L15. Cytoskeleton Flashcards
what are microtubules
- hollow cylinders
- made of the protein tubulin
- one end is attached to the centrosome
- is the most rigid and biggest
what are intermediate filaments
- very flexible and rope like
- forms the nuclear lamina
- used for mechanical and tensile support
what are actin filaments
- they are flexible fibers
- they are most highly concentrated in the cortex
- the most smallest
microtubules - what is the centrosome
- microtubule organizing center
- microtubules grow out of the centrosomes
- located near the cell center
- it consists of a pair of centrioles that is surrounded by a matrix of proteins
microtubules - what are its purposes
- create tracks for transport and organelle positioning
- during mitosis, they disassemble and reassemble as the mitotic spindle
- they also make up cilia and flagella (bacterial flagella is different)
microtubules - explain how the tubes are made
- each tubulin subunit is a dimer composed of 2 globular proteins (alpha and beta tubulin)
- the tubulin dimers are stacked into 13 parallel protofilaments
- these protofilaments have structural polarity
microtubules - explain its polarity
- alpha tubulin makes up the - end
- beta tubulin makes up the + end
microtubules - how does tubulin polymerize
- they polymerize from nucleation sites on the centrosome
- the centrosome organizes an array of microtubules that radiate outwards through the cytoplasm
- the centrosome matrix has gamma tubulin and it serves as a starting point for microtubules (- end is embedded in the centrosome)
microtubules - how do they grow
- dynamic instability permits rapid remodeling
- alpha/beta tubulin dimers are added to the + end as it grows
- alpha/beta tubulin dimers are lost from the - end as it shrinks
microtubules: dynamic instability - how can + and - ends be stabilized
- minus ends: being linked to the centrosome
- plus ends: stabilized by binding to specific proteins (capping proteins)
microtubules: dynamic instability - why do microtubules grow this way
it is easier to add gamma tubulin than to nucleate from scratch
microtubules: dynamic instability - what is it driven by
- GTP hydrolysis
- GTP-tubulin binds more tightly than GDP-tubulin
microtubules: GTP hydrolysis - growing microtubules
- polymerization
- each dimer binds to GTP
- GTP is hydrolyzed shortly after the addition of the dimer and GDP will remain tightly bound to beta-tubulin
- polymerization being faster than GTP hydrolysis creates a GTP cap
- this then packs the microtubule more efficiently
microtubules: growing microtubule - what is the GTP cap
it is where all of the tubulins are GTP-bound and can bind more strongly
microtubules: GTP hydrolysis - shrinking microtubules
- depolymerization
- GDP-tubulin associates less tightly and fall off
microtubules - create tracks for transport
- all the microtubules in the axon points in the same direction
- plus end is towards the termini and serves as a track for transport
- backward and outward transport is driven by different motor proteins
microtubules: create tracks for transport - motor proteins for forward and backward transport
- kinesins and dyneins use ATP hydrolysis to move cargo along microtubules via globular heads
- both are homodimers
- kinesins move towards the + end
- dyneins move towards the - end
microtubules: create tracks for transport - explain the motor proteins specificity
- kinesins and dyneins transport different cargo
- their tail determines cargo specificity
microtubules - organelle positioning
- kinesin pulls the ER outward and stretches it
- dyneins pull the Golgi inward towards the nucleus
microtubules: cilia - explain the cilia of an epithelial cell
- hair-like structures
- grows from a cytoplasmic basal body that serves as an organizing center
- has a core of bundled microtubules that grow from the basal body
microtubules: cilia - explain their movements
- whip-like
- power stroke: cilium is fully extended and fluid is driven over the surface of the cell
- recovery stroke: cilium curls back into position with minimal disturbances
microtubules: cilia and flagella - what are their uses
- respiratory tract (cilia): mucus swallowing
- oviduct (cilia): helps carry egg
- flagella: present in sperm and protozoa
microtubules: flagella - explain their movements
- dyneins causes flagella to beat
- they are present at regular positions and serve as cross-links to hold microtubule bundles
- other protein generate the force that causes the bending
- without dynein, the force is sliding instead of bending
microtubules: cilia and flagella - how are the microtubules arranged
in a 9 + 2 array: 9 double microtubules in a ring around a pair (2) of single microtubules
intermediate filaments - nuclear lamina
- it supports and strengthens the nuclear envelope
- the nuclear lamina is a network of filaments called lamins
- assembly and disassembly is done via phosphorylation and dephosphorylation
intermediate filaments - what are its uses
- has durable networks in the cytoplasm
- great tensile strength and helps cells withstand mechanical stress
intermediate filaments - what are they anchored to
- desmosomes
- enables connections between cells
intermediate filaments - what are they composed of
- the monomer is an alpha-helical rod domain
- pairs of monomers associate to form a parallel dimer
- 2 dimers form an antiparallel tetramer
- tetramers pack together to make the intermediate filament
intermediate filaments: what are the 4 classes - cytoplasmic
- keratin filaments (in epithelial cells)
- vimentin and vimentin-related filaments (in connective-tissue cells, muscle cells, and glial cells)
- neurofilaments (in nerve cells)
intermediate filaments: what are the 4 classes - nuclear
nuclear lamins (in all animal cells)
intermediate filaments: what are the 4 classes - keratin
- most diverse class
- in hair, feathers, and claws
- ends of filaments are anchored to desmosomes
- it creates a cabling tensile strength that absorbs stress from a stretching force
intermediate filaments: keratin - explain an abormaility
- mutant keratin gene: Epidermolysis bullosa simplex
- makes skin prone to blistering and rupture
intermediate filaments - what is plectin
- it is an accessory protein that reinforces intermediate filaments
- it cross-links filaments into bundles and links them to microtubules, actin, and desmosomes
actin filaments - explain the filaments
