10.2 Molecular motors: myosin, kinesin, and dynein
3 min read•august 1, 2024
Molecular motors are tiny protein machines that power cell movement and transport. , , and convert chemical energy from ATP into , enabling , cell division, and along cytoskeletal tracks.
These motors have unique structures tailored to their functions. Myosin works with actin filaments, while kinesin and dynein move along microtubules. Their cyclic interactions with cytoskeletal tracks, coupled with , generate force and directed movement within cells.
Molecular motors and cytoskeletal components
Types of molecular motors and their associated cytoskeletal components
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Myosin, kinesin, and dynein are the three main types of molecular motors
Myosin associates with actin filaments
Kinesin and dynein associate with microtubules
Molecular motors convert chemical energy from ATP hydrolysis into mechanical work
Enables movement along cytoskeletal tracks and cargo transport within the cell
Specific motor-cytoskeleton associations allow for directed movement and force generation in various cellular processes (muscle contraction, vesicle transport, cell division)
Role of molecular motors in cellular processes
Molecular motors are enzymes that convert chemical energy into mechanical work
Enable movement along cytoskeletal tracks and transport cargo within the cell
Specific associations between motors and cytoskeletal components allow for directed movement and force generation
Structure and function of myosin, kinesin, and dynein
Myosin structure and function
Large, multi-subunit protein with a globular and a long
Globular head domain binds to actin and hydrolyzes ATP
Long tail domain involved in cargo binding and thick filament formation
Responsible for muscle contraction and various other cellular movements
Examples: cytokinesis, cell migration, vesicle transport
Kinesin structure and function
Dimeric protein with two globular head domains, a neck linker, and a tail domain
Globular head domains bind to microtubules and hydrolyze ATP
Neck linker undergoes conformational changes
Tail domain binds cargo
Primarily transports cargo towards the plus end of microtubules in anterograde transport
Examples: transport of organelles, vesicles, and protein complexes
Dynein structure and function
Large, multi-subunit protein complex with two heavy chains and several intermediate, light intermediate, and light chains
Heavy chains contain globular head domains that bind to microtubules and hydrolyze ATP
Intermediate, light intermediate, and light chains involved in cargo binding and regulation
Mainly transports cargo towards the minus end of microtubules in retrograde transport
Examples: transport of organelles, vesicles, and protein complexes
Mechanisms of force generation and movement
Cyclic interaction between motor domains and cytoskeletal tracks
Molecular motors generate force and movement through cyclic interactions between motor domains and cytoskeletal tracks
Coupled with ATP hydrolysis
Myosin power stroke
Triggered by the release of inorganic phosphate from the myosin head
Causes a conformational change that pulls the actin filament, resulting in muscle contraction
Kinesin and dynein hand-over-hand mechanism
Two motor domains alternate in binding to the microtubule and undergoing conformational changes
Results in a stepping motion along the microtubule
Conformational changes and coordination of motor domains
Kinesin neck linker undergoes a conformational change upon ATP binding
Propels the unbound head forward to the next binding site on the microtubule
Leads to processive movement
Coordination of the two heads in kinesin and dynein is essential for efficient and directional movement
Regulation of their affinity for the microtubule track
ATP hydrolysis in molecular motor activity
ATP hydrolysis as the primary energy source
ATP hydrolysis provides the necessary chemical energy to drive conformational changes and force generation in molecular motors
Myosin
ATP binding causes dissociation from actin
ATP hydrolysis and subsequent release of inorganic phosphate and ADP lead to the power stroke and force generation
Kinesin and dynein
ATP binding and hydrolysis in the globular head domains cause conformational changes in the neck linker or linker domain
Drives the stepping motion along the microtubule
Coupling of ATP hydrolysis with mechanical cycles
Coupling of ATP hydrolysis with the mechanical cycles of molecular motors ensures efficient energy conversion into directed movement and force production
Rate of ATP hydrolysis and efficiency of energy transduction can regulate the speed and of molecular motors
Implications for their specific cellular functions (fast vs. slow muscle contraction, long-distance vs. short-distance transport)
Key Terms to Review (19)
ATP hydrolysis: ATP hydrolysis is the chemical reaction in which adenosine triphosphate (ATP) is broken down into adenosine diphosphate (ADP) and an inorganic phosphate (Pi), releasing energy that can be used for various cellular processes. This reaction is crucial for driving many biological activities, as it provides the necessary energy for functions like active transport, muscle contraction, and cell motility.
Bipolar kinesins: Bipolar kinesins are a type of molecular motor protein that can move along microtubules in opposite directions simultaneously, typically toward both ends of the microtubule. This unique feature allows bipolar kinesins to play crucial roles in processes such as cell division and intracellular transport by facilitating the movement of cellular cargo and maintaining proper organization of microtubules during mitosis.
Calmodulin binding: Calmodulin binding refers to the interaction between calmodulin, a calcium-binding messenger protein, and various target proteins or enzymes, which facilitates a range of cellular processes. This interaction is crucial for the regulation of molecular motors like myosin, kinesin, and dynein, as it helps mediate their activity in response to changes in intracellular calcium levels. The binding of calmodulin to its targets often induces conformational changes that activate or inhibit these proteins, linking calcium signaling to cellular movement and function.
