Cytoskeleton Flashcards
Eukaryotic cells have developed these spatial and mechanical functions in an intricate system of filaments called the
Cytoskeleton
The cytoskeleton’s varied functions depend on the behavior of three families of protein molecules which assemble to form three main types of filaments
1) Microfilaments: Actin,
2) Microtubules: tubulin
3) Intermediate Filaments
Microfilaments determine
cell shape and locomotion
For cells to function properly they must (2)
organize themselves in space and interact mechanically with their environment.
Microtubules determine
positions of organelles and direct intracellular transport
Intermediate filaments provide
mechanical strength
Actin is concentrated inside the
PM (anchors to PM)
In a happy and heathy cell the MTs will
extend all the way through the cell into the cortex
if a cell is not healthy you will see
retraction of the MTs.
Stress fibres
Thick and bundled actin that the cell can use to generate force for movement.
Cytoskeletal filaments are
dynamic and adaptable
Microtubules are often
in a star-like pattern emanating from the nucleus to the cell periphery
Microtubules are a major components of:
-the mitotic spindle during cell division,
-cilia and flagella for cell motility
-Structural pathways for the transport of materials within the cell
Actin filaments underlie the ________ and provide ______
PM, provide strength and shape to the thin lipid bilayer
Actin filaments also form
many types of cell-surface projections such as lamellipodia and filopodia that cells use to explore territory.
The actin-based contractile ring assembles to
divide cells in two.
Filopodia is used to
explore territory, they would have receptors on the tips that can interpret their environment.
Lamelopodia would be
a sheath that gives the cell the machinery it needs to move. It does this by polymerizing and depolymerizing
Intermediate filaments form a
protective cage for the cell’s DNA at the inner face of the nuclear envelope.
Intermediate filaments are twisted into
strong cables that can hold epithelial cell sheets together.
Intermediate filaments allow (2)
-nerve cells to extend long sturdy axons
-form tough appendages such as hair and fingernails
An example of an intermediate filament
Keratin
Cells that line the intestine and lung contain
microvilli and cilia (cytoskeletal-based cell surface protrusions) that maintain a constant location, length, and diameter.
The cytoskeleton also dictates
cell polarity during the lifetime of a cell.
Each type of cytoskeletal filament is constructed from
smaller protein subunits
What can small protein subunits do that large filaments cannot
diffuse rapidly across the cytoplasm
Actin exists in the cell as
small soluble subunits and as large polymers
Intermediate filaments are made up of _____ whereas actin filaments and microtubules are made of ________
smaller subunits that are themselves elongated and fibrous, subunits that are compact and globular (actin subunits and tubulin subunits)
What interactions hold subunits together
Non-covalent
Nucleation is
rate-limiting step in the formation of a cytoskeletal polymer
The tubulin and actin subunits assemble __________ to create _________
head-to-tail, polar filaments
Tubulin subunits are heterodimers composed of _________ and ________– tightly bound together by _________
α-tubulin and β-tubulin, non-covalent bonds.
Both α- and β-tubulin monomers have a GTP binding site, but
the GTP in the α-tubulin is trapped at the dimer interface while β-tubulin can have its GTP hydrolyzed to GDP or exchanged for a new GTP. GTP hydrolysis is thus important for microtubule dynamics
Longitudinal contact
between α- and β-tubulin
Microtubules are the stiffest and straightest structural elements found in most animal cells due to their
high persistence length (the property of a filament describing how long it must be before random thermal fluctuations cause it to bend)
The actin subunit is a _______ rather than a _______
Monomoer, dimer
Actin subunit binds ____ rather than _____
ATP, GTP
Because of the polarity of microtubules and actin filaments their ends
grow and shrink at different pace
The fast growing end, as well as the fast shrinking end, is called the
Plus end (+)
Whether filaments grow or shrink depend on whether the free subunit concentration is above or below the
Critical concentration
The critical concentration is. the
concentration of subunits where the filament grows and shrinks at the same rate= treadmilling
Treadmilling and dynamic instability are consequences of
nucleotide hydrolysis by tubulin and actin
newly incorporated subunits are in a
T form (GTP or ATP containing)
The longer a subunit is incorporated into a filament the more likely it is to be hydrolyzed, therefore in
D form (ADP- or GDP-containing).