- they are polymers of actin
- they also need accessory proteins for stability
- they have a cleft in the monomer that provides a binding site for ATP/ADP
- each filament is a 2-stranded helix
actin filaments - what cell shapes can they create
- microvilli
- dynamic protrusions
- lamellipodium
- contractile ring
actin filaments - how do they grow and shrink
- treadmilling
- actin monomers carry ATP and it is hydrolyzed after assembly and reduces the strength of the binding
- plus end receives addition much faster than the ATP is hydrolyzed, making it more stable and it grows
- at the same time, the - end contracts
treadmilling vs dynamic instability
- treadmilling: when rates are equal, the strand stays the same size
- dynamic stability: rapid switch from growth to shrinkage, thus microtubules undergo more drastic changes than actin filaments
actin filaments - what are actin binding proteins
- they bind and sequester actin
- they hold together bundles in microvilli and cross-link actin in the cell cortex
- they can also associate with myosin motors to form contractable bundles and form tracks for support
actin filament - examples of actin binding proteins
- formins and actin-related proteins (ARPs)
- they both promote polymerization
actin filaments - explain the concentration of actin in the cell
- 5% of all protein is actin
- 1/2 is assembled (other half are in reserve)
actin filament - explain how a cell moves (chemotaxis)
- they use actin mobilization
1. they push out protrusions at the leading edge
2. protrusions adhere to a surface
3. cell drags itself forward
actin filament: chemotaxis - how do they push out protrusions using lamellipodia
- it extends a thin sheet of lamellipodia which has a dense network of actin
- it is driven by actin polymerization
- plus end is close to the PM
actin filament - explain animal cell migration
- along with lamellipodia, the cell can form filopodia (thin stiff protrusions)
- these help probe the environment
- lamellipodia and filopodia both grow via rapid local actin growth
- the filaments will push out of the membrane without breaking it
actin filaments - explain lamellipodia formation
- actin related proteins (APRs) promotes actin web formation
- it promotes a continual assembly at the leading edge and disassembly at the back
actin filaments - explain filopodia formation
- they use formins
- these proteins attach to the + end and promote unbranched actin assembly
actin filaments - explain the myosin family
- they are all actin-dependent motor proteins
- they bind to and hydrolyze ATP
- they provide energy to move towards the plus end of actin
actin filaments: myosin family - myosin I
- in all cell types
- has a head and a tail domain
- the head binds to actin and hydrolyses ATP to generate motor activity to move the myosin
- the tail binds to the cargo
actin filaments: myosin family - myosin II
- in muscle cells
- this subfamily are all composed of dimers
- it has 2 globular ATPase heads and a single coiled-coil tail
- clusters of myosin molecules bind to each other through tails to form a myosin filament
- the 2 sets of heads face and pull in opposite directions
actin filaments: myosin II - explain it in muscle cells
- if organized in a bundle with actin filaments, myosin II can create a strong contractile force
- thus creating muscle contractions
- also seen in contractile bincles (non-miscle cells) during cell division
actin filaments - explain skeletal muscle cells
- skeletal muscle fibers are huge multicellular cells formed through fusion
- the nuclei is just beneath the PM
- most cells are made of myofibrils
actin filaments: skeletal muscle cells - what are myofibrils
- contractile elements
- they are formed from chains of sarcomeres
actin filaments: skeletal muscle cells - what are sarcomeres
- they are contractile units of muscle
- it gives the muscle its striped appearance
- they are a high organized assembly of 2 types of filaments
actin filaments: sarcomeres - what are the 2 filaments
- myosin II: thick filament
- actin: thin filament
- the actin filaments are anchored at the + ends at z discs and overlaps with myosin
actin filaments: myosin II - how does myosin walk along the actin filament
- attached
- released
- cocked
- force-generating
- attatched (then it repeats)
actin filaments: myosin II movement - attached (first one)
- rigor conformation
- myosin head that lacks a bound ATP or ADP is attached tightly to the actin filament
actin filaments: myosin II movement - released
- ATP binds and causes the dissociation of myosin
- actin steps forward
actin filaments: myosin II movement - cocked
- ATP binding causes the displacement of the myosin head
- movement of the myosin is driven by ATP hydrolysis and ADP and Pi binds
actin filaments: myosin II movement - force-generating
- the myosin head is bound weakly and Pi is released
- this triggers the power stroke: the force generating change in the shape of the myosin head
- this causes the head to regain its original rigor confirmation
actin filaments: myosin II movement - attached (second one)
- head is bound again but it is in a new position with no ADP
- cycle starts again
actin filaments: skeletal muscle cells - explain skeletal muscle contraction
it is triggered by the release of Ca2+ from the sarcoplasmic reticulum
actin filaments: skeletal muscle contraction - how is Ca2+ released
- motor neurons provides the signal (neurotransmitter) to the skeletal muscle and this triggers the action potential
- the action potential then goes into transverse (T) tubules that extend inward from the PM around each myfibril
- the electrical signal is then relayed to the sarcoplasmic reticulum
- this causes voltage gated channels to release Ca2+
actin filaments: skeletal muscle contraction - what happens after Ca2+ is released
Ca2+ activates accessory proteins tropomyosin and troponin complex
actin filaments: skeletal muscle contraction - what is tropomyosin
- it is rigid and rod-like
- it binds in the groove of the actin
- this prevents myosin from associating with actin
actin filaments: skeletal muscle contraction - what the troponin complex
- is also binds to Ca2+
- it is associated with tropomyosin
- in high levels of Ca2+, it triggers a change in the troponin complex
- this then causes a shift in tropomyosin and allows it to be accessed by actin
- once it is accessed by actin, contraction occurs
