Unit 9 - Cytoskeleton Flashcards
The cytoskeleton is:
A network of protein filaments extending throughout the cytoplasm in all eukaryotic cells
Two functions of the cytoskeleton:
- Provides a structural framework for the cell, serving as a scaffold that determines cell shape and the general organization of the cytoplasm.
- Responsibleforthemovementsofentire cells and for the internal transport of
organelles and other structures (such as mitotic chromosomes) through the cytoplasm.
1/2 Functions of the cytoskeleton:Provides a structural framework for the cell, serving as
a scaffold that determines cell shape and the general organization of the cytoplasm.
2/2 function of the cytoskeleton: Responsible for the movements of entire cells and for
the internal transport of
organelles and other structures (such as mitotic chromosomes) through the cytoplasm.
nucleus
The cytoskeleton is composed of 3 principal types of protein filaments:
- intermediate filaments
- microtubules
- actin filaments
The cytoskeleton is composed of ___ of protein filaments:
3
The cytoskeleton is composed of 3 principal types of protein filaments: These 3 types of protein filaments are held together and linked to subcellular organelles and the plasma membrane by:
a variety of accessory proteins
Intermediate filaments have a diameter of about
10 nm, which is intermediate between the diameters of the two other principal elements of the cytoskeleton
diameter actin filament
8nm
diameter microtubules
25 nm
In contrast to actin filaments and microtubules, intermediate filaments (IFs) are:
not directly involved in cell movement.
not directly involved in cell movement
intermediate filaments
main function intermediate filaments:
enable cells to withstand the mechanical stress that occurs when cells are stretched.
There are _ classes of intermediate filaments:
4 classes total
3 in the cytoplasm, 1 in the nucleus
Four classes of IFs –
3 in the cytoplasm, 1 in the nucleus
Four classes of IFs – 3 in the cytoplasm, 1 in the nucleus. Between all four classes there are nearly __ different types of IFS
50 different types of IFs.
What are the three types of intermediate filaments:
(1) keratin filaments in epithelial cells
(2) vimentin and vimentin related filaments in connective-tissue cells, muscle cells, and glial cells
(3) neurofilaments in nerve cells
What is a type of nuclear intermediate filament?
nuclear lamins in all animal ells
Intermediate filaments (IFs) appearance in the cell:
-IFs form a network throughout the cytoplasm, surrounding the nucleus and extending out to the cell periphery, where they are anchored to the plasma membrane.
-They are also INDIRECTLY CONNECTED TO NEIGHBOURING CELSS through a cellular structure called the DESMOSOME.
Assembly of intermediate filaments (3 steps)
(1)Each monomer has an extended, central a- helical domain, and unstructured carboxy- and amino-terminal domains.
(2)Two monomers wrap around each other IN PARALLEL using their a-helical domains to form a COILED COIL DIMER.
(3) Two dimers associate in a staggered and anti-parallel fashion to form a tetramer
do mature intermediate filaments have polarity?
no : one end of the IF resembles the other
Do microtubules have polarity?
yes
Do microfilaments have polarity?
yes
Of the three principle types of protein filaments, which are polar /nonpolar
- intermediate: nonpolar
- microtubules: polar
- actin filaments: polar
Assembly of intermediate filaments: after formation tetramer
Eight tetramers then laterally associate and are added to the growing IF. Unlike microfilaments and microtubules, there is no nucleation involved and IFs build onto existing IFs. Thus, the IF network is not very dynamic.
nucleation involved in formation of which 3 proteins?
Intermediate filaments - no nucleation involved
mirofilaments - yes
microtubules - yes
Mature cytoplasmic IFs have a
rope-like structure.
IF network is not
very dynamic
Mature cytoplasmic IFs have a __ structure
rope-like
Intermediate filaments underlying the inner face of the nuclear envelope form the
nuclear lamina
Intermediate filaments underlying the inner face of the nuclear envelope form the nuclear lamina, a fibrous network that (2):
- supports the nuclear membrane
- provides attachment sites for the chromatin
The nuclear lamina differs from the cytoplasmic IFs in
structure
The nuclear lamina differs from the cytoplasmic IFs in structure since
it forms a meshwork as opposed to a rope-like structure.
nuclear lamina shape
meshwork
The nuclear lamina disassembles with each cell division when
the nuclear envelope breaks down
The nuclear lamina disassembles with each cell division when the nuclear envelope breaks down. This is regulated by (2)
(1) phosphorylation of the lamina to cause disassembly
(2)dephosphorylation to allow for its reformation.
