Intermediate & Actin Filaments

Question 1

Define plectin and describe its structure.

Answer 1

Plectin is a type of elongated dimeric protein that composes the cross-bridges that connect intermediate filaments to other cytoskeletal filaments. Each plectin molecule has a binding site for an intermediate filament at one end and a binding site for another cytoskeletal filament at the other.

Question 2

Why do intermediate filaments lack polarity?

Answer 2

The two dimers that make up tetramers point in opposite directions, so the tetramer itself lacks polarity. Because the tetrameric building blocks lack polarity, so too does the assembled filament.

Question 3

What controls the assembly and disassembly of intermediate filaments?

Answer 3

Unlike the other two major cytoskeletal elements, assembly and disassembly of intermediate filaments is controlled primarily by phosphorylation and dephosphorylation of the subunits.

Question 4

What are neurofilaments?

Answer 4

Neurofilaments are loosely packed bundles of intermediate filaments whose long axes are oriented parallel to that of the nerve cell axon.

Question 5

Which direction do myosins move, and what type of filaments do they interact with?

Answer 5

Myosins interact with actin filaments and move toward the plus end of the actin filament.

Question 6

Describe the head domain of myosin.

Answer 6

The motor head domain contains a site that binds an actin filament and a site that binds and hydrolyzes ATP to drive the myosin motor.

Question 7

What are ciliopathies?

Answer 7

Oligogenic disorders involving proteins localized to the cilia or depend on the cilia.

Question 8

Describe the subtypes of cilia.

Answer 8

Cilia are divided into two groups: motile and non-motile. Non-motile cilia do not move at all and work to coordinate intracellular signaling. Motile cilia are divided into two subsections: those with planar motion and those with rotary motion.

Question 9

Where are intermediate filaments typically found?

Answer 9

Unlike microtubules, they do not have a central point but radiate throughout the cell. They are most abundant around organelles (especially the nucleus) and the membrane. They are abundant in tissue encountering high mechanical stress such as muscle, skin, and nerve cells.

Question 10

If a cytoskeletal filament is highly insoluble and resistant to salt and detergents, what type of filament is is likely to be?

Answer 10

An intermediate filament.

Question 11

What are the four major cell-type-specific families of cytoplasmic intermediate filaments?

Answer 11

1. Keratin (acidic type I)
2. Keratin (basic type II)
3. Vimentin (type III)
4. Neurofilaments (type IV)

Question 12

Describe the keratin family of cytoplasmic intermediate filaments.

Answer 12

There are two types: Type I is acidic and Type II is basic. They are usually found in epithelial or trichocytic cells. They are composed of from a diverse family of related proteins. The skin is mainly composed of keratins.

Question 13

Describe the vimentin family of cytoplasmic intermediate filaments.

Answer 13

The vimentin family (type III) of proteins is found in connective tissue, muscles, and glia cells. The subtype desmin is found in striated muscle sarcomeres and glial fibraillary acidic protein in nervous system glia.

Question 14

Describe the neurofilament family of cytoplasmic intermediate filaments.

Answer 14

These proteins (type IV) are found in nerve cells. They are composed of triplet proteins (made of heavy, intermediate, and light chains) that provide strong structural support.

Question 15

What function do nuclear lamins (type V) have?

Answer 15

Nuclear lamins support the nuclear envelope in all eukaryotes. Unlike the other four subtypes of intermediate filaments, nuclear lamins are not cytoplasmic proteins.

Question 16

What is the basic subunit of intermediate filaments?

Answer 16

a-helical monomers. The majority of these monomers is one long a-helix.

Question 17

Describe how coiled-coil subunits combine to form intermediate filaments.

Answer 17

Two a-helical monomers are linked to form a coiled-coil dimer. Four copies of the protein staggered next to each other form an anti-parallel staggered tetramer. Finally, you have eight tetramers in a cross-section of filament. So, there are 32 individual proteins in a cross-section.

Question 18

How does the speed of intermediate filament assembly and disassembly compare to the speed of assembly for microtubules and actin filaments?

Answer 18

It is slow compared to microtubules and actin filaments, but it exists. Previously, scientists thought intermediate filaments were immoveable and stable, but now we know they have a dynamic nature.

Question 19

Where does the strength of our skin come from?

Answer 19

Keratins (which are the primary protein found in skin) contain large amounts of cysteine for protein cross-linking. This creates the strength of skin.

Question 20

What is epidermolysis bullosa simplex, and what causes it?

Answer 20

It is a disease caused by mutations in K5 and K14 that disrupts keratin filaments. The basal cells that express these keratins lyse, leading to blistering after even mild mechanical trauma.

