Cytoskeleton I - Structure Flashcards
What does the cytoskeleton maintain and insure in cells?
- Maintains correctly shaped cells.
- Insures cells are properly structured internally.
List the functions of the cytoskeleton.
- Changes shape of the cell (e.g., RBCs)
- Moves the cell.
- Re-arranges the cellular compartment.
- Supports the plasma membrane.
- Provide the mechanical strength.
- Pulls chromosomes apart during cell division.
- Splits dividing cells during cell division.
- Guides intracellular traffic of organelles.
- Vesicles move around by using cytoskeleton as a sidewalk.
- Cells like sperm need to swim.
- WBCs and fibroblasts need to crawl.
- Muscle cell contraction.
What are the three families of cytoskeletal protein?
> Actin filaments (like Mardi-Gras beads)
Microtubules (slinky of life)
Intermediate filaments (like girders in building)
What is the function of actin filaments?
Determine the shape of cell’s surface and are necessary for whole-cell locomotion, secretion, and endocytosis.
What are actin filaments necessary for the cell to do?
- Whole-cell locomotion
- Secretion
- Endocytosis
List functions of microtubules within the cell.
> Form tube like structure.
Determine the positions of membrane-enclosed organelles and direct the intracellular transport.
Make up centrioles and mitotic spindle.
Cilia and flagella.
Sperm swim and cilia moves eggs in Fallopian tubes.
What are the functions of intermediate filaments?
> Provide mechanical strength.
Strong filament.
Resist mechanical stress.
Besides actin filaments, microtubules, and intermediate filaments, what other proteins are essential for assembly of the cytoskeletal filaments?
Accessory Proteins
True of False:
Accessory proteins include motor proteins which are machines for converting the evergy of ATP hydrolysis into mechaniscal movement (move organelles along filaments or move filaments themselves).
True
Give an example of accessory proteins interacting to assemble cytoskeletal filaments.
Hundreds of actin binding proteins that modify the dynamics and organization of filaments.
What is actin a bundle of?
Two proto-filaments.
What are actin filaments also known as?
microfilaments
What are the characteristics of actin filaments (microfilaments)?
- Are two-stranded helical polymers of the protein actin.
- Actin subunits are compact and globular (G-actin vs. F-actin).
> Actin is always G-actin in our cells. - Flexible structures 5-9 nm in diameter.
What are the characteristics of microtubules?
> long hollow cylinder
made of tubulin subunits that are compact and globular
long and straight
outer diameter 25 nm
more rigid than actin filaments
have one end attached to a single microtubule-organizing center called a centrosome
Are actin filaments or microtubules more rigid?
Microtubules are more rigid than actin filaments.
What are the characteristics of intermediate filaments?
- are rope-like fiber, diameter 10 nm
- large heterogeneous family
- are made of smaller subunits that are themselves elongated and fibrous
- extend across the cytoplasm to provide mechanical strength
- span from on cell-cell juction to another to strengthen the epithelial sheet
- do not readily polymerize
True or False:
The cytoskeleton is dynamic and adaptable.
True
Of the three cytoskeleton proteins, which two are polarized?
- actin filaments
- microtubules
Which cytoskeleton rearranges to form a bi-polar mitotic spindle and separate chromosomes?
microtubules
Although the cytoskeleton is dynamic and adaptable, give 2 examples of the cytoskeleton forming stable structures.
> Intestinal cells with microvilli must last a few days.
Actin bundles in hair cells of inner ear must last a lifetime.
However, actin filaments replaced every 48 hours
What maintains polarity for intestinal cells - apical surface vs. basolateral surface?
Cytoskeleton
What does the apical surface of epithelial cells do?
Absorbs nutrients.
What are the purpose of microvilli on epithelial cells?
Quadruple surface area and increase absorption rate.
What happens at the basolateral surface of epithelial cells?
Where cells transfer nutrients to bloodsteam.
What is the polarity of intestinal cells maintained by?
> microtubule
actin filaments
intermediate filaments
In an intestinal cell, what are intermediate filaments attached to?
Attached to desmosomes (adhesive structures) and to desmosomes (cell to cell contact) and hemidesmosomes (cell matrix contact).
