L12: Molecular motors II part 2 Flashcards
how to regulate the activity of motor proteins?
All motors use atp.
Cell has to find ways to control action to also preserve energy.
Reg track- make track available or not.
Reg motor- switch on or off motor function
Reg attachment to cargo
regulate the track —> action of tropomyosin in miscle. microtubule acetylation may promote kinesin-1 binding.
regulate the motor–> autoinhibited folded conformations. activated by cargo, stress-sensing
regulate attachment to cargo–> kinesin light chains and cargo binding
regulating the track?
Myosin II in muscle is regulated by troponin, tropomyosin, and calcium ions (Ca²⁺).
Tropomyosin wraps around actin filaments and regulates myosin binding.
The position of regulatory complexes (troponin & tropomyosin) on actin depends on calcium levels.
Calcium (Ca²⁺) is the trigger for muscle contraction:
At low Ca²⁺ concentrations, tropomyosin blocks myosin-binding sites on actin, preventing contraction (steric blocking mechanism).
When Ca²⁺ binds to troponin, it induces a conformational change, causing tropomyosin to shift, exposing myosin-binding sites on actin.
Myosin can then bind to actin, leading to muscle contraction.
Contraction in vertebrate striated muscle is regulated by Ca²⁺ binding to troponin, which relieves tropomyosin’s inhibition of myosin binding.
The “track” = actin filaments, which myosin moves along. This mechanism regulates when and where myosin can bind, essentially controlling access to the track.
myosin-based regulation of muscle contractility?
Regulation at the Level of the Thick Filament
Myosin can exist in an ON or OFF state.
In he OFF state of myosin
(called Interacting-heads motif,
IHM) the ATP-ase of the two
heads is switched off.
In the “blocked head” the Actin-
binding domain is inhibited.
This shut-down conformation conserves energy when the muscle is at rest.
Activation of Myosin Motors (ON State)
Thick filaments need to be activated along with actin and myosin motors for contraction.
Myosin motors are released from the OFF state through:
Mechanical stress on the thick filament (mechanosensing) – force applied on the filament may induce activation.
Phosphorylation of the regulatory light chain (RLC) – a post-translational modification, e.g., by myosin light chain kinase (MLCK), which disrupts the IHM and makes myosin motors available for contraction.
Thin filaments are regulated by Ca²⁺, while thick filament activation may depend on mechanosensing and phosphorylation.
The number of myosin motors activated during contraction determines the force produced.
i.e:
In resting muscle most of the myosin motors
are inhibited (OFF) by folding onto the filament
surface.
Force depends on the number of myosin
motors activated during contraction.
Myosin motors are activated by the force
applied on the filament (mechanosensing) or
by phosphorylation of the regulatory light
chain.
structure of native myosin filament in the relaxed cardiac sarcomere?
Reconstructions of the thick filament reveals the three-dimensional organization of myosin, titin and myosin-binding protein C (MyBP-C, yellow).
regulating the motor?
Regulate the motor - kinesins are controlled by folded inactive conformations
* Motors often exist in folded inactive states
* Transition to activate state can be controlled by cargo binding and
post-translational modification
chat:
Inactive state: The cargo-binding domain folds onto the motor domain, blocking its activity.
Activation:
Cargo binding can induce a conformational change that releases the inhibition and activates the kinesin motor.
Another possibility is that phosphorylation of the cargo-binding domain may release the folded inhibition, activating the kinesin motor.
subunit composition of cytoplasmic dynein?
Processive motor. Need 2 dynein motor associated together by the tail? Overall motor has 2 stoke regions to work on microtubule.
But motor needs cargo bound to it, accessory proteins, particularly dyein and dynactin complex. Forming a large complex allowing motor to walk along microtubule.
Composed of two motor domain containing heavy chains
* Many light and intermediate chain involved in cargo binding and
regulation (IC, LIC, LC7, LC8, TCTEX)
* Cytoplasmic dynein form often associated with the dynactin
complex to link to cargo
activation of cytoplasmic dynein motility?
Activation of cytoplasmic dynein motility by dynactin-cargo adapter complexes
* Dynein by itself is inactive
* Dynein does not stably associate with dynactin
* Binding to cargo (adaptor), allows the formation of a tri-partite complex
* Only the tri-partite dynein, dynactin, cargo complex is capable of processive
Motility
Molecular motors are complicated. Assembly of this machinery reg function of this motor in the cell.
