Molecular Motors Flashcards
Define the properties of a motor protein:
Energy source - hydrolysis of ATP
Movement along cytoskeletal filaments - either microtubules or actin filaments
Motor proteins can bind to specific cargo like organelles and transport them around the cell
Some exhibit processivity, which refers to their ability to take multiple steps along a filament without detaching
What are kinesins?
Microtubule based motor proteins
Typically move towards the plus (+) end of microtubules (away from the center of the cell, towards the periphery)
Function - involved in transporting vesicles, organelles, and proteins toward the cell’s periphery in mitosis
Light chains are associated at the cargo-binding domain
Different isoforms appear to specialise in particular types of cargo
The globular ATPase heads of the heavy chains may be located at either the N or C-terminus of the protein
What are dyneins?
Microtubule based motor proteins
Move towards the minus (−) end of microtubules (toward the cell center)
Function - responsible for transporting cellular cargo toward the center of the cell and cilia + flagella movement
2 or 3 heavy chains and a variable number of associated intermediate and light chains
What are the 2 subtypes of dyneins?
Cytoplasmic dynein - Involved in intracellular transport.
Axonemal dynein - Powers the beating of cilia and flagella for movement
What are actin based motor proteins?
Proteins involved are myosin’s which typically move towards the plus (+) end of actin filaments.
Function - muscle contraction, cell motility + cargo transport
What are the 2 types of myosin and their functions?
Myosin II - Drives muscle contraction by pulling actin filaments in sarcomeres.
Myosin V - Involved in cargo transport (vesicles and organelles) along actin filaments.
What is the difference between myosin II and myosin V movement along actin?
Movement of Myosin-V along actin is processive, meaning that myosin V remains attached to an actin filament as it walks along that filament
Needed to transport organelles along actin filament
Myosin-II is a non-processive motor that detaches from actin at a stage of each reaction cycle
Describe the difference between cargo attachment in kinesins and dyneins:
Kinesins are specialised for different cargos and bind directly to their cargo
Dyneins bind their cargo through a multi-molecular complex called dynactin
Dynactin utilises a small actin-like filament composed of Actin-Related Protein-1 (Arp-1) to locate a range of other cargo-related proteins for attachment
Describe the mechano-chemical cycle of kinesin:
1 - At the start of each step the ATP-bound rear head is tightly attached to the track and the ADP-bound front head is loosely attached
2 - In the motor-stroke the rear head is propelled forward by the exchange of ADP for ATP in the front motorhead
3 - This binding of ATP in the front motorhead causes a small “neck-linker” peptide that links the head to the coiled coil, to change it direction
4 - This action pulls the rear motorhead forward as by this time hydrolysis of it’s ATP has released it from the microtubule
What are the 3 typical domains of motor proteins?
Motor (head) domain - binds to ATP and interacts with the cytoskeletal filament to generate motion
Linker/ neck region - transmits force from the head domain to the cargo-binding domain, helping with the stepping mechanism
Tail (cargo-binding) domain: Attaches to the cargo, ensuring the motor protein carries its load to the appropriate location
Describe the movement of myosin II in muscle contraction:
In muscle the coiled tails of Myosin-II are immobilised within the thick filaments of the sarcomere and the lever-arm action of their heads move the actin filaments past them
Myosin II heads bind to actin filaments, undergo conformational changes powered by ATP hydrolysis, and pull the actin filaments during the power stroke
This process repeats, driving the sliding of actin filaments over myosin, which shortens the sarcomere and contracts the muscle
What is the structure of myosin?
Myosin is a dimer consisting of 2 globular heads and a supercoiled tail
Globular myosin heads exhibit - intrinsic ATPase activity and an actin binding site
A set of light chains are associated with each head
Where do actin filaments insert?
Actin filaments insert at Z discs with plus ends
Compare the different modes of movement in motor proteins in relation to their cellular function - kinesins and dyneins
Kinesins’ movement is a processive walking
Suited for anterograde transport moving cargo like organelles and vesicles toward the cell periphery
Ideal for long-distance, uninterrupted transport.
Dyneins’ use a power stroke mechanism for movement moving towards minus end (centre)
Handle retrograde transport moving organelles to cell centre, organising mitotic spindle and flagella + cilia movement
Compare the different modes of movement in motor proteins in relation to their cellular function - myosin II and myosin V
Myosin II specialises in force generation through a power stroke mechanism
Role is muscle contraction via binding of actin filaments
Moves towards plus end
Myosin V, in contrast, is a highly processive motor protein that moves along actin filaments without detaching for long distances. This makes it ideal for intracellular cargo transport, ensuring that vesicles and organelles are delivered to specific locations within the cell
Also moves towards plus end of cell