Cargo transport: Cargo transport refers to the movement of various substances, such as organelles, proteins, and other materials, within cells facilitated by molecular motors. This process is crucial for maintaining cellular organization and function, ensuring that essential components reach their appropriate destinations in an efficient manner.
Dynein: Dynein is a type of molecular motor protein that moves along microtubules in cells, primarily transporting cellular cargo towards the minus end of the microtubule. This movement is essential for various cellular functions, including organelle transport, mitosis, and flagellar motion, highlighting its importance in both muscle contraction and cell motility.
Hand-over-hand model: The hand-over-hand model describes a mechanism of movement where one molecular motor protein binds to a filament and pulls another motor protein along, effectively resembling a relay system. This model is essential for understanding how molecular motors like myosin, kinesin, and dynein transport cellular cargo along cytoskeletal filaments in a coordinated manner.
Head domain: The head domain refers to a specific structural region of molecular motors like myosin, kinesin, and dynein that is primarily responsible for binding to the cellular cargo and facilitating movement along cytoskeletal filaments. This domain is crucial because it contains the ATP-binding site, which provides the energy needed for the motor protein's movement. The head domain's interaction with the filamentous structures of the cytoskeleton is essential for processes like muscle contraction, intracellular transport, and cell division.
Intracellular transport: Intracellular transport is the process by which substances, such as proteins, organelles, and other cellular components, are moved within a cell to their specific locations. This transport is essential for maintaining cellular organization and function, relying on specialized structures and molecular motors that facilitate movement along the cytoskeletal framework.
Kinesin: Kinesin is a type of molecular motor protein that moves along microtubules in cells, playing a crucial role in transporting cellular cargo such as organelles, vesicles, and proteins. This movement is essential for various cellular processes, including cell division and maintaining the organization of the cytoplasm, connecting kinesin to the functions of microtubules and other cytoskeletal components.
Mechanical work: Mechanical work refers to the process of energy transfer that occurs when a force acts upon an object, causing it to move in the direction of that force. In biological systems, this concept is crucial as it underpins how molecular motors like myosin, kinesin, and dynein convert chemical energy into mechanical movement, enabling essential cellular processes such as muscle contraction and intracellular transport.
Muscle contraction: Muscle contraction is the process by which muscle fibers generate force and shorten, enabling movement in the body. This physiological event is tightly linked to electrical signals, energy production, molecular interactions, and structural components that work together to facilitate movement and maintain cellular integrity.
Myosin: Myosin is a type of molecular motor protein that interacts with actin to facilitate muscle contraction and various forms of cellular movement. This protein plays a crucial role in converting chemical energy from ATP into mechanical work, making it essential for processes such as muscle contractions and cell motility. Myosin exists in several forms, but the most well-known is myosin II, which is primarily involved in muscle contraction.
Optical Tweezers: Optical tweezers are a powerful scientific tool that uses focused laser beams to trap and manipulate small particles, such as biological molecules and cells, at the nanoscale. This technique allows researchers to apply forces, measure interactions, and study dynamic processes in biomolecular recognition, cellular mechanics, and molecular motor function.
Phosphorylation: Phosphorylation is a biochemical process where a phosphate group is added to a molecule, typically a protein, which can change the molecule's function and activity. This modification is crucial for regulating various cellular processes, such as signaling pathways, energy metabolism, and the function of molecular motors and ion channels. It acts as a switch to turn activities on or off, making it vital for the dynamic regulation of cellular functions.
Power stroke model: The power stroke model is a concept describing the mechanism by which molecular motors, such as myosin, kinesin, and dynein, generate force and movement in cells. This model explains how these proteins utilize ATP hydrolysis to undergo conformational changes that enable them to 'walk' along cytoskeletal filaments, ultimately facilitating various cellular functions like muscle contraction and intracellular transport.
Processivity: Processivity refers to the ability of molecular motors to perform multiple consecutive actions without releasing their substrate. This characteristic is crucial for the efficient transport of cellular cargo and plays a significant role in cellular mechanics and organization. Processivity ensures that these motors can move along cytoskeletal filaments while attached, facilitating continuous movement and enabling effective functioning in various biological processes.
Single-molecule imaging: Single-molecule imaging is a powerful technique that allows researchers to visualize and analyze the behavior and interactions of individual molecules in real-time. This method provides insights into molecular dynamics and processes, particularly in the context of molecular motors like myosin, kinesin, and dynein, which are essential for cellular transport and movement. By tracking single molecules, scientists can uncover detailed mechanisms of action and the efficiency of these motors in biological systems.
Tail domain: The tail domain is a structural component of certain molecular motors, like myosin, kinesin, and dynein, which helps in cargo binding and determines the specificity of the motor's function. This region extends from the motor's head, enabling interaction with cellular structures or other proteins. The tail domain plays a crucial role in linking the motor to its cargo and can also influence the motor's stability and activity within cellular environments.
Unconventional myosins: Unconventional myosins are a class of myosin proteins that do not primarily function in muscle contraction but instead play crucial roles in various cellular processes such as vesicle transport and cell motility. Unlike conventional myosins, which mainly interact with actin filaments in muscle tissues, unconventional myosins can move along actin filaments in non-muscle cells, serving essential functions in intracellular transport, cell shape maintenance, and signaling.