Muscle & Non-Muscle Motility

Question 1

Compare and contrast coventional (Type II) myosin with non-conventional (Type I) myosin.

Answer 1

Both are F-actin binding proteins and ATPases. Type II is a two-head complex that can bind to two actin microfilaments at the same time. Type I is a single-headed myosin that can bind to only a single actin filament.

Question 2

Define cytoplasmic streaming.

Answer 2

Cytoplasmic streaming is the process by which vesicles, ER, and other cellular organelles move along thick actin bundles in the subcortical cytoplasm. This movement is triggered by light and powered by unconventional type I myosin on F-actin. This process occurs in green algae.

Question 3

What is the purpose of cytoplasmic streaming?

Answer 3

It equalizes light exposure for the process of photosynthesis and is important for the delivery of nutrients and small metabolites.

Question 4

What type of work do type I myosins perform in non-muscle settings?

Answer 4

They can bind to actin filaments while rooted in a membrane. This allows the myosins to move the filament relative to the membrane and is important in the migration of cells.

Question 5

What type of work do type II myosins perform in non-muscle settings?

Answer 5

They can bind to two actin filaments and move them relative to each other. This is important in cellular movement and creating the “noose” of cellular division.

Question 6

Describe the location and function of type I and II myosin in Dictyostelium.

Answer 6

Type II myosin is found in the tail of the migrating cell. If disrupted, cellular division is impaired but not the motility of lamellipodium. Type I myosin is found in the leading edge of the lamellipodium. The disruption of unconventional myosins stops motility.

Question 7

What forces cause the cell to move in the model for motility using actin assembly and myosin motors?

Answer 7

Actin polymerization in lamellipodium: When actin is assembled at the leading edge, it provides a force that allows the cell to project forward. This force is not sufficient, however, so it needs ATP in the form of myosin I. Myosin I binds to the actin filaments as they are being assembled and provides a force to push them forward. Myosin II works to contract the tail end of the cell as well, but the force generated by myosin I is much more important.

Question 8

In a nerve cell, what types of proteins perform long distance and short distance transport, respectively?

Answer 8

Long distance transport occurs via microtubule motors (kinesins) and short distance via actin motors (myosin).

Question 9

Describe the location and function of smooth muscle.

Answer 9

Smooth muscle is found in the gastrointestinal tract, bladder, and uterus. It is used for slow, steady, and regular contraction.

Question 10

Describe the location of myoepithelial cells.

Answer 10

These cells are associated with multiple secretory glands (mammary glands, sweat glands, etc).

Question 11

How long has striated muscle been around from an evolutionary perspective?

Answer 11

About 500 million years.

Question 12

Why are muscle cells multi-nucleate?

Answer 12

During development, individual myoblast cells fuse together to form huge cells with many different nuclei.

Question 13

What type of imaging technique reveals the striations in muscle?

Answer 13

Interference microscopy.

Question 14

Define sarcomere.

Answer 14

A sarcomere is the contractile unit of the myofibril and muscle cell.

Question 15

Define titin and describe its function.

Answer 15

Titin is a protein that connects the Z and M lines in the sarcomere, acting as a giant “spring scaffold.” It is the 3rd most abundant muscle protein after actin and myosin. It is also the largest known protein, about 30,000 amino acids in length.

Question 16

What are the thick filaments of the sarcomere composed of?

Answer 16

Bundled myosin II. These type II myosins are large enough to see with an electron microscope.

Question 17

What is the S1 fragment of the myosin II complex?

Answer 17

The S1 fragment is the head domain (ATPase) of the myosin.

Question 18

How are myosin filaments assembled?

Answer 18

To form a myosin filament, 30 or 40 myosin II complexes are bundled together. Filaments form via lateral aggregation of coiled-coil tails of myosin heavy chains. The thick filament is bipolar (meaning the heads stick in two different directions). These myosin filaments are held together by weak noncovalent bonds.

Question 19

Describe the process of S1 decoration.

Answer 19

S1 decoration is used to prove experimentally that the head groups of myosin bind to actin filaments. First, remove the head groups with a protenase named papain, generating liberated S1 fragments. These freed head groups will bind to actin with a specific orientation.

Question 20

Describe the experiment performed to reveal the walking movement of myosin II that is powered by ATP.

Answer 20

1. Bind myosin (or fragments of myosin) to glass slide.
2. Add fluorescently-labeled F-actin filaments.
3. Add excess ATP
4. The filaments move.

Question 21

Which direction do myosin heads walk towards?

Answer 21

Myosin heads always walk towards the plus-end of an actin filament.

Question 22

Describe the steps of the swinging cross-bridge hypothesis.

Answer 22

1. No ATP is bound by myosin head group, and head is tightly bound to actin.
2. ATP binds myosin, releasing it from actin (conformation change 1)
3. ATP hydrolysis cocks myosin head (conformation change 2)
4. Myosin binds actin weakly. Phosphate bond cleaved but still bound to ADP (conformation change 3).
5. ADP bond cleaved (ADP released). Myosin again binds actin tightly.

Question 23

Which step of the swinging cross-bridge hypothesis generates the force of the muscle contraction?

Answer 23

The fifth step in which the ADP bond is cleaved and myosin binds actin tightly. It is also called the power stroke.

Question 24

Describe the sliding filament model of myofibril contraction.

Answer 24

This model describes the myofibril contraction that results from ATP-dependent sliding of thin and thick filaments. In this model, none of the filaments change in length; they only move past each other and slide. Sarcomeres shorten and filaments slide towards each other, caused by each myosin II head group pulling towards its respective Z band. The I-bands shorten but the A-bands do not change.

Question 25

How is muscle contraction controlled?

Answer 25

Myofibril contraction is stimulated by the release of calcium ions from the sarcoplasmic reticulum.

Question 26

Explain the role of the T-system in the control of muscle contraction.

Answer 26

T-tubules are continuations of the plasma membrane. They extend into the muscle fiber and carry a nerve impulse (action potential) into the muscle fiber.

Question 27

Explain the role of the sarcoplasmic reticulum in the control of muscle contraction.

Answer 27

A derivative of the ER, the SR serves as a calcium ion reservoir. Receipt of an electrical signal causes calcium ion release via voltage-gated calcium channels. The calcium ion spikes stimulates myofibril contraction; terminated by pumping calcium ions back into the SR.

Question 28

What is the purpose of the calcium signal in the control of muscle contraction?

Answer 28

The purpose of the calcium signal is to move tropomyosin out of the way of the interaction between the myosin head group and the microfilament.

Question 29

Describe how calcium ions interact with tropomyosin to control muscle contraction.

Answer 29

In the sarcomere, the tropomyosin dimer binds along an actin filament (lies in the a-helical groove). The troponin complex binds to tropomyosin. In the absence of calcium ions, tropomyosin blocks the myosin head binding on the F-actin filament; it physically gets in the way. With a calcium ion influx, troponin C binds calcium ions. An allosteric change in the structure of troponins and tropomyosin uncovers the myosin binding site, allowing the myosin to walk on actin and myofibril contracts. Removal of calcium ions restores the inhibition.