Cytoskeleton Flashcards
what is the cytoskeleton
an intricate network of protein filaments that extend throughout the cell
what are the different cytoskeleton filaments
- intermediate filaments
- microtubules
- actin filaments
what is the role of the intermediate filaments
enables cells to withstand mechanical stress
where are intermediate filaments found
- throughout the cytoplasm
- within the nucleus
what is the toughest and most durable of the cytoskeletal filaments
intermediate filaments
describe the structure of intermediate filaments
- rope-like structure with many strands twisted together
- two alpha helical monomers are wrapped together to form a coiled-coil dimer
- two coiled-coil dimers run in opposite directions to form a tetramer
what holds intermediate filaments together
- noncovalent bonding between subunits
- combined strength of overlapping interactions provides tensile integrity
what allows for interactions between other components in the cell and intermediate filaments
- central rod domains are similar in size and AA sequence
- variance in the terminal ends allows interactions
what are keratin filaments and where are they found
- cytoplasmic intermediate filaments
- found in epithelial cells
what are vimentin and where are they found
- cytoplasmic intermediate filaments
- found in connective tissue, muscle, glial cells
what are neurofilaments and where are they found
- cytoplasmic intermediate filaments
- found in nerve cells
what are nuclear lamins and where are they found
- nuclear intermediate filaments
- found in all animal cells
what are nuclear lamina
intermediate filaments that form a 2D meshwork on the inner surface of the nucleus
how are nuclear lamina formed
phosphorylated (and dephosphorylation) of lamins allow for the disassembly and reassembly during cell division
how are intermediate filaments stabilized
- by accessory proteins
- allows for cross-linkage and connect them to other cytosolic cell components
what do accessory proteins do
- interact w complexes that link the cytoplasmic cytoskeleton to structures in the nucleus
- aids w organization and positioning of the nucleus
- stabilize intermediate filaments
what do microtubules do
- crucial role in organization in eukaryotic cells
- system of tracks along which vesicles, organelles and macromolecules can be transported
- can be rapidly dis/reassembled depending on needs
describe the structure of microtubules
- comprised of tubulin molecules (dimer of alpha and beta tubulin)
- dimers stack to form a hollow cylindrical microtubule of 13 parallel protofilaments
- have structural polarity
describe the structural polarity of microtubules
- plus end (beta)
- minus end (alpha
where are microtubules formed
- begins at specialized organization centers
- the most prominent is the centrosome (located near the nucleus in non-dividing cells)
describe centrosomes
- made up of 2 centrioles surrounded by a gel-like matrix of proteins containing gamma-tubulin rings (which are the starting points/ nucleations sites for microtubules)
- negative ends of the microtubules are embedded in the centrosome
what controls the location, number, and orientation of microtubule
microtubule organization centers
what do all methods of microtubule organization use
- gamma-tubulin rings to initiate growth
- manage microtubule formation through the [ ] of free alpha/beta dimers
how do microtubules grow and shrink
through dynamic instability
what does dynamic instability allow for in microtubules
rapid remodeling and organization of microtubules
the action of dynamic instability is possible how
- through GTP hydrolysis by tubulin dimers
- each free tubulin dimer contains 1 GTP bound to beta-tubulin
- GTP is hydrolyzed shortly after dimer is incorporated into a growing microtubule
what happens when tubulin dimers are added before hydrolysis can take place
- accumulation of GTP-associated dimers
- forms GTP-cap
how does the GTP cap form
accumulation of GTP-associated dimers
what is special about GTP-bound tubulin
- form stronger bonds with neighbouring dimers
- pack together more efficiently and promote growth
what happens when hydrolysis takes place before new tubulin dimers are added
- loss of GTP-cap
- weaker bonds between GDP-associated dimers favours disassembly
- GDP-associated tubulin molecules rejoin cytosolic pool where they can exchange GDP for GTP and used for future polymerization
can cells modify dynamic instability of microtubules to suit their needs
yes
microtubules can direct traffic along their length _______(faster/slower) than the rate of free diffusion
much faster
movement along microtubules and other filaments is guided by what
motor proteins
what are motor proteins
- dimers
- have 2 ATP-binding heads, which facilitate movement through ATPase activity
- have a single tail that bids to their cargo
how do motor proteins move
- the walking thing
- the back leg gets ATP and conformational change and moves forward and then reattaches and repeat? idek but just remember conformation changes
what are the two motor protein families
- kinesins travel to plus end
- dyneins travel to minus end
what do kinesins do
- motor proteins that move to plus end
- diff kinds transport diff kinds of cargo
- sometimes a single one can transport multiple
what do dyneins do
- motor proteins that move to minus end
- always use adaptor proteins to interact w cargo
what are responsible for positioning organelles
microtubules
what are the roles of kinesins and dyneins as a cell grows
- kinesins distribute the ER
- dyneins keep golgi inwards towards the nucleus
what are cilia
- hairlike structures extending from the cell surface
- contain a core of microtubules that can beat to move fluid or propel cells
what are flagella
