Cytoskeleton And Cellular Architecture Pt. 3

Front 1

Of the three cytoskeleton filaments, which is the most dynamic and highly regulated?

Back 1

Actin filaments. They are also the smaller filaments in diameter and are of intermediate flexibility between IFs and MTs.

Front 2

Do actin filaments (AFs) have structural polarity?

Back 2

Yes. Like MTs, they have a plus and a minus end.

Front 3

What is another name for AFs?

Back 3

Microfilaments

Front 4

What creates the flexibility for AFs?

Back 4

Many weak longitudinal and lateral bonds produce a flexible double helix.

Front 5

In AFs, which end is barbed and which is pointed?

Back 5

Positive is barbed

Negative is pointed

Front 6

Why is the polarity of AFs important?

Back 6

It promotes treadmillong, which is how these filaments move.

Front 7

How are AFs structured?

Back 7

They are made up of a twisted chain of identical actin monomers, which point in the same direction along the chain.

Front 8

What is G-actin? F-actin?

Back 8

1. A soluble monomer for AFs.

2. A Filamentous monomer for AFs.

Front 9

How does treadmilling work? How is it different from the way MTs dynamic instability works?

Back 9

Treadmilling depends on the simultaneous gain of monomers at the plus end and loss of monomers at the minus end. Each monomer is bound to ATP (plus end) or ADP (minus end) instead of GTP/GDP like MTs.

AFs can maintain their size if loss and gain are equal, and the monomers from the minus end move to the plus end. In MTs, monomers are only added to or removed from the plus end.

Front 10

What are the 5 classes of proteins that regulate actin?

Back 10

1. Monomer-sequestering protein

2. Nucleating protein

3. Capping (plus-end blocking) protein

4. Severing protein

5a. Cross-linking protein (in cell cortex)

5b. Bundling protein (in filopodia)

Front 11

What is profilin? What does it do specifically?

Back 11

A monomer-BINDING protein. It binds to the plus end of an actin monomer.

1. Binds to G-actin monomer to contribute to polarity of actin polymerization

2. Blocks addition of G-actin to minus end of AFs

3. Doesn't interfere with addition of bound actin monomers to the plus end of AFs.

Front 12

What are formins and Ena/VASP

Back 12

NUCLEATING proteins.

Protein complexes that stimulate polymerization at the plus end of the AF. Profilin introduces G-actin to the filament and formins or Ena/VASP introduces it to the AF plus end.

Front 13

What is ARP2/3?

Back 13

An actin-related NUCLEATING protein that initiates branching. This allows networks of actin filaments to form in parts of cells.

ARP2/3 binds to AF. Monomers bind to ARP2/3 to create a branch, and elongation occurs at the positive end.

Front 14

What is FACP?

Back 14

F-actin CAPPING protein. It binds to the AF plus end. This stabilizes the plus end and blocks both addition and loss of actin subunits.

It also competes with formin and Ena/VASP, which allows networks of AFs to grow specifically in one region of the cell, but not others.

Front 15

What is ADF/Cofilin?

Back 15

A SEVERING/DEPOLYMERIZING protein. It stabilizes spontaneous breaks in filaments, which causes the filaments to sever, producing ends that lead to depolymerization.

Front 16

What is gelsolin?

Back 16

A SEVERING/DEPOLYMERIZATION protein. It severs AFs but remains in contact with the plus end, stabilizing it rather than allowing it to break down like ADF/Cofilin does.

Front 17

What do cross-linking and bundling proteins do? What are some examples, and what do they do?

Back 17

They organize actin filaments into bundles and networks.

1. Fimbrin: forms tight bundles (ex: filopodia)

2. Alpha-actinin: forms loose bundles (ex: plasma membrane attachment & special role in muscle cells)

3. Filamin: forms actin filament networks (ex: cell cortex)

Front 18

What is the cell cortex?

Back 18

The specialized layer of actin-filament-rich cytoplasm just beneath the plasma membrane. In this region, AFs are linked by actin-binding proteins (filamins) into a meshwork that supports the plasma membrane and gives it mechanical strength.

Front 19

What are the functions of the Actin cytoskeleton?

Back 19

1. Providing cell structure

2. Providing cell mobility

3. Helping in cell division

4. Helping with muscle contraction

5. Trafficking along the periphery of the cell

Front 20

What are filopodia?

