3. Intermediate filaments
What are the 3 main types of cytoskeletal elements in eukaryotic cells?
What cytokeletal elements emanate from a single organizing center within interphase cells?
1. Positioning organelles within the cell
2. Transport of vesicles for intracellular trafficking
3. Partioning of chromosome during division
What are the functions of microtubules?
What cytoskeletal element is a large and heterogenous family of proteins and provides mechanical strength?
The cytoskeleton allows for cellular remodeling during periods of what?
Muscle contraction and axon/dendrite extension in neurons
The cytoskeleton provides the machinery used for what?
Generates and/or maintains organelle structure
The cytoskeleton does what for organelle structure?
Lamellipodia or filopodia which are essential for cell motility
During interphase, the actin filaments beneath the plasma membrane form dynamic structures called what?
What exists as long filaments that project from the microtuble organizing center which is located adjacent to the nucleus?
The structural organization of microtubule and actin filaments undergo dramatic changes when the cell undergoes what?
The mitotic spindle
After chromosomes duplication the single microtubule array will form a bipolar structure called what?
The contractile ring
Actin will reform, with the help of associated motor proteins, to form a 'belt' around the middle of cell called the what?
Microtubule-dependent chromosome segregation
The contractile ring will 'pinch' the cell in half after what?
The cytoskeletal filaments
What will reassemble to form the structures first described in interphase?
Bundles of actin filaments form what to increase the cell surface area thereby increasing nutrient uptake?
Do microvilli have a constant location, length and diameter or variable location, length and diameter?
The contents of the intestinal lumen from leaking into the body
Actin filaments also form below the microvilli; these cell-cell junctions prevent what?
Actin and microtubule filaments have what? It enables cells to
differentiate between top and bottom and left from right.
In polarized epithelial cells, microtubules run vertically (apical to the basal) or horizontally which is important for the trafficking of newly synthesized material?
Intermediate filaments, attached to what, link adjacent epithelial cells together to form a sturdy sheet of cells
A sturdy sheet of cells
Intermediate filaments, attached to desmosomes, link adjacent epithelial cells together to form what?
Intermediate filaments, attached via what, link epithelial cells to the underlying extracellular matrix on the basal lateral side of the cell?
The underlying extracellular matrix on the basal lateral side of the cell
Intermediate filaments, attached via hemidesosomes, link epithelial cells to what?
1. They provide dynamic structures for cellular reorganization,
2. They are stable structures for the maintanance of cellular structure
3. They establish polarity in a cell
Name three functions of cytoskeletal elements
α- and β-tubulin (γ tubulin also exists)
Microtubules are formed from two tubulin subunits (dimers); What are they?
Hydrolysis; (α-tubulin always has GTP bound)
Organization of the tubulin heterodimer is such that the GTP bound to the alpha subunit is protected from what?
Hydrolysis; (β-tubulin can have either GTP or GDP bound)
GTP bound to the beta subunit can undergo what? (This effects MT dynamics)
α-tubulin always has GTP bound and β-tubulin can have either GTP or GDP bound. This difference is important for the property known as what?
What are hollow tubes formed by 13 protofilaments containing alternating α- and β-tubulin heterdimers?
Microtubules are stiff and difficult to bend due to what kind of interactions?; This makes MT the straightest structural element in cells (up to millimeters in length!)
Tubulin subunit assemble into what in the same direction giving the microtubule a distinct polarity.
At the other end
Tubulin subunit assemble into a protofilament in the same direction giving the microtubule a distinct polarity; α-tubulin subunits exposed at one end and β-tubulin subunits exposed where?
Microtubules nucleate from the microtubule organizing center (MTOC) located near the what?
Microtubules nucleate from the microtubule organizing center located near the nucleus. This sets up polarity such that all microtubles are "what"?, that is the plus ends of the microtubule emanate outwards from the center of the cell
6 isoforms (each of alpha and beta tubulin)
There are multiple isoforms of tubulin: how many isoforms are in humans?
Each isoform has a distinct location within the cell and performs subtly different what?
A head-to-tail fashion
Actin subunits assemble in what fashion to generate a filament with polarity?
To the "minus end" of the actin filament
ATP binds in a deep cleft in the center of an actin monomer which is open to what end of the actin filament?
Pointed versus barbed nomenclature refers to the arrowhead
appearance of what heads when bound to actin filaments?
Actin filaments are a single helix of monomers that forms what looks like two parallel protofilaments that twist around each other in what kind of helix?
They are typically crosslinked by accessory proteins making actin structures stronger than an individual filament
Why are actin filaments flexible and easily bent, but strong?
1. They differ only slightly in their amino acid sequence
2. The alpha isoform is expressed only in muscle cells
3. The beta and gamma isoforms are found in nonmuscle cells
What are the differences between the three isoforms of actin?
What kind of filaments are only found in some metazoans including vertebrates, nematodes, and mollusks but not in all cell types of these organisms
Cells subjected to mechanical stress
Intermediate filaments are prominent in the cytoplasm of what kind of cells?
A rigid exoskeleton
Intermediate filaments are not found in animals that have what kind of exoskeleton?
What form the meshwork beneath the inner membrane of the nuclear envelope and are the site of chromosome and nuclear pore complex attachment?
