module 5 Flashcards
what is the cytoskeleton ?
network of structural proteins which will occupy the cytosol and extends throughout the cytoplasm and will allow signalling and transport to occur , will also give cell shape
what are the 3 classes of proteins in the cytoskeleton ?
intermediate filaments, actin and microtubules
microtubules primary role
support trafficking within cells
intermediate filaments primary role
add mechanical strength to cells
actin primary purpose
support cellular motility and large scale movements like contractions
what are the intermediate filaments like int he body ?
bones in the body, in where they will provide strength allowing them to resist shape change , are the strongest filaments and provide the greatest mechanical strength
why are intermediate filaments divided into different classes ?
different cells in the body will face different amounts of mechanical stress where different cells will have different types of intermediate filaments
class 1 intermediate filaments
acidic keratins , epithelial cells , used for tissue strength and integrity
class2 intermediate filaments
basic keratins , epithelial cells , tissue strength and integrity
class 3 intermediate filaments
Desmin, GFAP, vimentin , periphevin found in muscle cells , glial cells and are needed for sarcomere organization and integrity
class 4 intermediate filaments
neurofilaments protein, found in neurons and are used for axon organization
Class 5 intermediate filaments
LAMINS protein, found in nucleus needed for nuclear structure and organization
where does the strength of intermediate filaments come from ?
how individual proteins are packaged and assembled into polymers , wont gain strength until fully assembled
role of secondary structures in intermediate filaments
intermediate filaments are rich in a helices where al lot of there strength comes from and are responsible for the long coiled structures of the filaments while the hydrogen bonds will stabilize the structure
tertiary and quaternary structures of intermediate filaments
tertiary- contains coiled monomers
quaternary- contains coiled dimers
coiled monomers will come together to form what ?
will form a dimer where the they will wrap around each other to create a coiled coil - this will allow for maximum hydrogen bonding between 2 peptides
what will 2 dimers create ?
will create tetramers , where the dimers will assemble in antiparallel fashion , aligned lengthwise , the hydrogen bonding and strength of the filament increases
known as the building block of intermediate filaments
what are the 3 stages that the tetramer building blocks come together ?
1)formation of a unit length filament ( 8 tetramers join )
2) immature filament is created by unit length filaments coming together
3) mature filament is formed by compacting
what is the role of post translational modifications in intermediate filaments
will control the shape and functions of the intermediate filament
Phosphoryaltion and glycosylation will occur in the head and tail domains
phosphorlyation of an intermediate filament will result in
dissolution of an intermediate filament into unit length filaments, this is important because filaments need to be able to be assembled and dissasembled for cellular processes like cell division
what are the 3 specialized intermediate filament
Lamin,Desmin and keratin
what is a lamin ?
intermediate filament found in the nucleus which forms the nuclear matrix which will protect the chromatin
what are desmins ?
intermediate filament that doesnt for long thin filaments but acts by coonnecting structures together and is needed for muscle integrity
what is keratin?
intermediate filament that binds to desmosomes to forma complex
What is the primary role of microtubules ?
they are used in cellular trafficking and will determine how things are trafficked in the cytoplasm
microtubule assembly happens where and requires ?
many proteins and will be assembled in the microtubule organization centre, MTOC can be found in different areas of the cell
what are microtubules made out of
tubulins made of dimerized proteins
what is tubulin a and tubulin B ?
globular proteins which will bind in head and tail to
fashion to form a dimer
both tubulin a and B can bind to ?
GTP AND B tubulin can cleave its GTP to GDP , and there will be a shape change
STEPS in microtubule assembly
1) dimers form unstable polymers
2) Polymer growth -polymer with 6 subunits, it will be more stable and grow longitudinally and form a protofilament
3) formation of protofilament tubes - will form a sheet where 13 protofilaments will come together and becomes the nucleation site for microtubule elongation
4) assembly and disassembly - ends of microtubules , dimers will continue to come and go, if rate of assembly is higher than dissassmebly, the mircotubule will grow
Assembly in microtubules
a tubules will always have GTP while B tubulin will have GTP or GDP –> GTP bound to B tubulin, dimer polymerization will be favoured and they will be attached to each other
dissassembly in microtubules
B tubulins GTP IS HYDROLYSED to GDP, there will be a conformational change that promotes depolymerization–> leads to dissassembly
what is meant by polarity of microtubules ?
because of the end to end polymerization of dimers , the ends will have different polarity , both ends can grow but it wont occur at the same rate
what is dynamic instability
microtubules ability to shrink or grow very quickly depending on the environment of the cell and what it needs
what is a GTP cap?
growing microtubule ha s
what will hydrolysis do in dynamic instability
GTP hydrolysis can expose GDP bound subunits at the tip
what happens after hydrolysis
catastrophic depolymerization
what is recapping ?
