L11: Essay format Flashcards
what are motor proteins and their tracks
Proteins that convert chemical energy into movement or mechanical work.
Cytoskeletal motors: Intracellular transport of proteins, nucleic
acids and membrane bound organelles
Muscle contraction
Cell migration
Mitosis / meiosis
Cell division (cytokinesis)
Chemical energy
ATP → ADP + Pi drives function of molecular motors
Other motors:
DNA and RNA polymerases, helicases,
ribosomes
Bacterial flagellar motor
Microtubules serve as tracks for kinesin and dynein motor proteins.
Microtubules extend from the nucleus to the cell periphery, forming a crosslinked network that helps in intracellular transport, cell shape, and organization.
Actin filaments serve as tracks for myosin motor proteins.
Actin filaments exist in two main arrangements:
Cortical actin: Just beneath the plasma membrane, providing structural support and helping in cell movement.
Inner actin bundles: Found deeper within the cell, often forming stress fibers for maintaining tension and structure.
These filaments act as scaffolds where motor proteins function.
Structure:
Microtubules are made of tubulin dimers (α-tubulin and β-tubulin). These dimers polymerize to form protofilaments, which then interact laterally to form a hollow cylindrical structure (microtubule).
Actin filaments are made of globular (G-actin) subunits that polymerize into filamentous (F-actin), forming a helical structure.
polarity of m and actin
Both microtubules and actin filaments have intrinsic polarity, meaning they have distinct plus (+) and minus (-) ends.
This polarity influences motor protein movement, ensuring that motors travel in a specific direction.
Actin Polymerization & ATP Hydrolysis
Actin monomers (G-actin) bind ATP before polymerizing into F-actin (actin filaments).
Once incorporated into the filament, ATP is hydrolyzed to ADP, providing energy for filament dynamics.
The actin filament forms a helical structure with a periodic repeat of ~36 nm.
Actin monomers assemble head-to-tail, giving the filament its polarity:
Plus (+) end: Faster-growing end.
Minus (-) end: Slower-growing, more depolymerization occurs here.
Motor proteins move directionally:
Most myosins move toward the plus end of actin filaments.
The movement direction for kinesins and dyneins (on microtubules) depends on their structure/ individual properties
common features of motors
a cargo binding region (top), linker region (often alpha helical coiled coil) (middle) and a cytoskeletal binding/motor domain (bottom).
Three Main Classes of Motor Proteins
Kinesins – Move along microtubules, mostly toward the plus (+) end.
Dyneins – Move along microtubules, but toward the minus (-) end.
Myosins – Move along actin filaments, mostly toward the plus (+) end.
General Structure of Motor Proteins
Cargo-binding region → Attaches to vesicles, organelles, or other cellular cargo.
Motor domain → Interacts with the cytoskeletal track (either microtubules or actin).
This is where ATP hydrolysis occurs, converting chemical energy into mechanical movement.
Linker region → Connects the cargo-binding region to the motor domain, allowing force transmission.
Dynein Specifics
Dynein’s motor domain is much larger than those of kinesin and myosin.
It has light chains that help stabilize a longer alpha-helical domain (possibly part of the “lever arm”).
Dynein moves toward the microtubule minus end, playing key roles in organelle positioning and intracellular transport.
kinesin family
N-kinesins (motor domain at the N-terminus) – mostly move toward the plus end of microtubules (e.g., Kinesin-1, Kinesin-4).
C-kinesins (motor domain at the C-terminus) – move toward the minus end of microtubules.
some kinesins (motor domain in the middle)
some are involved in microtubule depolymerization rather than transport. (research bipolar kinesins)
Examples of kinesin functions:
Kinesin-1 – specialized for vesicle & mRNA transport along microtubules.
Kinesin-4 – involved in chromosome positioning during mitosis.
Kinesin-13 – depolymerizes microtubules, important in mitotic spindle dynamics.
myosin family
Myosin Superfamily Overview
There are ~39 myosins in the human genome, each adapted for specific functions.
Myosins are actin-based motor proteins that use ATP hydrolysis to generate movement.
Different myosin families have structural and functional specialisations.
Myosin-2 Family (Muscle Contraction & Contractile Forces)
Structure:
Has two motor domains (heads) connected by an α-helical coiled-coil tail.
Forms filaments by polymerisation (multiple molecules work together).