The individual polypeptides of intermediate filaments are elongated molecules with a central α-helical domain that assemble to form what?
The individual polypeptides of intermediate filaments are elongated molecules with a central α-helical domain that assemble to form parallel dimers. Dimers associate in a staggered antiparallel manner to form what?
Since tetramers are composed of antiparallel dimers, intermediate filaments do not have what?
Intermediate filaments have a rope-like characteristic making them easily bent but extremely difficult to what?
For what, phosphorylation may have a role in triggering their disassembly similar to what is seen for nuclear lamins during mitosis?
Anchoring the intermediate filaments at sites of cell-cell contact sites
How do keratin filaments impart mechanical strength to epithelial tissues?
Defective keratin molecules can result in what in response to minor mechanical stress?
At the ends of neurofilament polymers and along the length of a filament
Filament addition occurs where?
Neurofilament gene expression
Axonal diameter is controlled by what, which influences how fast an electrical signal travels down the axon?
ALS or Lou Gegrig's desease
Mutations in neurofilament genes that impact filament assembly have been linked to what disease?
Abnormal assembly of neurofilaments in motor neurons and in the axon interferes with normal axonal transport leading to what?
The assembly and disassembly of actin and tubulin
Eukaryotic cell survival depends on the balance between what two things?
By plants, fungi or sponges
Toxins that specifically target actin and microtubule filaments are produced in self-defense by what (they can not run away from predators)?
Either filament formation or its disassembly
The toxins produced bind tightly to either the filament form or the free subunit forms to drive what opposing actions?
Latrunculin and swinholide
What two toxins (from different marine sponges) depolymerize actin filaments by binding to actin subunits to prevent their assembly or severing actin filaments?
What toxin (fungal metabolites) binds to the plus end of actin filaments to prevent actin polymerization?
What toxin ("death cap" fungus) stabilizes actin filaments by binding at the interface between actin subunits?
1. Cholchicine (autumn crocus)
2. Vinblastine (Madagascar periwinkle plant)
What three toxins all bind to tubulin subunits to prevent microtubule formation?
What is the rapid interconversion between a growing and shrinking state, in the presence of a uniform concentration of free subunits
Actin filaments undergo length flutuations, but they are on such a small scale that in eukaryotes, dynamic instability refers mainly to what?
The key factor influencing dynamic instability is the type of nucleotide present at what end of the filament?
GTP bound form
For free subunit dimers this rate of hydrolysis is so slow that the free subunits are primarily in what form?
If what occurs faster than GTP hydrolysis, a microtubule filament will have GTP "cap" at its plus end?
If polymerization slows down, the GTP cap will undergo hydrolysis to GDP causing what?
GTP bound tubulin subunits form a linear microtubule protofilament that is stabilized by what kind of bonds?
The curling at the ends of the shrinking microtubules
With GTP hydrolysis the protofilament adopts a curved conformation, this leads to the progressive disruption of the microtuble protofilament; evidenced by what?
The free subunit concentration and the rate constant
The rate of subunit addition to the ends of filaments is the product of what?
The structural differences between the two ends
The rate constant (kon) is much faster for the plus end of a filament than the minus end due to what?
At an intermediate concentration
At what concentration of free subunits will the rate of subunit addition be faster than nucleotide hydrolysis at the plus end, but slower than nucleotide hydrolysis at the minus end?
The subunit loss at this end
The minus end will therefore have a GDP/ADP bound nucleotide which promotes what?
Filament stability and subunit addition
The plus end will be in its GTP/ATP bound state which promotes what?
At the minus end
Thus the filament will add subunits at the plus end while simultaneously losing subunits where?
The property of adding subunits at the plus end while simultaneously losing subunits at the minus send is referred to as what?
A filament whose net total length does not change
(Treadmilling) At a particular subunit concentration, filament growth at the plus end exactly balances filament shrinkage at the minus end generating what kind of filament?
Single actin filaments in vitro and may occur in live cells for microtubules
Treadmilling has been seen for what kind of filaments in vitro? Where has treadmilling been seen in live cells?
A tubulin homolog called what was identified that could assemble into ring-like structure at the site of septum formation during cell division?
A conformation change in the filament structure
FtsZ polymerization causes GTP hydrolysis leading to what?
The FtsZ ring
What ring forms at the middle of a cell and becomes smaller as the cell pinches into two; similar to the actin based contractile ring in eukaryotes?
The FtsZ ring may act as a docking site for cell division enzymes required for building the what?
Rod-shaped or spiral shaped cells
Bacterial homologs Mbl and MreB are found in what kind of cells?
Abnormalities in cell shape and defects in DNA segregtion
Disruption of expression causes what kind of abnormalities and/or defects?
Cells grow into irregular twisted tubular shapes and eventually die
What changes in cell morphology occur due to loss of Mbl in Bacillus substilus? What is the eventual end result?
A plasmids origin of replication (shown in blue)
In vivo, ParM (shown in green) assembles into filaments that associate with another protein that binds to what?
Opposite ends of the cell ensuring that each daughter cell gets a copy
ParM polymerization pushes the replicated plasmids where? For what purpose?
Homologs to intermediate filament proteins have been identified in what bacterial species?
What forms a fiber that runs down the inner side of the curving bacterial cell wall to influence cell shape?