6: Cytoskeleton Flashcards

Question

Describe how actin filaments are assembled.

Answer 1

Head-to-tail assembly The entire filament is polar with + ends and - ends (not due to electrical charge) Looks like two intertwined strands Multiple interactisns between subunits (each subunit directly contacts four others) Internal subunits more stable than end subunits because of more bonds

Answer 2

This is because taking away a phosphate group from ATP (to form ADP) changes the chemical environment, and therefore leads to ADP-actin having a different conformation There is a higher Kd at the ADP-actin end than the ATP-actin end Ultimately, ATP hydrolysis promotes instability of the filament and allows the cytoskeleton to be dynamic

Answer 3

Accessory proteins Leads to different types of actin cytoskeleton: e.g. motor proteins, bundling proteins, etc.

Answer 4

The actin cortex This is a dense actin meshwork under the plasma membrane This region is under tension, keeping it anchored to the substrate

Answer 5

A flat, sheet-like projection at the leading edge of a migrating cell The "front wheel" of cell migration Actin polymerisation occurs within this projection, causes forward protrusion Allows attachment to extracellular matrix via focal contacts The rear contracts via myosin-based contractions, also contributing to migration

Answer 6

Filopodium is slender, spike-like projections extending from the cell surface often found in combination with lamellipodium Allows pathfinding, filopodia can act like antennae for the cell to sense the extracellular matrix Stabilises the protrusive activity of the lamellipodium, ensuring more precise movement

Answer 7

They 'staple' together actin filaments at random locations, creating a dense meshwork with gel-like properties, anchored to the plasma membrane Water molecules are bound to the filaments via hydrogen bonds, causes elastic deformation

Answer 8

Multi protein complex Binds to actin filaments at their minus ends Nucleates the formation of new actin filaments Promotes the polymerisation of actin to create a branched framework Polymerisation at the front causes protrusive force needed for cell migration

Answer 9

Proteins that bind to the plus ends of actin filaments and prevent further polymerisation (promote depolarisation) The oldest assembled filaments are less stable, so they release free actin monomers They ensure that actin polymerisation is focused at the leading edge of the cell in structures like the lamellipodium, ensuring directionality of migration This also allows actin to be recycled

Answer 10

Net assembly at the leading edge Net disassembly at the rear of the leading edge "Front-wheel drive"

Answer 11

Actin polymerisation factors that promote nucleation and elongation of actin filaments Nucleate new actin filaments at the plus end Formins remain at the plus end and help elongate unbranched filaments Critical for filopodia formation

Answer 12

Bundling proteins (e.g. fascin) help stabilise and organise filaments into tightly packed bundles in structures like filopodia Works alongside forming to ensure that actin filaments are structured properly for migration

Answer 13

Myosin I: singular globular head and short tail Typically involved in intracellular transport and membrane association Moves along actin filaments using ATP hydrolysis Tail can bind to membrane or cargo Myosin II: two globular heads and a long coiled tail (dimer) Responsible for muscle contraction and generating contractile forces Forms bipolar filaments that slide actin filaments past each other Tail forms thick filaments and interacts with other myosin II molecules

Answer 14

1) Rigor state: myosin globular head tightly binds to actin filament without ATP to form a stable rigor complex 2) ATP binds to myosin: causing a conformational change that weakens the interaction with actin and myosin releases from the actin filament 3) ATP hydrolysed: causes another conformational change, myosin head moves into a "cocked" state, ready to bind to adjacent actin filaments 4) Power stroke: release of Pi and ADP triggers the power stroke, where myosin reverts its original conformation, pulling the actin filament towards the centre of the sarcomere 5) One ATP molecule is consumed in each cycle, and the myosin head moves one step along the actin filament, generating physical force

Answer 15

Composed of muscle fibres, formed by the fusion of individual myoblast cells Fibres contain multiple myofibrils, which are highly organised structures responsible for muscle contractoin Myofibrils are made of sarcomeres Sarcomeres are made up of actin and myosin filaments arranged in a highly organised manner The Z-line anchors the plus ends of the actin filaments at each end of the sarcomere Thick myosin II filaments are arranged in a bipolar configuration

Answer 16

Myosin heads move towards the positive ends of the actin filaments, pulling the actin filaments inwards --><-- This slides the actin filaments relative to the myosin, resulting in the shortening of the sarcomere (sliding filament model)

Answer 17

Calcium signalling Calcium binds to enable exposure of actin binding sites to allow myosin to bind