If the rate of subunit addition is low then the filament tip will be in the
D form
D form has a ______ critical concentration than the T form.
higher
what happens when the concentration of free subunits is in an intermediate range? That is, lower than the critical concentration of the D form but higher than that of the T form?
Treadmilling
In the D form you need ________ subunits available to add to be able to maintain the length
more
GTP cap
the rate of hydrolysis was faster than the rate of subunit addition
(as it grows we begin to hydrolyze the GTP to GDP, and as this happens if we ran out of available soluble subunits ) (Plus end)
Loss of GTP cap
rapid shrinkage at the plus end- this is called MT Catastrophe
(rate of hydrolysis faster than rate of subunit addition)
MT rescue
as plus end shrunk it would release tubulin dimers, dimers could exchange their GDP for GTP and the concentration of GTP would increase beyond the CC and we would get rapid growth and the GTP would form again
When tubulin is GTP bound it is more
stable
A GTP bound tubulin specific biochemical structure
Straight
GDP bound tubulin in the filament causes
conformational change, slight curvature
A MT is made up of ____ protofilaments
13
Why continuously spend energy hydrolyzing nucleoside triphosphate?
allows cells to maintain a fluid structure that is primed for a rapid response to external stimuli or the environment.
Intermediate filament structure depends on
Lateral bundling and twisting of coiled coils
Actin and tubulin is present in
all eukaryotes
Intermediate filaments are only present in
some metazoans such as vertebrates, nematodes, and mollusks.
Intermediate filaments are predominantly found in cells
exposed to mechanical stress
IF assembly
-You have an N terminal and a c terminal, one monomer coils with another monomer, forming a dimer. -Dimer interacts with another dimer forming a staggered tetramer,
-They interact head to tail, in that the first dimers N terminal will be closer to the second dimers C terminal, and they interact in this staggered manner.
-Two tetramers pack closely together and then 8 tetramers twist into a ropelike structure called a filament
Is there directionality and polarity in assembled intermediate filaments
no (nothing can move along, no way to tell which way to go)
A second family of intermediate filaments are the
neurofilaments
Neurofilaments are found in ____ concentrations along ________
high concentrations along axons
Adherens proteins and junction
connect branched actin to next cell, giving it strength from cell to cell (in epithelial cells)
Desosomes
Connections between cells via IF, provide stability and strength to cells and therefore lining of bl vessel or intestine
Why is minus end of MTs toward apical and plus end towards basal surface
Cellular machinary can move things in proper direction
-ex. blood vessel: apical side next to inside of vessel where blood will be, cell needs to move nutrient s from blood into cells and epithelial layer
Cytoskeleton maintains _________ of cell that allows specialized cells to perform their functions
Polarity
Filaments formed from multiple __________ have advantageous properties
protofilaments
Nucleation experiment what happens to soluble actin subunits when adding salt
They polymerize
Lag phase
Nucleation
Growth phase
Elongation
Equilibrium phase
Steady state
Steady state
same number of subunits coming off the actin filament that you are adding to the other end
Concentration at which steady state occurs
critical condition
What happens if we added oligomers at the neginning (nucleation experiment)
Reach critical concentration in less time, same rate of growth, build fibril faster, bypass nucleation step
What happens if you fully depolymerize actin
The rate at which you could build a new fibril would reduce
Lateral contact
α-α and β-β
What do lateral and longitudinal contacts do
Strengthen microtubules
Structure of MT: where is GTP present
Both the alpha and beta subunit
GTP in the alpha subunit(MT)
Buried in the core to make the interactions between 2 units very strong
GTP in the beta subunit
readily accessible for hydrolysis
Alpha and beta subunits bind to eachother to form ________ which gives structure
protofilaments polarity
What do protofilaments have on their ends
A plus end and a minus end
The GTP of what subunit is readily accessible for hydrolysis
beta
Polymerization will happen more readily at the
plus end
It is slower to add a subunit to the minus end because it is in
D form
What happens to MT when you lose GTP bound cap
Fraying, no strength at end (less interactions), less energy needed to break bonds
Amyotrophic lateral sclerosis is associated with the
Accumulation and abnormal assembly of neurofilaments in motor neuron cell bodies and in the axon
