depletion on muscle contraction?
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In the sarcomere, what does the lattice-like arrangement of multiple sarcomeres enable?
In the sarcomere, what does the lattice-like arrangement of multiple sarcomeres enable?
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How do myosin heads interact with actin filaments during contraction?
How do myosin heads interact with actin filaments during contraction?
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What initiates the release of calcium from the sarcoplasmic reticulum during muscle contraction?
What initiates the release of calcium from the sarcoplasmic reticulum during muscle contraction?
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Which protein complex is responsible for regulating the binding of myosin to F-actin in striated muscle?
Which protein complex is responsible for regulating the binding of myosin to F-actin in striated muscle?
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What happens to tropomyosin when calcium binds to troponin in the sarcomere?
What happens to tropomyosin when calcium binds to troponin in the sarcomere?
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In smooth muscle, what is activated by calcium-bound calmodulin to trigger contraction?
In smooth muscle, what is activated by calcium-bound calmodulin to trigger contraction?
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What role does phosphatase play in muscle contraction and relaxation?
What role does phosphatase play in muscle contraction and relaxation?
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When the calcium levels fall, what happens to the tropomyosin in the sarcomere?
When the calcium levels fall, what happens to the tropomyosin in the sarcomere?
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In smooth muscle, how is F-actin anchored throughout the cell?
In smooth muscle, how is F-actin anchored throughout the cell?
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What does myosin light chain kinase (MLCK) do once activated?
What does myosin light chain kinase (MLCK) do once activated?
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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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Description
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.