Answer 18

1) Myosin I bound to the plasma membrane can 'walk' along actin filaments 2) Myosin II bipolar mini-filaments contract the actin filament mesh at the rear of the cell, causing the back of the cell to contract and allowing the cell to move forward

Answer 19

Cytoskeletal systsems Made of alpha and beta tubulin, a very stable dimer Forms a long, hollow tube More rigid than actin filaments Dynamic structures

Answer 20

GTP-bound beta tubulin promotes polymerisation of microtubules Beta tubulin hydrolyses GTP to GDP after polymerisation, reducing stability and promoting disassembly Drives dynamic instability where microtubules can rapidly grow and shrink Alpha tubulin keeps GTP bound in that state, it does not hydrolyse to GDP

Answer 21

Guanine triphosphate Related to dynamic microtubule structure

Answer 22

Polar polymers with 13 protofilaments (linear chains of tubulin dimers) Plus and minus ends (beta is at the plus end) Each tubulin dimer forms two lateral and two longitudinal contacts Tubulin dimers form heterodimers (subunits of microtubules) Lateral exchange of dimers is difficult, longitudinal is easier Assembly easier at the plus end than the minus end.

Answer 23

Initial process by which microtubules begin to form from tubulin dimers Involves assembly of a small number of tubulin dimers into a stable structure which serves as a 'seed' for further growth Requires high concentrations of tubulin Typically occurs at specialised regions in the cell like the centrosome

Answer 24

Centrosomes are loose assemblies built around centrioles Gamma-tubulin ring complexes are concentrated at the centrosome Gamma-tubulin ring complexes facilitate the nucleation phase of microtubule asembly Minus ends of MTs are anchored at the centrosome, as elongation is easier at the plus end

Answer 25

GTP-bound tubules Have a GTP-tubulin cap, so stabilises the plus end and allows dimers to be added longitudinally GDP-bound tubules Have no GTP-tubulin cap, so leads to 'self-unpeeling' and 'catastrophe' where the lateral interactions between dimers weaken, and the microtubule disassembles

Answer 26

Enables microtubules to rapidly grow and shrink, allowing cells to respond to internal and external signals Facilitates 'search and capture' of specific targets in the cell, helping to organise internal structures and guide transport Allows cell polarity and directional organisation by stabilising certain MTs in response to spatial cues

Answer 27

Allows microtubules to search the intracellular space to find and attach to kinetochores on chromosomes Selective stabilisation of MTs helps to form the mitotic spindle, ensuring accurate chromosome alignment and segregation Stabilised MT bundles can exert force on chromosomes, pulling them apart to opposite poles during anaphase

Answer 28

1) Colchicine: binds to tubulin dimers and prevents incorporation into microtubules 2) Taxol: stabilises tubulin in the MT lattice leading to cell death - potent anti-cancer drug

Answer 29

1) Internal organisation of the cell: moving vesicles and positioning organelles 2) Chromosome segregation: mitotic spindle 3) Moving fluids or moving cells in fluids: cillia and flagella

Answer 30

Microtubules form polarised arrays (plus and minus ends) creating an internal “road map” for cell organisation and directional transport Intracelllular membranes and organelles are positioned and shaped by microtubules. E.g. the endoplasmic reticulum extends along microtubules, colocalising with them Microtubules act as tracks for motor proteins (e.g. kinesins and dyneins) to move organelles, vesicles and other cargo

Answer 31

Motor proteins bind to MTs and use energy from ATP hydrolysis to generate conformational change (power strokes) that drive movement Each motor protein will have a directional preference

Answer 32

Kinesin: moves towards the plus end (usually the cell periphery) Dynein: moves towards the minus end (usually the cell centre, near the nucleus)

Answer 33

Movement often likened to a “walking” mechanism, with motor domains working in pairs for processive motion Cargo binds to specific motors, determining direction of transport Switching direction requires switching the motor protein attached to the cargo Motors may work alone or in small teams depending on cargo

Answer 34

Centrosomes replicate and nucleate microtubules to form the mitotic spindle Microtubules grow in radial arrays and are selectively stabilised to orient the spindle Kinetochore microtubules attach to chromosomes and are stabilised for chromosome segregation Interpolar microtubules are cross linked and stabilised by microtubule association proteins (MAPs), helping to structure the spindle Motor proteins at kinetochores pull the chromosomes apart Other motors push inter polar microtubules apart, elongating the spindle to separate daughter cells

