Week 10 Textbook Flashcards
what is the cytoskeleton
an intricate network of protein filaments that extends throughout the cytoplasm
- allows organization of internal components
- plants = stiff
- animals = built on IF, MT, AF
what forms intermediate filaments
family of fibrous proteins entwines to form the intermediate filaments
- forms a meshwork called the nuclear lamina under the nuclear membrane
- distributing the mechanical stress of the cell
- very flexible and great tensile strength
what forms the microtubules
globular tubulin subunits come together
- hollow cylinders
- long and straight
- rupture when stretched + more rigid
what forms the actin filaments (microfilaments)
globular actin subunits
- flexible
- most abundant beneath the plasma membrane or in the villa of the epithelial cells
explain the structure of intermediate filaments more
- thinner than actin but thicker than myosin filaments
- toughest and more durable
- remain intact even after the cell dies
- form a network + anchored to the plasma membrane via desmosomes
- form nuclear lamina which reinforces the nuclear envelope
form rope with many strands twisted together
the rod domain has a alpha-helical region (coiled coil)
- they run in opposite directions and are staggered tetramers
what is the process of IF ropes forming
the alpha-helical region of the monomer coil with another monomer = coiled coil dimer
twisted with other coiled coils, they become staggered and anti parallel = tetramer
8 tetramers come together LATERALLY (stacking on top of each other) to form noncovalent bonds (H bonds, LDF, ionic, dipole)
then they twist together to form an array of 8 tetramers
are both ends of the IF the same
bc the dimers are paired in opposite directions both ends of the tetramer are the same
no polarity (no difference in ends, not talking about charge)
what are the 4 classes that intermediate filament can be grouped into
- keratin filaments in epithelial cells
- vimentin and vimentin-related filaments (connective tissue, muscle cells and supportive glial cells of the NS
- neurofilaments in nerve cells
(these 3 = cytoplasm found) - nuclear lamins - strengthen the nuclear envelope (only found in nucleus)
explain keratin filaments
the most diverse of the intermediate filaments
- every epithelium in the body has its own keratin proteins
- specialized keratins = hair, feathers, claws
- the ends of the keratin filaments are anchored by desmosomes - to associate laterally with other cells
- these strong cables distribute the stress when the skin is stressed
what are neurofilaments
they are IF that are found along the axons of vertebrate neurons - they provide strength and stability to the axons that transmit information
explain the nuclear lamina
fibrous layer on the inner surface of the inner nuclear membrane
formed as networks of IF made from nuclear LAMINS
- the nuclear lamina disassembles and re-forms at each cell division and re-appears in the daughter cells
- chromatin binds to the nuclear lamina
explain the process of the nuclear lamina collapsing and reassembling
it is controlled by phosphorylation and dephosphorylation of the lamins
phosphorylated by protein kinases
- this weakens the interaction between the lamin tetramers and cause the filaments to fall apart
dephosphorylation causes the protein phosphatases to reassemble
what is progeria
rare disorder that causes ppl to age fast
- caused by defects in a particular nuclear lamin
t/f MT can bundle together to form cilia and flagella
true
they bundle together to form this
explain the structure of microtubules
built from the subunits of tubulin
each dimer is composed of alpha-tubulin and beta-tubulin
- they are bound together by noncovalent bonds
- the tubulin dimers stack together again by noncovalent bonding to form a wall of hollow cylinders
each protofilament has a structural polarity with alpha on one end and beta on the other end
t/f the beta end is the - end of MT
false, the beta end is the plus end
the alpha end is the - end
explain the alpha and beta dimers and how they form the MT
each beta and alpha subunit is binded to GTP which are linked together to form a dimer
they stack vertically to make 1 string = protofilament
t/f dimers add to the plus end when growing MT
true
what are centrosomes
located in most prominent microtubule-organizing center
consists of a pair of centrioles surrounded by a gel-like matrix of proteins
inside of the gel matrix, there are ring-shaped g-tubulin which are the complexes that serve as the starting point or nucleation site for the growth of one microtubule
what is the function of centrioles
the pairs sit perp to one another
they are made of a cylindrical array or short microtubules
- they have no role in the nucleation of microtubules from the centrosome
- the y-tubulin ring complexes are efficient on their own
although the centrioles in the cilia and flagella nucleate the growth of microtubules - called basal bodies
t/f the arrangement of microtubules varies in different cell types
true
in animal cells
what is dynamic instability
behaviour of switching back and forth between polymerization and depolymerization
- undergo rapid remodelling for their function
they can hydrolyze GTP –> GDP
each tubulin dimer has GTP binded and the beta tubulin hydrolyzes GTP into GDP after the dimer was added to the growing chain
what happens when the tubulin dimers are growing faster than the GTP can hydrolyze
the end of the rapidly growing microtubule is composed entirely of GTP tubulin dimers to make a GTP cap
- GTP dimers bind more strongly to their neighbors in the microtubule than dimers with GDP
what happens when the GTP hydrolysis is faster than the addition of new GTP tubulin dimers
GTP cap can be lost
the protofilaments containing GDP tubulin peel away from the microtubule wall
GDP tubulin is released and broken - released into cytosol = shrinking MT
in a typical fibroplast - half of the ublin in the cell is in the MT and the other is in the cytoplasic pool rapidly exchnaginging their bound GDP for GTP
t/f MT rapidly reassemble and disassemble
true
cytoplasmic microtubules are taken apart and rebuilt to form an intricate structure = mitotic spindle = machinery that segregates the chromosomes equally into daughter cells before divison
why is the cells polarity so important?