Works in ensembles, meaning does not function as a single motor.
Function:
Generates contractile forces in muscle cells and non-muscle cells.
Moves hand-over-hand along actin filaments, pulling them for muscle contraction.
Found in sarcomeres of skeletal and cardiac muscle.
Myosin-5 (Intracellular Transport & Vesicle Movement)
Structure:
Two-headed myosin with an extended α-helical coiled-coil domain.
Has a cargo-binding tail that interacts with organelles, vesicles, and mRNA.
Function:
Vesicle and organelle transport along actin filaments (e.g., moves melanosomes in pigment cells).
Unlike myosin-2, it functions as a single molecule rather than in filaments.
Uses a hand-over-hand stepping mechanism with large steps (~36 nm).
Myosin-1 (Membrane & Cytoskeletal Linkage)
Structure:
Single-headed myosin with a short tail domain.
Lacks a coiled-coil domain, meaning it does not form filaments.
Function:
Membrane-cytoskeleton interactions, helping with endocytosis, exocytosis, and membrane tension regulation.
Often found in microvilli and cell surface projections
a
motors bind to and pull cytoskeletal filaments
Muscle contraction driven by the relative sliding of the interaction of mysoin with actin.
Myosin II forms filaments overlapping with actin filaments
* Myosin motors attach to actin and pull it in one direction
One motor working at a time.
Uses atp hydrolysis
Big change in conformation of the motor that pulls the actin filament in this reaction.
Gen force and shortening in the muscle
Molecular event responsible for muscle contraction.
dyenin family
Cytoplasmic dynein: The main function is in minus-end directed intracellular transport. It moves cargo toward the minus end of microtubules.
Axonemal dyneins: These are involved in the movement of cilia and flagella. They help in the beating motion of cilia and flagella, which are structures made of microtubules and other proteins.
motors and translocation
Motors can bind to and translocate on cytoskeletal filaments
motor domain interaction:
One motor domain of a motor protein (such as kinesin or myosin) binds to the microtubule or actin filament.
The other motor domain is not directly bound to the track and is free to move (flopping around).
ATP Hydrolysis Cycle:
The ATP binding and hydrolysis cycle drives the movement.
When ADP is replaced by ATP, the motor protein undergoes a conformational change.
This change allows the lagging motor domain (the one that was previously detached) to bind to the track, while the leading motor domain (the one ahead) detaches.
Hand-Over-Hand Movement:
The motor domains move in a hand-over-hand fashion, where one head detaches and moves forward, and then the cycle repeats. This results in the cargo being moved along the filament.
Myosin V on Actin Filaments:
Myosin V moves along actin filaments in a similar hand-over-hand manner.
The position of the myosin molecule changes over time during movement.
This motion can be visualized using techniques like Atomic Force Microscopy (AFM), where changes in the position of the head and the overall molecule can be captured and analyzed.
*check if right
principles of motor protein mechanism
Tracks are polar; polarity drives cargo movement in one direction.
Motors perform cyclical, asynchronous attachment and detachment cycles (one ATP used per cycle).
Processive motors: Stay attached for many steps (100s) without detaching (e.g., Myosin V).
Non-processive motors (e.g., muscle myosin): Detach after one step.
* Fuel: ATP + H20 ADP + Pi
Free energy from ATP hydrolysis (ΔG = 10^-19 J)
Work = force x step size
If efficiency is 50%: (efficiency of conversion of chemical energy into mechanical work)
0.5. 10-19 J = force x 10nm (order of magnitude displacement gen by motor in a single interaction?)→ force = ca 5pN (pN is 10-12 N)
Every motor uses 1 atp molecule in a single interaction
atpase cycle of myosin
- Myosin II is tightly bound to the actin filament in the absence of ATP
- The working stroke is linked to release of the hydrolysis products (ADP and Pi) from the
active site and drive filament sliding - Muscle myosin II is a ‘low duty ratio’ motor; for most of the ATPase cycle it is detached
from actin
Cyclical Interaction with Myosin Filament
In rigor condition (without ATP), myosin is strongly bound to actin but cannot generate force.
The next step: ATP binding triggers a loss of affinity between myosin and actin.
ATP hydrolysis occurs while the products (ADP + Pi) are still bound to the myosin motor domain, causing a conformational change and displacement of the myosin.
