Intermediate & Actin Filaments Flashcards
Define plectin and describe its structure.
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.
Why do intermediate filaments lack polarity?
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.
What controls the assembly and disassembly of intermediate filaments?
Unlike the other two major cytoskeletal elements, assembly and disassembly of intermediate filaments is controlled primarily by phosphorylation and dephosphorylation of the subunits.
What are neurofilaments?
Neurofilaments are loosely packed bundles of intermediate filaments whose long axes are oriented parallel to that of the nerve cell axon.
Which direction do myosins move, and what type of filaments do they interact with?
Myosins interact with actin filaments and move toward the plus end of the actin filament.
Describe the head domain of myosin.
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.
What are ciliopathies?
Oligogenic disorders involving proteins localized to the cilia or depend on the cilia.
Describe the subtypes of cilia.
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.
Where are intermediate filaments typically found?
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.
If a cytoskeletal filament is highly insoluble and resistant to salt and detergents, what type of filament is is likely to be?
An intermediate filament.
What are the four major cell-type-specific families of cytoplasmic intermediate filaments?
- Keratin (acidic type I)
- Keratin (basic type II)
- Vimentin (type III)
- Neurofilaments (type IV)
Describe the keratin family of cytoplasmic intermediate filaments.
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.
Describe the vimentin family of cytoplasmic intermediate filaments.
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.
Describe the neurofilament family of cytoplasmic intermediate filaments.
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.
What function do nuclear lamins (type V) have?
Nuclear lamins support the nuclear envelope in all eukaryotes. Unlike the other four subtypes of intermediate filaments, nuclear lamins are not cytoplasmic proteins.
What is the basic subunit of intermediate filaments?
a-helical monomers. The majority of these monomers is one long a-helix.
Describe how coiled-coil subunits combine to form intermediate filaments.
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.
How does the speed of intermediate filament assembly and disassembly compare to the speed of assembly for microtubules and actin filaments?
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.
Where does the strength of our skin come from?
Keratins (which are the primary protein found in skin) contain large amounts of cysteine for protein cross-linking. This creates the strength of skin.
What is epidermolysis bullosa simplex, and what causes it?
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.
What is the subunit of actin microfilaments?
Globular G-actin. It polymerizes into filamentous F-actin.
What is dynamic treadmilling?
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.
What conditions are favorable to the formation of actin microfilaments from G-actin?
When G-actin is in an environment of salt and Mg2+ ions, it leads to the formation of F-actin.
How are the plus and minus ends of F-actin different?
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.
What state must actin be in to form polymers?
The actin must be bound to ATP because actin-ADP is unstable. When bound to ATP, the actin is stable.
For both actin filaments and microtubules, the structure is built on a foundation. What is that foundation for actin filaments?
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.
How does the ARP2/3 complex nucleate an actin filament?
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.
What is the relationship between nucleating proteins and monomer-binding proteins?
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.
Thymosin B4 is an example of what type of actin-binding protein?
A monomer-binding protein.
State the function of side-binding proteins, and give an example of one.
Side-binding proteins stabilize the actin filament and prevent it from disassembling. An example is tropomyosin.
State the function of motor proteins, and give an example of one.
Motor proteins are involved in the movement of actin filaments and cells. An example is myosin.
State the function of cross-linking proteins, and give an example of one.
These proteins link actin filaments together but rather than bundling them, they cross-link them into a net-like structure. An example is filamin.
State the function of bundling proteins, and give an example of one.
Bundling proteins make higher order actin filaments by cross-linking them into bundles. An example is a-actinin.
State the function of severing proteins, and give an example of one.
Severing proteins work against end-capping proteins. They act like a scissors to quickly deplanarize actin filaments. An example is gelsolin/villin.
State the function of end-capping proteins, and give an example of one.
These proteins prevent the disassembly of microfilaments. An example is CapZ.
How are the microvilli of the intestinal wall built and maintained?
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.
Every time an ARP2/3 complex is added to an actin filament, how does the structure change?
It creates a branched point. The nucleation and cap on the filament branches to stabilize the structure.
What are filopodia, and how are they maintained?
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.
What are lamellipodia, and how are they maintained?
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.
Where is F-actin abundant?
The membrane, actin cytoskeleton, microvilli, stress fibers, filopodia, lamellipodia, and the contractile ring essential for the division of the cytoplasm in animal cells.
