MCBL Cytoskeleton II: Muscle Contraction Flashcards by Daryl Traylor

What do actin’s contractile properties come from?

Actin’s contractile properties come from its interactions with Myosin motor proteins.

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What cell motions does actin perform?

Cell contraction/cell crawling

Cleavage furrow during cytokinesis

Cargo transport within the cell

Muscle cell contractions

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What does the myosin family of motor proteins do?

All myosin proteins bind actin and hydrolyze ATP to provide the energy for the movement of myosin along the actin filament

Most myosin motors walk along actin towards the + end

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How many different types of myosin are there?

At least 14 different types

All myosins share high homology in their amino acid sequence for the motor domain

Myosins vary in the C-terminus and some vary in the N-terminus

Most families of myosin are common to all/most eukaryotic cells

Most myosins function as a dimer but three familes do function as a monomer

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Describe the myosin I subfamily.

Myosin I is found in almost all eucaryotic cells

Has a single head w/ ATPase activity and a tail that binds cargo

Head interacts with actin filament at the F-actin binding domain and hydrolyzes ATP so that it can move along the actin filament

Tail binds to specific cargos

Different myosin I proteins vary in their C-terminal region and bind different cargos

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Describe the myosin II subfamily.

Myosin II is responsible for binding to muscles but can be found in non-muscle cells as well

Two subunits of myosin II interact to form a homodimer

Has two ATPase heads and long rod-like tails that form a coiled-coil

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What is myosin II always associated with?

Myosin II is always associated with contractile activity in muscle and non-muscle cells

In skeletal muscle, clusters of myosin II dimers bind at the tails to form the thick myosin filament

Myosin monomers and dimers show polarity

In the thick filament, the two sets of heads are pointing in opposite directions and are bipolar

One set of heads binds actin and pulls in one direction while the other binds actin and pulls in the opposite direction

This allows oppositely oriented actin filaments to slide past one another and allows for the generation of contractile forces in muscle

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Picture of myosin I and II.

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How is skeletal muscle organized?

Skeletal muscle is formed from parallel arrangements of multiple muscle fibers

A single skeletal muscle cell (fiber) runs the entire length of the muscle

Can be cm’s long and up to 50 um in diameter

Each muscle cell is formed by the fusion of many precurser cells

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True or false: Muscle fibers are multinucleated.

True; each muscle fiber retains the nuclei of the original cells

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Describe the orientation of the plasma membrane (sarcolemma) that surrounds each muscle fiber.

Each muscle fiber is surrounded by a plasma membrane known as a sarcolemma

The sarcolemma contains deep invaginations called T-tubules that run to the center of the fiber

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Describe the composistion of a muscle fiber.

Each muscle fiber is composed of multiple myofibrils that run parallel to one another along the long axis of the muscle

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What are myofibrils?

Myofibrils are the contractile elements of the muscle cell/fiber

Each can be as long as the muscle fiber itself and 1 to 2 um in diameter

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What is each myofibril composed of?

Each myofibril is composed of repeating units called sarcomeres

Sarcomeres are the individual units that make up the myofibril

Each sarcomere is 2.5 to 3 um in length

The sarcomeres give muscle its classic striated appearance

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Describe the composition of a sarcomere.

Each sarcomere is highly organized and made up of actin and myosin

Actin filaments make up the thin fibers

+ end of actin filaments interacts with Z-disc via the CapZ actin binding protein. This helps to stabilize actin filaments and prevent depolymerization. Holds them in regularly spaced orientation.

end of actin filaments orient towards center of sarcomere and overlap w/ myosin II filaments

Myosin II filaments are bipolar in arrangement and make up the thick filaments

M-line is the midpoint of the thick filament where the myosin heads begin to be arranged in the opposite directions

Z-disc is the point of attachment of actin filaments

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Picture of structure of sarcomere.

What are the purposes/functions of some of the actin binding proteins that are involved in the sarcomere?

Cap Z - Attachment point of end of actin filament at the Z line

Nebulin - Actin side binding protein that determines the length of each actin filament

Tropomodulin - Caps - end of actin filament and prevents depolymerization

Titin - Myosin II side binding protein that helps determine length of myosin and acts as a “spring” to help bring sarcomere back to appropriate length after too much stretch. Stretches Z disc into the thick filament where it interacts with myosin tails up to the M lines.

What is the Z-disc built from?

The Z-disc is made up of Cap Z proteins and alpha actin.

What is the I-Band?

This is a light band that spns two sarcomeres. It runs from Z line to the outer edge of the thick myosin filament.

What is the A-band?

The A-band is a dark band in the center of the sarcomere. It spans the length of the thick myosin filaments and may overlap with thin actin filaments.

What is the H-band?

The H band is a light band in the center of the sarcomere and interacts with only myosin. There is no actin filament overlap with myosin in this region.

What is the sliding filament model of muscle contraction?