This displacement resets the motor, repriming it and getting it ready for the next force generation step.
The motor can then attach to actin, release Pi, and release ADP.
This release triggers the power stroke (or working stroke) — the step where the myosin motor generates force.
At the end of the power stroke, myosin returns to the rigor condition.
The different states (ADP + Pi and rigor) are elucidated by crystallizing myosin in the ADP + Pi state or rigor state.
the models
- Atomic Model = Detailed Structural View
This refers to the crystal structure of myosin bound to actin at different stages of the ATPase cycle.
It’s about atoms and angles: how parts like the converter domain, lever arm, and light chains shift at the atomic level.
It shows exactly how the 70° tilt happens and which domains change shape.
📌 Think of it like: “Zoomed-in molecular blueprint of the working stroke.”
🦾 2. Lever Arm Model = Functional / Mechanical View
This is a simplified, big-picture model focusing on how the lever arm acts like a rowing paddle.
It’s used to explain how movement and force are generated (displacement of 5–40 nm depending on the lever length).
It emphasizes the role of the lever arm in producing linear motion along actin.
📌 Think of it like: “The practical model of how the myosin ‘walks’ along actin.”
- Initial Setup
Myosin binds to actin with one motor domain and adopts a conformation where the lever arm is “up.”
- Initial Setup
The motor domain grabs onto actin.
The lever arm is in a “cocked” position, ready to swing.
🔹 2. ATP Hydrolysis Powers Movement
ATP is split, and Pi is released first. Then, ADP is released in a sequential manner.
ATP → ADP + Pi (this releases energy).
Pi leaves first, which triggers the power stroke (lever arm swing).
ADP leaves second, finalizing the movement.
🔹 3. The Power Stroke
The lever arm tilts by ~70°, causing the actin filament to slide.
The lever arm swings down by 70°, like a rowing oar.
This pulls the actin filament towards the center of a muscle cell (the M-line).
🔹 4. Conformational Changes
ATP hydrolysis is coupled with a conformational change in the converter domain…
The converter domain (a part of the motor) changes shape.
This change moves the lever arm and attached light chain.
The catalytic domain (the part that breaks ATP) stays the same.
🔹 5. What Actually Moves
The motor domain binds to actin and stays still. The lever arm swings.
The part that touches actin doesn’t move.
The lever arm is what swings, pulling the actin filament.
🔹 6. Size of the Movement
Lever arm tilts = 70°, displacement = 5–10 nm (or 30–40 nm for myosin V).
Myosin II (in muscle): ~5–10 nm step.
Myosin V (in cargo transport): longer lever arm = ~30–40 nm step.
More lever arm = bigger step = faster or farther movement.
🔹 7. Coordination of Two Heads
Leading head & lagging head work asynchronously (hand-over-hand).
Myosin has two heads.
One head pulls, while the other prepares for the next step.
They take turns, just like walking.
ATP causes the lagging head to release, swing forward, and reattach.
Each step is powered by ATP.
The heads communicate so they don’t both detach at the same time.
atp conditions
ADP-Vi and Pi Conditions
cytoplasmic dynein: rotation of the microtubule-binding stalk
ADP-Vi condition: In this state, the motor is “frozen” or blocked in a specific conformation, which prevents the motor from proceeding further in its cycle.
Pi condition: When Pi is released, it triggers a conformational change that leads to the motor’s movement, particularly in the power stroke.
Apo State (No Nucleotide Bound)
Rigor state: This is a condition where no nucleotide (ATP, ADP, or Pi) is bound to the motor, causing it to be in a strongly bound state to actin or microtubules, often preventing movement.
Merged State (Regions Overlapped)
In this state, certain regions of the motor protein are merged or overlapped. However, due to different conformations, not all regions can overlap, leading to specific shifts and tilting that are part of the stalk region.
The tilting in one domain induces a shift in the position of the stalk region, contributing to the motor’s movement.
Tail Domain and Stalk
Ring/AAA domains: These are found in the motor head and are involved in the ATPase cycle, coordinating with the movement of the stalk.
Stalk region: This part of the motor protein attaches to the microtubule and rotates, driven by the motor protein’s ATP hydrolysis.
Rotation in the ring induces tilting in the stalk, which shifts the microtubule by approximately 8nm. This movement is then amplified by the lever arm, generating a linear displacement of about 8nm per cycle