In intestinal cells, what is the function of the microtubules?
Form tracks to get newly syntehsized proteins to proper locations.
Give an example of a cellular function that would cause the reassembly of the cytoskeletal subunits to occur to reshape the cytoskeleton in a cell response to an external signal.
Neutrophil chasing a bacterium.
What type of bond holds cytoskeletal filaments (or polyers) together?
Weak noncovalent interactions, which means that their assembly and disassembly can occur rapidly.
What kind of subunits are used to assemble actin filaments?
actin subunits
What kind of subunits are used for formation of microtubuels?
tubulin subunits
How do actin subunits and tubulin subunits self-associate?
By using a combination of end-to-end and side-to-side protein contacts.
What are protofilaments?
Long linear strings of protein subunits joined end to end.
Are single protofilaments thermally unstabe?
Yes
You know that a single protofilament is thermally unstable, but what about cytoskeletal filaments from multiple protofilaments bound side to side?
Requires breakage of several bonds in middle of stable filaments - resists breakage by ambient thermal conditions.
Are mutiple protofilaments thermally stable?
Yes
What does staggered side to side binding of filaments allow filaments to tolerate?
Bending and stretching forming rope-like structures (intermediate filaments).
What is polymerization?
Assembly of actin or tubulin subunits (monomers) into a linear polymer.
What is depolymerization?
Removal of monomers at the ends of the polymer.
How must subunits assemble for new large filaments to form?
Assemble into initial aggregate or nucleus.
For new large filaments to form, subunits must assemble into initial aggregate or nucleus. What is this initial process called?
Nucleation
What is the formation of an actin nucleus caused by?
Random collision of 3 subunits.
What can the time span for nucleation be tested in?
A test tube which examines the time course of actin polymerization.
In the formation of an actin filament, what is the rate-limiting step?
> Filament nucleation - lag phase:
- a process of formation of initial aggregate or nucleus
- the rate-limiting step
In which phase of actin filament formation are actin subunits quickly added onto the ends of nucleated filaments?
Filament elongation - growth phase.
In the formation of cytoskeletal filaments, what is the critical concentration?
Steady state in which the rate of monomer addition equals the rate of monomer loss.
What are the 3 courses of formation in cytoskeletal filaments?
1) Filament nucleation - lag phase
2) Filament elongation - growth phase
3) Steady state - equilibrium phase
What is tubulin?
A hetero-dimer of alpha-tubulin and beta-tubulin with non-covalent bonds.
What do both alpha-tubulin and beta-tubulin both have a binding site for?
GTP
- GTP in the alpha-tubulin is never hydrolyzed *
How many proto-filaments does 1 microtubule consist of?
13 - aligned in parallel.
What does arrangements of alpha and beta tubulins create?
Structural polarity.
Is beta-tubulin or alpha-tubulin considered the + end?
beta-tubulin
In a microbule, what subunits are in longitudinal contact?
alpha-beta
In a microtubule, what subunits are in lateral contact?
alpha-alpha or beta-beta
How are actin filaments arranged to generate structural polarity?
Head-to-Tail to generate structural polarity.
What binding site does an actin monomer contain?
Binding site for ATP or ADP.
What makes up an actin filament?
Consists of 2 protofilaents, held by lateral contacts.
Are actin filaments flexible and easily bent?
Yes
What are the two distinct and dynamic ends that actin filaments or microtubules have?
plus and minus end
How are such different ends (plus and minus end) made possible in actin filaments or microtubules?
by changes in the conformation of each subunit as it enters the polymer.
What are the characteristics of the plus end?
> Fast-growing or shrinking end.
> Has beta-tubulin or referred as barbed end of actin filament.
What are the characteristics of the minus end?
> Slow-growing or shrinking end.
Has alpha-tubulin or the GTP binding cleft on the actin monomer point towards to minus end, also referred as pointed end.
True of False:
Elongation proceeds spontaneously when deltaG for addition of the monomer is less than zero, due to the [monomer] exceeds the critical concentration.
True
What do actin and tubulin enzymes catalyze?
ATP (actin) or GTP (tubulin)
What is bound to tubulin or actin in the T form?