IFs and human disease
Mutations in a nuclear lamin protein are associated with progeria (premature aging in children) leading to death at a very young age.
Mechanism is unknown – may involve impaired cell division, increased cell death, inability to repair tissue.
Microtubules: shape:
Microtubules are rigid, hollow rods approximately 25 nm in diameter.
microtubules: dynamicity
microtubules are dynamic structures that continually undergo assembly and disassembly.
Main functions of microtubules (3)
- the separation of chromosomes during mitosis
- the intracellular transport of membrane-bound vesicles and
organelles - cell movement
Microtubules (MTs) are composed of
a single type of globular protein, called tubulin.
tubulin
globular protein
Tubulin is a
heterodimer consisting of 2 closely related proteins, a-tubulin and b- tubulin.
tubulin - b-tubulin interactions (non- covalent) to form a protofilament. This gives the filament polarity: a- tubulin is exposed at one end (the minus end) and b-tubulin is exposed at the opposite end (the plus end).
what is the growing end of microtubules:
The plus end is considered the growing end
d b-tubulin is exposed at the opposite end (the plus end).
The plus end is considered the growing end. This polarity plays a role in determining
the direction of movement along MTs.
__ arrange in a tube structure to give the microtubule.
13 protofilaments
MTs are highly
dynamic
The dynamic nature of MTs is dictated by
the ability of GTP to bind to tubulin.
growing MTs have a
GTP cap
growing MTs have a “GTP cap” but
shrinking MTs do not.
major microtubule-organizing center
centrosome
The centrosome is
the major microtubule-organizing center
The microtubules in eukaryotic cells extend outward from a
microtubule organizing centre (MTOC).
The microtubules in eukaryotic cells extend outward from a microtubule organizing centre (MTOC). The __ of the MT is anchored to the MTOC.
In animal cells the major MTOC is called
the centrosome
centrosome, located
adjacent to the nucleus
The centrosome is composed of (3):
- 2 centrioles
- Pericentriolar material
- y-tubulin ring complexes
1/3 of the elements that compose the centrosome are the 2 centrioles:describe
a unique arrangement of MT protofilaments at right
angles to each other
describe pericentriolar material (an element of the centrosome)
an amorphous collection of several proteins
describe y tubulin ring complexes (an element of the centrosome)
composed of a special form of tubulin called g-tubulin and accessory proteins. Serves as the nucleation site for MT assembly.
Movement of membrane-bound vesicles and organelles along microtubules is based on
the action of motor proteins.
Motor proteins utilize __to travel steadily along the microtubule in a single direction.
utilize energy derived from ATP hydrolysis
Two families of motor proteins:
- kinesins 2. dyneins
kinesins
move toward the plus (+) end of a microtubule (away
from the centrosome)
dyneins
movetowardtheminus(-)endofamicrotubule
(toward the centrosome)
Microtubule motor proteins:The cargo is bound to
the tail domain
Microtubule motor proteins:MT binds to
the globular head of the motor protein.
The movement of a kinesin motor protein
Trailing head has ADP bound, leading head has no nucleotide.
Binding of ATP by the leading head induces a conformational change causing the former trailing head to move forward and become the leading head.
New leading head binds to MT ~16 nm ahead of its previous site. Therefore the cargo moves in 8 nm increments.
New leading head releases ADP. Trailing head hydrolyzes ATP to ADP and Pi. The cycle is now ready to repeat.
Actin filaments are highly concentrated at
the periphery of the cell, where they form a three-dimensional network underlying the plasma membrane.
function network of actin filaments (3):
- provides mechanical support 2. determinescellshape
- enablescellstomigrate
Actin filaments are made of the protein
actin
actin exists as (2):
(1)globular monomer called G- actin
(2)filamentous polymer called F- actin
Actin monomers (G-actin) polymerize to form
actin filaments (F-actin)
Actin filaments have the appearance of
a double-stranded helix
Because all the actin monomers are oriented in the same direction, actin filaments have a
distinct polarity and their ends (called the plus (+) and minus (-) ends) are distinguishable from one another.
Like microtubules, new monomers are added to __ of the filament
plus
Like MTs that nucleate at the microtubule organizing centre, actin microfilament assembly begins by
the nucleation of actin monomers.