Question 21

What is the subunit of actin microfilaments?

Answer 21

Globular G-actin. It polymerizes into filamentous F-actin.

Question 22

What is dynamic treadmilling?

Answer 22

Dynamic treadmilling is the microfilament version of dynamic instability in microtubules. ATP binding favors polymerization. ATP hydrolysis (to ADP) favors depolymerization. These processes are constantly going back and forth, creating this state of dynamics.

Question 23

What conditions are favorable to the formation of actin microfilaments from G-actin?

Answer 23

When G-actin is in an environment of salt and Mg2+ ions, it leads to the formation of F-actin.

Question 24

How are the plus and minus ends of F-actin different?

Answer 24

In a concentration of actin subunits, the F-actin exhibits polarity. The plus end (the barbed end) shows rapid assembly while the minus ends show very little change. After the concentration has dropped sufficiently, there is only addition to the plus end. At some point, you will see a loss of actin subunits from the minus end while the plus end continues to assemble.

Question 25

What state must actin be in to form polymers?

Answer 25

The actin must be bound to ATP because actin-ADP is unstable. When bound to ATP, the actin is stable.

Question 26

For both actin filaments and microtubules, the structure is built on a foundation. What is that foundation for actin filaments?

Answer 26

There are actin-related proteins (ARP2/3) that facilitate the assembly of actin filaments by forming a macromolecular complex composed of 7 proteins called bovine ARP2/3 complex.

Question 27

How does the ARP2/3 complex nucleate an actin filament?

Answer 27

It provides a foundation/template and then elongates the actin filament by subunit addition. This complex generates polarity in the actin filament by capping the minus-end and inhibiting its growth. This allows G-actin subunit addition onto the plus end, which creates rapid growth changes. This system is essentially the same as the regulating system of dynamic instability in microtubules.

Question 28

What is the relationship between nucleating proteins and monomer-binding proteins?

Answer 28

These two types of proteins both modify actin assembly and function, but they work against each other. Nucleating proteins positively assist in the formation of actin filaments from the subunits. Monomer-binding proteins bind to G-actin and act as a sponge to take it out, preventing it from forming new actin filaments.

Question 29

Thymosin B4 is an example of what type of actin-binding protein?

Answer 29

A monomer-binding protein.

Question 30

State the function of side-binding proteins, and give an example of one.

Answer 30

Side-binding proteins stabilize the actin filament and prevent it from disassembling. An example is tropomyosin.

Question 31

State the function of motor proteins, and give an example of one.

Answer 31

Motor proteins are involved in the movement of actin filaments and cells. An example is myosin.

Question 32

State the function of cross-linking proteins, and give an example of one.

Answer 32

These proteins link actin filaments together but rather than bundling them, they cross-link them into a net-like structure. An example is filamin.

Question 33

State the function of bundling proteins, and give an example of one.

Answer 33

Bundling proteins make higher order actin filaments by cross-linking them into bundles. An example is a-actinin.

Question 34

State the function of severing proteins, and give an example of one.

Answer 34

Severing proteins work against end-capping proteins. They act like a scissors to quickly deplanarize actin filaments. An example is gelsolin/villin.

Question 35

State the function of end-capping proteins, and give an example of one.

Answer 35

These proteins prevent the disassembly of microfilaments. An example is CapZ.

Question 36

How are the microvilli of the intestinal wall built and maintained?

Answer 36

Protein complexes recruit ARP2/3 to the plasma membrane (at lipid raft subdomains) and promote the nucleation of actin filaments. These filaments elongate with additional G-actin subunits at the growing plus end. This elongation drives plasma membrane extension.

Question 37

Every time an ARP2/3 complex is added to an actin filament, how does the structure change?

Answer 37

It creates a branched point. The nucleation and cap on the filament branches to stabilize the structure.

Question 38

What are filopodia, and how are they maintained?

Answer 38

Filopodia are the result of actin filament growth in a membrane extension. They are long, finger-like projections maintained by the creation of densely bundled arrays of actin filaments.

Question 39

What are lamellipodia, and how are they maintained?

Answer 39

Lamellipodia are the result of actin filament growth in a membrane extension. Rather than the finger-like projection of filopodia, lamellipodia are more like the palm of a hand. They move in mass migration and are maintained by cross-linking actin filaments.

Question 40

Where is F-actin abundant?

Answer 40

The membrane, actin cytoskeleton, microvilli, stress fibers, filopodia, lamellipodia, and the contractile ring essential for the division of the cytoplasm in animal cells.