Back 20

Stiff, thin protrusions many cells extend. Each contains a loose bundle of 10-20 actin filaments with the plus ends pointing outward. They are exploratory, motile structures generated by rapid local growth of AFs.

Front 21

What are lamellipodia?

Back 21

Dynamic sheetlike extensions on the surface of an animal cell, especially one migrating over a surface.

Contain a dense meshwork of actin filaments oriented so most have plus end close to plasma membrane. Motile structure that expands and contracts with great speed.

Front 22

What are four examples of AF function?

Back 22

1. Microvilli

2. Cell cortex

3. Filopodia and lamellipodia

4. Contractile ring in dividing cells

Front 23

True or false: microtubules underlie unidirectional growth of projections from the plasma membrane?

Back 23

False. Actin filaments underlie unidirectional growth of projections from the plasma membrane.

Front 24

What are myosin motor proteins?

Back 24

All actin-dependent motor proteins belong to the myosin family. They bind to hydrolyze ATP, which provides energy for their movement along actin filaments toward the PLUS end. Myosin-I and Myosin-II (the latter of which is specific to muscle cells), are the most abundant.

Front 25

What are some of the functions of myosin motor proteins?

Back 25

1. To help vesicles move to the plus end on the cell cortex.

2. To help filaments move relative to the plasma membrane

3. To help contractile ring filaments slide passed each other and disassemble as the ring shrinks.

Front 26

How does myosin-II work?

Back 26

The tail domains interact with each other. The heads are shaped like a double-barbed fish hook. One side of each head interacts with actin filaments in muscles to move them.

Hint 26

Front 27

What are the steps to cell migration as it relates to actin remodeling? (How does actin help the cell move, yo).

Back 27

1. Focal contacts (which contain integrin) help the cell adhere to a surface.

2. Polymerization of actin filaments causes plasma membrane to distort into lamellipodium or filopodium.

3. The leading edge reorganizes AFs to stretch the cell and form new integrin/ECM focal contacts.

4. Lagging end reorganizes AFs to form stress fibers.

5. Stress fibers stretch and then snap back, releasing integrin from lagging end and allowing lagging end to snap forward. (This is currently a theory).

The stress is generated by a myosin motor.

Front 28

Name three different types of muscle. Which has the best understood contraction method?

Back 28

Skeletal muscle, smooth muscle, and cardiac muscle. Contraction of skeletal muscle is best understood.

Front 29

What does skeletal muscle contraction depend on?

Back 29

Interaction between actin and myosin-II

Front 30

Describe a skeletal muscle.

Back 30

Huge, multinucleated cells formed by the fusion of many separate smaller cells. The nuclei of the contributing cells are retained in the muscle fiber and lie just beneath the plasma membrane.

Front 31

What are myofibrils?

Back 31

The contractile elements of a muscle cell. They fill the bulk of the cytoplasm. Cylindrical structures that can be as long as the cell itself. Each one is made up of identical tiny units called sarcomeres.

Front 32

Describe the structure of a sarcomere?

Back 32

It is made up of thin filaments (actin) connected to thick filaments (myosin-II) in a staggered fashion. The thin filaments are anchored by their plus end to the Z disc (made of alpha actinin). The minus ends of the thin filaments overlap the ends of the thick filaments. Sarcomeres make up myofibrils, which make up muscle cells.

Hint 32

Front 33

How does contraction work in sarcomeres/myofibrils?

Back 33

The actin (thin) filaments slide along the myosin-II (Thick) filaments without shortening. The sliding motion is driven by the myosin motor protein heads walking toward the plus end of the adjacent actin filaments.

Front 34

How is muscle contraction regulated?

Back 34

1. Tropomyosin surrounds actin (thin) filament, which prevents the myosin heads of the thick filament from associating with actin filament.

2. Troponin, a protien complex that includes a Ca2+-sensitive protein, is associated with the tropomyosin molecule.

3. Signal from neurons releases Ca2+ in myocyte (muscle cell)

4. Ca2+ binds to troponin, changing its conformation, causing tropomyosin fiber to shift.

5. Myosin binding site exposed, allowing myosin/actin association.

Front 35

In a myofibril, where is the ER found, and what is it called?

Back 35

It surrounds the myofibril and is called the sarcoplasmic retuculum.

Front 36

What is this an image of?

Back 36

Top is a relaxed sarcomere. Bottom is a contracted sarcomere.