Like MTs that nucleate at the microtubule organizing centre, actin microfilament assembly begins by the nucleation of actin monomers. However, the nucleation is
not restricted to a particular region of the cell. It is stabilized by a protein complex called Arp2/3. This complex also allows for branch-points in the network.
Microfilaments then grow by
the reversible addition of monomers to the nucleated microfilaments.
Like MTs, actin microfilament growth requires
energy, in this case it requires ATP.
Two states of G-actin:
- ATP-bound (predominant state) 2. ADP-bound
ATP-bound G-actin __ and __ than the ADP- bound form
ATP-bound G-actin polymerizes faster and dissociates slower than the ADP- bound form
Both the assembly and disassembly of actin filaments are regulated by
actin-binding proteins
The turnover of actin filaments plays a critical role in
a variety of cells movements.
The key protein responsible for actin filament disassembly is
cofilin
Cofilin binds to actin filaments and
increases the rate of dissociation of actin
monomers (bound to ADP) from the minus (-) end.
Cofilin remains bound to the ADP-actin monomers, preventing
their reassembly into filaments.
profilin can
stimulate the incorporation of actin monomers into filaments.
Profilin acts by
stimulating the exchange of bound ADP for ATP
Profilin acts by stimulating the exchange of bound ADP for ATP, resulting in __
the formation of ATP-actin monomers
rofilin-bound ATP-actin monomers can be
re-polymerized into filaments.
the balance between cofilin and profilin activity determines
the state of actin polymerization
the balance between cofilin and profilin activity determines the state of actin polymerization and this is controlled by
signaling pathways
At steady-state, where actin filament assembly = actin filament disassembly, there is
no net change in the actin filament length but the filament is said to “treadmill” as the newly-added actin moves down the filament.
All actin-dependent motor proteins belong to
the myosin family.
Myosins bind and hydrolyze ATP, which provides
energy for their movement along actin filaments from the minus (-) end of the filament toward the plus (+) end of the filament.
Two subfamilies of the myosin family of motor proteins:
- myosin I - found in all types of cells
- Myosin II - most abundant in muscle
Myosin I is a
single molecule with one globular head and a tail that attaches to another molecule or organelle in the cell.
Myosin I is a single molecule with one globular head and a tail that attaches to another molecule or organelle in the cell.
In this way the attached molecule or organelle can be moved along actin filaments by
the motor activity of the myosin I head.
The HEAD DOMAIN of myosin I interacts with:
ACTIN FILAMENTS and has ATP-hydrolyzing motor activity that enables it to move along the filament
the __ varies between the different types of myosin I and determines __
The TAIL VARIES between the different types of myosin I and DETERMINES what cell components will be moved along by the motor
Myosin I can move
vesicles along actin filaments towards the plus end.
Myosin I can move vesicles along actin filaments towards the plus end.
It can also _
attach the plasma membrane to the cortical actin filament network, pulling the plasma membrane into a different shape.
Myosin II is a
dimer with two globular heads and a tail that forms a coiled-coil structure.
Myosin II dimers associate through their coiled-coil tails forming a
myosin II filament.
the head domain of myosin 2 interacts with __ and has ATP hydrolyzing motor activity
The HEAD DOMAIN interacts with ACTIN FILAMENTS and has ATP-hydrolyzing motor activity that enables it to move along the filament
The head domain interacts with actin filaments and has ATP-hydrolyzing motor activity that enables it to move along the filament. In this way, myosin II can
cause actin to contract, which happens in muscle cells
Muscle contraction: Myofibrils are made up of structures called
sarcomeres
Muscle contraction:Each sarcomere is
a highly- organized assembly of actin microfilaments and myosin II filaments.
Muscle contraction:The plus (+) end of the actin filament is attached to a structure called
the Z disc.
During muscle contraction, the myosin filaments start
moving towards the plus end, pulling the actin filaments closer together.
muscle contraction is triggered by:
a rise in intracellular Ca2+ produced by stimulation from an adjacent neuron
Contraction is triggered by a rise in intracellular Ca2+ produced by stimulation from an adjacent neuron. The calcium is released from
a specialized region of the ER in skeletal muscle called the sarcoplasmic reticulum.
Actin filaments: Actin polymerization is __ dependant
ATP
Microtubules: Tubulin polymerization is __ dependent.
gtp