GTP subunits will bind to recap the microtubule and stop depolymerization
microtubule growth will happen when ?
will continue growing when GTP bound dimers are available
tubule dimers are preferentially added to ? when will they always grow ?
the plus end and as long as there is GTP bound tubulins dimers present
when will they stop growing ?
When GTP is converted to GDP on the tubule dimers at one end they will rapidly fall off and can initiate catastrophe which is rapid depolymerization of tubule dimers at the plus end creating a shortened microtubule
what can be done against microtubule catastrophe ?
catastrophe aversion : Capping
catastrophe reversal : rescue
what is capping in catastrophe ?
microtubule is desired length, plus end will be bound to capping proteins that will add stability to microtubules and keep them polymerized even in GDP bound dimers
when can rescue occur ?
spontaneously if there is enough GTP bound dimers present but can also happen in the presence of other proteins
what are the 2 reasons dynamic instability are needed ?
allows cells to rapidly create new pathways for trafficking depending on its changing needs
- allows cells to exert force , any molecule near plus end of the microtubule will be transported through the cell as it grows or shrinks
microtubule based motor proteins are used for?
to control trafficking , they can bind to cargo that needs to be trafficked and will then bind to microtubule and walk along it, walking requires ATP
most common motor proteins are?
kinesin and dynein
kinesins will move towards which end ?
kinesin will move towards the plus end while dynein will move towards the minus end
what are the 5 steps in the walking of motor proteins ?
1) head on is bound to microtubule and head 2 is bound to ADP
2) walking movement is started by ATP binding to head 1creating a conformational change that allows head 2 to swing around
3) Head 2 will be over a binding site , it will bind to the microtubule and releases ADP
4)ATP in head 1 will undergo hydrolysis turning it to ADP causing it to be released from microtubule
5) entire process is repeated but with ATP binding to head 2 , creating head 1 to swing around
how do actin and microtubules differ?
they differ in the networks they form- actin network contributes to the structure of the cell and to large scale movements like muscle contractions , actin cytoskeleton can move the cell itself
composition of actin ?
globular proteins
movement of actin ?
motor proteins are used to initiate movement
what is the building block of the actin cytoskeleton ?
monomeric actin protein (actin monomers)
what is the structure of actin filaments?
bind to each other longitudinally and laterally which create high tensile strength
polarization and actin filaments
ends are not the same charge where the plus end is called the barbed end and the minus end is he pointed end
describe polymerization with actin
actin monomers will bind ADP/ ATP, binding of ATP to an actin monomer will promote assembly/ polymerization whereas binding of ADP will encourage dissassembly
what are the stages of actin polymerization
nucleation, elongation and steady state
nucleation
2 actin monomers dimerize however nucleation will take place when the 3rd actin monomer binds to the dimer to form nucleus trimer, serves as the same purpose as MTOC
ELONGATION
elongation can happen in both directions but polymerization is favoured in the plus end , actin polymerization is dynamic with actin monomers being added and removed
STEADY STATE
when rate of assembly is the same as rate of disssembly there will be a steady state and elongation will stop
what is actin treadmilling ?
favoured addition of monomers to one end with the same rate of removal from the other end and will keep actin filament the same length also resulting in the filament moving within the cell
how is treadmilling regulated ?
ATP rich actin concentration compared to ADP bound actin where ATP actin to polymerize is lower in the plus end than the minus end, ifATP concentration is perfect,
what does treadmilling allow ?
ability to rapidly adjust the actin cytoskeleton faster than intermediate filaments that require dissassembly
what role do actin binding proteins play ?
will modulate the structure and function of actin cytoskeleton and play important role in disassembly
monomer binding proteins
abp WHICH will bind directly to the actin monomers and influence polymerization
nucleating proteins
Abp which binds to actin polymers to increase stability
capping proteins
ABP that bind to the plus or minus end and can stabilize the polymer to prevent dissasembly and further assembly
severing and depolymerization proteins
proteins that bind to the actin polymer and sever or induce dissassembly
cross linking proteins
will create side to side linkage of actin polymer to form actin filament bundles
membrane anchors
link actin to non actin structures behave like integrins
Actin Binding motor proteins
proteins that bind to the actin filament and allow movement
most popular type of actin binding motor protein ?
myosin which has 18 different families which are a multi sub unit protein that contains either light chains or heavy chains based on size
what are the 3 domains of myosin ?
the motor ,regulatory and tail
motor domain in myosin
formed by a heavy chain, bind to actin filament and ATP
regulatory domain in myosin
heavy chain and 2 light chains which moves back and forth as myosin moves
tail domain in myosin
binds other proteins or myosins
how does myosin move ?