This is the contraction (shortening) of a muscle due to the simultaneous shortening of all sarcomeres in series in a myofibril as actin filaments slide past the thick myosin filaments.

The length of actin and myosin does not change

The amount of overlap between the filaments changes. A band length stays constant; this is the length of myosin filament

I and H-band lengths decrease as more actin filament overlaps thick myosin filament.

These changes bring the Z-lines closer together and the length of the sarcomere is reduced from 3 um to 2 um

Sliding filament model of muscle contraction picture.

What is the sliding motion in the sliding filament model due to?

This is due to the interaction of myosin head groups with the actin filaments.

Multiple myosin head groups move along the actin filament towards the plus end.

This occurs rapidly and in repeating cycles.

Cycling is ATP ad Ca²⁺ dependent

At end of contraction, myosin head groups lose contact w/ actin filament and sarcomere relaxes back to original length

Describe, in detail, myosin cross bridge cycling.

1. ATP binds to myosin and is hydrolyzed by ATPase into ADP and phosphate. The energy released by this process activates the myosin head and cocks it into a high-energy, extended position. 2. The cocked myosin head binds to a newly exposed active site on the thin filament, generating a cross-bridge between actin and myosin. 3. Myosin releases the ADP and phosphate and returns to a low-energy position, pulling the thin filament along, and this movement is called a power stroke. Shortening occurs when the extensible region pulls the filaments across each other (like the shortening of a spring). Myosin remains attached to the actin. 4. The binding of [ATP](http://en.wikipedia.org/wiki/Adenosine_triphosphate "Adenosine triphosphate") destabilizes the myosin-actin bond, allowing myosin to detach from actin. While detached, ATP hydrolysis occurs "recharging" the myosin head. If the actin binding sites are still available, myosin can bind actin again. 5. The collective bending of numerous myosin heads (all in the same direction), combine to move the actin filament relative to the myosin filament. This results in muscle contraction.

Picture of myosin cross bridge cycling.

When ATP is NOT bound to myosin, what happens?

Myosin will stay bound to actin in a state of rigor.

How rapid is the muscle contraction?

A 30 cm muscle will shorten to 20 cm in 0.1 sec This occurs because of the simultaneous movement of all myosin heads in a sarcomere There are about 300 myosin heads per myosin thick filament all cycling at a high rate Each sarcomere in the myofibril is shortening at a synchronus fashion

Describe the morphological features of muscle fibers and their role in excitation-contraction coupling.

The plasma membrane (sarcolemma) of muscle contains deep invaginations that run to the center of the muscle fiber at regular intervals along each sarcomere. These invaginations are known as T tubules The sarcoplasmic reticulum (SR) is the main Ca²⁺ storage site in muscle cells The terminal cisternae are sacs on the SR that associate with the T tubules and form a structure known as a triad The terminal cisternae are sites of Ca²⁺ release

What is excitation-contraction coupling?

Excitation-contraction coupling is the induction of a muscle contraction via the depolarization of the plasma membrane.

What are the steps to excitation-contraction coupling?

1) Incoming action potential causes release of Ach into neuromuscular junction 2) Plasma membrane is depolarized and an action potential is triggered. voltage gated Na⁺ and K⁺ are activated 3) The AP moves deep into the muscle fiber down the t-tubules 4) The AP stimulates voltage sensitive dihydropyridine (DHP) receptors in the t-tubules 5) The DHP receptor is in close contact with RYR Ca²⁺ receptors in the terminal cisternae in the SR 6) The DHP receptor changes conformation and this pushes the RYR receptor open so that Ca²⁺ is released. This causes the muscle contraction. 7) The Ca²⁺ - ATPase pumps on the SR remove Ca²⁺ from the cytoplasm back into the SR. Ca²⁺ concentration is kept low when the muscle cell is at rest.

Picture showing steps of excitation contraction coupling.

What role does Ca²⁺ play in inducing a contraction in skeletal muscle?

Ca²⁺ regulates tropomyosin position.

Picture showing myosin and tropomyosin interactions.

What is tropomyosin and what does it do?

Tropomyosin is a long, rod-shaped protein that fits into a groove on the actin filament. It blocks the myosin head from binding actin when the muscle is at rest This is a physical block (steric hinderance)

What is troponin and what does it do?

Troponin is a complex of three protein subunits that associate with the ends of tropomyosin. They include a Ca²⁺ sensitive subunit that regulates tropomyosin position When the muscle cell is excited (depolarized), Ca²⁺ binds to troponin C This causes a conformational change in the troponin complex Troponin pulls tropomyosin out of the way and this exposes the myosin binding site on actin This allows myosin +ATPP to bind and move the actin filament

Picture showing Ca²⁺ , troponin and tropomyosin interactions in muscle cells.

If Ca²⁺ levels stay high within the muscle cell, what will happen?

So long as Ca²⁺ levels remain high in the cytoplasm, cross bridge cycling and muscle contractions will continue.