- used primarily for locomotion
- enable movement by propogating regular waves along their length
what is the similarity of cilia and flagella
share a similar core structure
describe the core structure of cilia and flagella
- ring of 9 doublet microtubules around a pair of single microtubules in a 9+2 array
- specialized form of dynein attaches one microtubule to another in each outer doublet
- cross-linkage through accessory proteins holds the bundle together, leading to microtubule bending
describe the structure of actin filaments
- made up of a twisted chain of actin monomers
- each monomer points in the same direction, making distinct polarity
- filaments are thinner, shorter, and more flexible than microtubules, however there are more of them
what are actin filaments used for
maintaining shape and movement
how are actin filaments formed
actin monomers carry ATP which is hydrolyzed after it is incorporated into the filaments
is polymerization quicker on the minus or plus end of actin filaments
plus
what happens when the [ ] of actin monomers is high
filament can grow
what happens when the [ ] of actin monomers is intermediate
- treadmilling occurs
- the rate of growth at the + end is matched by the rate of dissociated at the - end
- this allows the filament to move forward in space
how is polymerization of actin monomers regulated
protein binding
what happens when actin filaments are needed
proteins like formins and actin-related proteins (ARPs) promote polymerization
what happens when actin filaments are not in demand
proteins like thymosin bind to monomers to prevent polymerization
what are myosins
- belong to a family of motor proteins
- bind and hydrolyze ATP to provide energy for movement along an actin filament
what are the common myosin subfamilies
- myosin I
- myosin II
describe myosin I
- has a head domain that interacts w actin filament
- has a tail that determines what type of cargo it can transport
describe myosin II
has 2 heads that interact w the actin filament to form contractile bundles driving changes in shape, movement, and division
where is actin found
- throughout the cytoplasm
- high [ ] just beneath the plasma membrane in the cell cortex
what drives changes in cell shape and movement
rearrangement of actin filaments and associated myosin motors
describe the steps of cell crwaling
- actin polymerization the extension of exploratory motile structures at the leading edge
- attachment transmembrane integrins adhere to molecules in the extracellular matrix and actin filaments in the cortex
- contraction myosin motor proteins slide along actin filaments to drag the cell body forwward
what are 2 examples of exploratory motile structures
- lamellipodia
- filopodia
what is lamellipodia
- the “sheet feet” that extend out at the front of the cells when they crawl
- contain a dense meshwork of actin filaments
how do filipodia add new monomers
formin binds to the growing + end of actin filaments
how do lamellipodia add to the cell
ARPs form complexes that bind to the side of existing filaments and nucleate the formation of new filaments which grow out at an angle
what controls the location, organization, and behaviour of actin filaments
actin-binding proteins
what are actin binding proteins controlled by
- extracellular signaling molecules
- which activate intracellular pathways including monomeric GTPases from the Rho family
what does Rho focus on
bundle assembly
what does Rac focus on
lamellipodium formation
what does Cdc2 focus on
filopodia formation
what are the monometic GTPases in from the Rho family
- Rho
- Rac
- Cdc42
myosin involved in muscle contraction belongs to which myosin subfamily
myosin II
describe the structure of skeletal muscle fibers
- large, multinucleated cells formed by the fusion of many smaller cells
- bulk of the cytoplasm is made up of myofibrils
- each myofibril is made up of sarcomeres
what are myofibrils
cylindrical contractile structures
what are sarcomeres
- contractile units
- organized assemblies of actin and myosin filaments
- actin + end anchors to Z discs
- actin - end overlaps the myosin filaments
how does muscle cell contraction happen
- shortening of the cell sarcomeres
- actin filaments slide along the myosin filaments w no change in length
what are the steps of myosin walking for muscle contraction
- attached at the rigor conformation as the myosin head lacks ADP or ATP
- released ATP binds to the myosin head reducing its affinity for actin
- cocked myosin head wraps around the ATP to facilitate hydrolysis, displacing the myosin head forward
- rebinding and power stroke binding of the myosin head to a new site on the actin filament releases the phosphate group and allows myosin to return to its initial conformation and release ADP
when does muscle contraction occur
when a skeletal muscle receives a signal from a motor neuron
how is the signal from a motor neuron sent to a skeletal muscle
- relayed through transvers tubules (t-tubes), that extend inward from the plasma membrane around each myofibril
- signal is relayed to the SR
what is the SR
- sarcoplasmic reticulum
- sheath of interconnected flattened vesicles that surround each myofibril
what happens when the SR receives a signal
- SR has high [ ] of Ca2+, which is released into the cytosol through specialized ion channels upon receiving a signal
- the influx of Ca2+ activates calcium-sensitive molecular switches that are closely associated w actin filaments
what does tropomyosin do
binds in the actin helix to prevent myosin heads from associating w actin filaments
what does troponin complex do
undergoes conformational changes in response to Ca2+ which shifts tropomyosin molecules, allowing myosin heads to interact w actin filaments