it is a reflection of the polarized systems of microtubules in its interior to help to position organelles in their required location within the cell and to gde the streams of vesicular and macromolecular traffic moving between one part of the cell to the other
t/f free diffusion movement is faster than movement guided by microtubules
false
materials transported along axonal microtubules = very fast
some axons extend along the limbs of animals are so long, it may take a week to move but it is still better than simple diffusion
what are microtubule-associated proteins
- stabilize MT against disassembly
- others link microtubules to other cell components including the cell cortex or other types of cytoskeletal filaments
- nucleate the growth of new microtubules including the y-tubulin ring complex
- some proteins binds to the filaments free plus end accelerating the addition of tubulin dimers to the plus end
explain the branch-nucleating protein complex called augmin
it is a protein complex that is involved in nucleation which binds along the sides of existing MT and recruits y-tubulin ring complex which initiates the growth of new microtubule that branches off the og filament
- enables the assembly of dense microtubule arrays
- it is important in the organization of the MT network in plant cells which do not have centrosomes that nucleate MT in animal cells
(depletion of augmin causes decreased plant growth)
what are some chemicals that influence microtubule dynamics
some drugs like cochicine and nocodazole destabilize MT by interfering with the polymerization of tubulin dimers
the drug paclitaxel binds tighly to microtubules and prevents their depolymerization (they have different effects)
sometimes when you disrupt the microtubule dynamics it interferes with proper assembly of the mitotic spindle which moves the chromosomes to their proper ends - this causes issues in cell division
t/f antimitotic drugs are used as cancer therapy
true
they can be used to cause the tumor cells to die but they might also kill the dividing cells in healthy tissue
saltatory movements
moving for a brief period, stopping and then moving again
brownian movement
movement caused by random thermal motion
what are motor proteins
proteins like myosin or kinesin that use energy from the hydrolysis of a ATP molecule bound to them to propel themselves along a protein filament or other polymeric molecules
what are the 2 families of microtubule-associated motor proteins
kinesin
- motor proteins that uses the energy of ATP hydrolysis to move toward the plus end of a microtubule
dyneins
- motor protein that uses energy from hydrolyzing ATP to move towards the minus end of the microtubule
- another form of the protein is responsible for bending cilia
- most of them are dimers that have two globular ATP binding heads and a single tail
t/f depending on what cell component the motor protein attaches to is the type of cargo that it will transport
true
- the heads of each of the proteins binds to the components in a specific way to ensure only one direction movement
explain how the motor proteins move
the globular heads of the proteins are enzymes with ATPase activity - which allows them to have a conformational change that enables the motor proteins to move hand over hand along the MT
- atp hydrolysis looses the attachment of head 1
- the ADP release and ATP binding conformational change pulls head 1 forward so that the back one always has ATP and the front one (in the direction of movement) has ADP
what is the protein called that connects cargo to the motor protein
adaptor protein
explain how MT and motor proteins extend all over the cell
the ER extends out from points of connection with the nuclear envelope along microtubules
as the cell grows kinesins attached to the outside of the ER membrane (via adaptor proteins) pull the ER outward along microtubules stretching like a net
cytoplasmic dynein attached to the golgi membrane pull the golgi apparatus along microtubules in the opposite direction - inward towards the nucleus
t/f colchicine drug causes ER and golgi and MT to disassemble
the ER collaspses around the nucleus
the golgi which is not attached to anything is broken into small vesicles across the cytoplasm
but when the drug is removed the organelles go back to normal
what are actin filaments
they are polymers of actin which is a protein
without actin filaments the animal cell cannot crawl or engulf by phagocytosis or divide into 2
- they interact with the actin binding proteins
- they can form stiff and stable structures such as the microvilli on the epithelial cells lining the intestine or the small contractile bundles that can contract and act like tiny muscles in most animal cells
- they can form temporary structures or contractile ring that pinches the cytoplasm in two when an animal cell divides
- actin dependent movements usually require actins association with a motor protein = myosin
explain the formation of the actin filament via actin monomers
twisted chain of identical globular actin monomers that all point in the same direction
the actin filament has structural polarity which a plus and minus end
each monomer has a cleft where they bind for ATP or ADP
