MCBL Cytoskeleton II: Muscle Contraction

Question 1

What do actin’s contractile properties come from?

Answer 1

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

Question 2

What cell motions does actin perform?

Answer 2

Cell contraction/cell crawling

Cleavage furrow during cytokinesis

Cargo transport within the cell

Muscle cell contractions

Question 3

What does the myosin family of motor proteins do?

Answer 3

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

Question 4

How many different types of myosin are there?

Answer 4

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

Question 5

Describe the myosin I subfamily.

Answer 5

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

Question 6

Describe the myosin II subfamily.

Answer 6

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

Question 7

What is myosin II always associated with?

Answer 7

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

Question 8

Picture of myosin I and II.

Answer 8

Question 9

How is skeletal muscle organized?

Answer 9

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

Question 10

True or false: Muscle fibers are multinucleated.

Answer 10

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

Question 11

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

Answer 11

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

Question 12

Describe the composistion of a muscle fiber.

Answer 12

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

Question 13

What are myofibrils?

Answer 13

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

Question 14

What is each myofibril composed of?

Answer 14

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

Question 15

Describe the composition of a sarcomere.

Answer 15

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

Question 16

Picture of structure of sarcomere.

Answer 16

Question 17

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

Answer 17

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.

Question 18

What is the Z-disc built from?

Answer 18

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

Question 19

What is the I-Band?

Answer 19

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

Question 20

What is the A-band?

Answer 20

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.

Question 21

What is the H-band?

Answer 21

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.

Question 22

What is the sliding filament model of muscle contraction?

Answer 22

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

Question 23

Sliding filament model of muscle contraction picture.

Answer 23

Question 24

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

Answer 24

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

Question 25

Describe, in detail, myosin cross bridge cycling.

Answer 25

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 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.

Question 26

Picture of myosin cross bridge cycling.

Answer 26

Question 27

When ATP is NOT bound to myosin, what happens?

Answer 27

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

Question 28

How rapid is the muscle contraction?

Answer 28

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

Question 29

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

Answer 29

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

Question 30

What is excitation-contraction coupling?

Answer 30

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

Question 31

What are the steps to excitation-contraction coupling?

Answer 31

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.

Question 32

Picture showing steps of excitation contraction coupling.

Answer 32

Question 33

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

Answer 33

Ca²⁺ regulates tropomyosin position.

Question 34

Picture showing myosin and tropomyosin interactions.

Answer 34

Question 35

What is tropomyosin and what does it do?

Answer 35

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)

Question 36

What is troponin and what does it do?

Answer 36

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

Question 37

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

Answer 37

Question 38

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

Answer 38

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