GTP/ATP
What is bound to tubulin or actin in the D form?
GDP/ADP
True or False:
Each monomer carries a tightly bound ATP or GTP molecule that is hydrolyzed to ADP or GDP soon after the monomer assembles into the polymer.
True
What are the two types of subunit structures?
T form and D form.
What form does the tip of the polymer remain in if the rate of polymerization is faster than the rate of hydrolysis of the bound nucleotide?
T form - as an ATP cap or GTP cap.
What is treadmilling?
The plus end grows while the minus end shrinks.
In treadmilling, what form does the plus end remain in?
Plus end remains in T formation.
In treadmilling, what form does the minus end remain in?
Minus end adopts D formation.
In treadmilling, at which ends are subunits added and removed?
Subunits added at plus end and removed at minus end.
In treadmilling, does the polymer maintain a constant length?
Yes
Does treadmilling predominate in actin filaments or microtubules?
actin filaments
What is dynamic instability?
The rapid inter-conversion between a growing and shrinking state at a constant concentration of free subunits.
What causes catastrophe?
If nucleotide hydrolysis proceeds more rapidly than subunit addition, the cap is lost and the microtubule begins to shrink.
What causes rescue?
GTP-containing subunits may still add to the shrinking end, and if enough add to form a cap, then microtubule growth resumes.
What are the two different structures that microtubules can take during the addition or hydrolysis of GTP?
> Adding tubulin-GTP produces straight proto-filaments.
> Hydrolysis of GTP after assembly changes the conformation of subunits, making the proto-filaments curved.
What makes the proto-filaments curved?
Hydrolysis of GTP after assembly changes the conformation of subunits.
What makes the proto-filaments straight?
Adding tubulin-GTP produces straight proto-filaments.
Is dynamic instability predominate in microtubules or actin filaments?
Microtubules
What allows the GDP-protofilaments to relax into the more curved conformation?
loss of GTP cap
On rapidly growing microtubules, protofilaments containing GDP-subunits are forced into a linear conformation by what?
Many lateral bonds, given a stable cap of GTP-subunits.
What is each monomer of an intermediate filament?
Each monomer is an elongated molecule with an extended central alpha-helical domain.
How do monomers connect to each other in constructing an intermediate filament?
Monomer forms a parallel coiled-coil dimer with another monomer.
How do dimers associate in constructing an intermediate filament?
A pair of dimers associate in an antiparallel manner (N to C; C to N) to form a staggered tetramer (C).
True or False:
Intermediate filaments have no nucleotide binding sites and no structural polarity.
True
- only polarity is the N- and C-terminus *
In the construction of intermediate filaments, how is each tetramer structured to allow association with another tetramer?
Within each tetramer, the two dimers are offset, allowing it to associate with another tetramer.
Each parallel tetramers (proto-filaments) pack together lateraly to form the filament
How many parallel tetramers (proto-filaments) pack together laterally to form the filament?
8 parallel tetramers (proto-filaments) pack together laterally to form the filament.
What is the most diverse group of intermediate filaments?
Keratins
In the human genome, how many distinct keratin genes are there?
50 distinct keratin genes in the human genome.
How many of the keratin genes are found in human epithelial cells?
about 20 found in human epthelial cells
How many of the keratins genes are specific to hair and nails?
10 specific to hair and nails.
How do keratins impart mechanical strength?
By anchoring intermediate filaments at site of cell-cell contacts, desmosomes, or cell-matrix contacts, called hemidesmosomes.
What type of protein imparts mechanical strength by anchoring intermediate filaments at site of cell-cell contacts, desmosomes, or cell-matrix contacts, called hemidesmosomes?
Keratin
What are the 3 types of cytoskeleton proteins?
- actin filaments
- microtubules
- intermediate filaments
List the features of the cytoskeleton.
- Cytoskeleton are built from protein subunits.
- Cytoskeletal filaments are formed from proto-filaments.
- Nucleation is the rate-limiting step in the formation of cytoskeletal filament.
- Polymerization of protein subunits creates structural polarity.
- Nucleotide hydrolysis by tubulin and actin results is filament treadmilling and dynamic instability.