1) hydrolysis of ATP to ADP and inorganic phosphate causes a conformational change in regulatory domain swinging it like a lever
2) motor domain will bind to the actin filament where the inorganic phosphate is released from the myosin causing the myosin to be pulled along the actin filament , binding of new ATP will cause the myosin to unbind from actin filament
3)myosin will move towards the barbed or plus end of the actin filament
what is cellular migration ?
when the cell actually moves around not just cargo
how does the cell carry out migration ?
the cel will use actin filaments and motor proteins to create pushing and pulling forces
cell migration is initiated how?
actin filaments will polymerize near plasma membrane which will push it outwards
what are the 3 actin responsible for cell migration ?
filopodia, stress fibres and lamellipodia
what are filopedia?
thin parallel bundles of filaments which will extend in the direction of intended movement
lamellipodia
larger sheet like bundles of actin filaments, which are polar structures that are broader and distend a wider amount of the plasma membrane
stress fibres
actin filaments that are rich in motor proteins and are anchored to integrins which allows the cells to move forward
what is interphase ?
G1 , S and G2 where most of the cells life will be in G1- interphase is when the cell is actively living or preparing to divide
what is mitosis ?
when cells are dividing where it will go through a cell cycle where it will continue to divide in its lifetime
why are cell cycle checkpoints needed ?
cells have developed certain checkpoints at certain transition phases in cell division - division requires lots of energy with mass rearrangements taking place
what is the functional unit of checkpoints ?
cyclins and cyclin dependant kinases which are proteins that are associated with progression in the cell cycle
CDK and cyclins will bind together to get activated in which other proteins will undergo phosphorlyation
how do cyclins and CDK work in the cycle ?
CDK and cyclins will bind together to get activated in which other proteins will undergo phosphorlyation that will trigger the next stage of the cell cycle
G1 phase in cell cycle
not actively dividing, , G1 cells will be active and growing but have not committed yet to dividing and need to pass checkpoint to begin
G0
not part of the cycle but is when cells are not dividing and can be described as being in a resting state
G1/S checkpoint
Dna damaged is checked out before it goes to S phase which is known as the start. point and commits to division, activation of signals allowing cell to divide
S phase
cell replicates entire genome and centrosome is duplicated, the cell is getting ready to divide
S/G2 checkpoint
check of DNA integrity before it goes to G2
G2 phase
last chance for cells to grow before division takes place and cellular contents are increased
G2/M checkpoint
tigers large scale rearrnagement in the cell structures that allows for mitosis –> increase in cell volume will allow cells to pass this checkpoint
M phase
Stage where mitosis and cell division take place
mitotic spindle checkpoint
makes sure all chromosomes are properly separated preventing chromosome imbalance –> makes sure that cytokinesis only takes place after mitosis
P 53 protein
tumour suppressor proteins that makes sure that cells with damaged DNA dont divide and can start apoptosis within cells that are damaged- protein doesnt work ,cell will divide uncontrollably
Interphase and mitosis
not a phase in mitosis but a state that the cell stays in preceding mitosis( where G1, S AND G2 take place )
prophase
first stage of mitosis where chromosomes become condensed and packed into chromatids and there will be 2 copies of each chromosome known as sister chromatids and are connected by the centromere
- gene transcription stops and endomembrane system dissolves where mitochondria remain intact
what happens after chromosome condensation ?
nuclear envelope will dissolve that will release chromosomes into the cytosol and the microtubules will reorganize into form mitotic spindle that will form around the centrosomes
what are centrosomes ?
centrioles and proteins form together to make centrosomes which are used as MTOCS
Prometaphase in mitosis
formation of the kinetochore which is protein complex that binds to chromatids on either side of the centromere each sister chromatids will have 2 kinethochores
what is the function of kinetochores ?
molecular motors that will use ATP to polymerize spindle fibres letting the chromosome move in the cell
Metaphase in mitosis
chromosomes will arrive at the spindle equator which will be equidistant to the 2 centrosomes and chromosomes will attach to the kinetochore microtubules which will pull in both directions
mitotic spindle checkpoint is here and only when they are properly aligned will the cell move into anaphase
anaphase in mitosis
proteins that bind the sister chromatids are cleaved dividing each into 2 identical daughter chromosomes and kinetochore microtubules are shortened moving the chromosomes apart and towards opposite ends , the max condensation level is reached for the chromosomes and microtubules will begin to move around the spindle equator to prepare for the contractile ring
telophase and mitosis
reorganization of the cell final stage of mitosis , rearangement from prophase are reversed and the nuclear membrane is reformed around the chromosomes along with the cytoskeleton reforming and endomembrane system
cytokinesis
final step in cell division ( not a mitosis step ) contractile ring will form where the spindle equator was located and the cell will divide in half as the ring tightens, plasma membrane will snap and reform due to the cells hydrophobic nature