- they are more flexible, thinner, usually shorter than microtubules
actin filaments don’t occur in isolation in the cell, they found in cross-linked bundles and networks which are stronger than individual filaments
what end does the growth of actin filaments occur the fastest
rate of growth is faster at the plus end
a actin filament with no associated protein is unstable and can disassemble from both ends
- actin monomers usually carry a tightly bound nucleoside triphosphate (NTP, d, t, a, c, )
the actin monomer hydrolyzes its bound ATP to ADP after it is inside the filament
*nucleotide hydrolysis promotes depolymerization bc they decrease the stability of the polymer - this helps the disassembly of the MT and actin filaments
what happens if the conc of free actin monomers is high
the adding of monomers on both ends and filament will grow rapidly
what happens at intermediate concentration of free actin monomers
monomers add to the plus end at a rate faster than the bound ATP can be hydrolyzed so the plus end grows
and at the minus end, the ATP is hydrolyzed faster than new monomers can be added because ADP-actin destabilized the structure, the filament loses subunits from its minus end at the same time as it adds them to the plus end - it happens simultaneously = TREADMILLING
what happens when the rate of the monomer addition equals the monomer loss in actin filaments
remains the same length
what are some drugs that affect actin filaments
phalloidin = binds to filaments and prevents depolymerization
cytochalasin = caps filaments plus ends and prevents polymerization
latruncilin - binds actin monomers and prevents their polymerization
what are actin binding proteins
proteins that interact with actin monomers or filaments to control the assembly structure and behavior of actin filaments and networks
formins and actin-related proteins (ARPs) promote actin polymerization
- thymosin is a actin binding protein and it binds to monomers and prevents them from adding to the ends of actin filaments by keeping actin monomers until they are required
what is the function of actin-bundling proteins
they hold actin filaments together in paraellel bundles in microvilli
some others cross link actin filaments together n the cell cortex that supports the plasma membrane
filament severing proteins cut actin filaments into fragments to convert into actin gel to a more fluid state
what is the most familiar actin-binding protein
myosin
- it is apart of the family of motor proteins that bind to and hydrolyze ATP which provides energy for their movement along an actin filament towards the plus end
- there are many types of mysosins in cells
myosin 1 and 2 are the most abundant
explain myosin 1
has a head domain and a tail
the head domain binds to the actin filament and has the ATP hydrolyzing motor activity which gives them the ability to move along the filament
- the tail varies among the different kinds of myosin 1 and determines what kind of cargo they will carry depending on their tails
- sometimes depending on the cell they will carry vesicle cargo or pull the plasma membrane to make it into different shapes
explain myosin 2
all cells have it
a 2 headed myosin motor that interacts with actin filaments to form contractile bundles, driving dynamic changes in cell shape that contribute to cell movement and division
- muscule cells have a specialized form of myosin 2
t/f the assembly and polarity of actin and MT can give the cell polarity too
true
and it plays a large role
EX: the epithelial cells in the tissue that lines the inner surface of the intestines are highly polarized - the polarity is essential for its function bc food molecules are absorbed from the gut lumen by transported in the cells membrane
MT allign their plus end towards the basal surface and together with motor proteins they organize the cell interior and idrect vesicle traffic
- the formation of these domain requires Rho GTPases
- distruption of polarization can = malignant tumors
how does cell polarization impact reproduction and development of other cells
budding yeast undergo highly polarized form of cell division
a new yeast cell will cluster at the site where Rho GTPases cluster and assemble actin filaments + MT to mediate the transport of materials to the new cell
polarization in the embryos of some animals can make the organisms body plan and enhance them for specialization of different tissues, cells, etc
explain the structure of myosin 2
these proteins are dimers and they have 2 globular ATPase heads at one end and a coiled coil tail at the other
clusters of myosin 2 molecules bind to each other thru their coiled coil taisl forming a bipolar myosin filament
- the filament is like a double headed arrow with both of the heads poiting outward and the tails are touching in the middle
- one head binds to actin filaments in one orientation and moves the filaments one way and the others set binds to other actin filaments in the opposite orientation and moves the filaments in the opposite direction
thus if the myosin and the actin filaments are arranged with one another they can make a bundle = strong contractile force - seen in muscle contraction