Answer 35

Cilia and flagella are microtubule-based structures that move cells or fluids across cell surfaces They are extended from centrioles, which act as basal bodies at the base of each structure 9+2 arrangement - 9 outer doublet microtubules and 2 central single microtubules Movement is powered by dynein motors on the A tubule, walking along adjacent B tubules Nexin links connected doublets, converting sliding into bending, creating a waving motion that propels fluids or the cell

Answer 36

Intermediate filaments are important in organisms and tissues that experience mechanical stress They enable cells to collect and form layers, which stabilises them Helps to withstand mechanical stress

Answer 37

Vertebrates, nematodes and some mollusks have them Arthropods, echinoderms do not (they have hard shells and exoskeletons instead), because evolutionarily they don’t need them because their hard shells help minimise mechanical stress

Answer 38

Intermediate in diameter between actin and MTs They have a very high tensile strength They are resistant to many treatments that disrupt actin filaments and MTs No nucleotide binding Assemble by principle of ‘coiled-coil’ interactions

Answer 39

A coiled-coil is made of two or more right-handed alpha helices wrapped around each other Each helix is amphipathic, positions a and d in the 7-residue (heptad) repeat are hydrophobic, forming a stripe These hydrophobic stripes align and twist around each other, creating a left-handed supercoil This structure stabilises protein-protein interactions through a hydrophobic interface

Answer 40

Hydrophobic residues at positions a and d in the heptad drive tight helix packing Side chains interlock to form strong, stable interactions Coiled-coils are versatile motifs found in many unrelated proteins (e.g. myosin and kinesin) Can be homo or heteromeric, forming dimers, trimers, or tetramers Provide strength and flexibility ideal for forming rope-like intermediate filaments

Answer 41

Built from central alpha helical coiled-coil dimers Dimers pair anti-parallel to form tetramers Tetramers pack laterally and end-to-end into rope like filaments N- and C-terminal extensions vary, enabling diverse functions No polarity so motor proteins cannot act on them Very stable due to many subunits but flexible from small breaks in coiled-coils

Answer 42

Cytoplasmic: Keratins (in epithelia) Vimentin and vimentin-related (connective tissue, muscle cells, glial cells) Neurofilaments (in neurones) Nuclear: Nuclear lamins (in all animal cells, sheet under nuclear envelope)

Answer 43

Interactions mostly via head/tail regions Different proteins have different heads and tails so will not interact and copolymerise

Answer 44

Found in neurones Stabilise fragile neuronal processes Very long extensions  Keeps the axon as a stable structure Full of neurofilament proteins and microtubules

Answer 45

Found in epithelia Allows sheets of cells to stretch without rupture Most diverse class of intermediate filament proteins Links to neighbouring cells at desmosomes Like webbing in rip-stop fabrics Other IF proteins (desmin, vimentin, etc.) have similar roles in muscle and connective tissue 

Answer 46

Present in nearly all animal cells Found right beneath the nuclear envelope Meshwork of filaments = nuclear lamina Maintains integrity of the nucleus Links chromatin to the nuclear envelope Can be destabilised when cells divide

Answer 47

Through phosphorylation of nuclear pore proteins and lamin proteins Disassembles the entire lamina network Free lamins can then engage with microtubules in the cytoplasm When division is complete, phosphate groups are removed, and lamina is reassembled to reform the meshwork

Answer 48

Hutchinson-Gifford Progeria Syndrome Extremely rare Childhood onset of premature aging Growth retardation, baldness, facial hypoplasia, aged skin, delayed tooth formation, osteoporosis, arthritis, teenage mortality, almost always from cardiovascular disease, caused by a point mutation in gene encoding lamin A

Answer 49

Actin: forms apical microvilli, a contractile apical belt, supports the cell cortex Microtubules: enables directed transport between apical and basolateral regions Intermediate filaments: anchor cells to each other and the basal lamina giving mechanical strength

Answer 50

Actin initiates the turn by forming filopodia and lamellipodia at the growth cone Microtubules invade these new protrusions, stabilising direction and delivering membrane vesicles Microtubules can also stimulate local actin polymerisation Neurofilaments fill in behind, providing structural support as the axon extends

Answer 51

Actin filaments assemble at the cell equator, recruiting myosin to form a contractile ring, contracts to pinch the cell in two (cytokinesis) Microtubules form the mitotic spindle to segregate chromosomes Intermediate filaments (lamins) are phosphorylated to break down the nuclear envelope

Answer 52

Plectin is a linker protein that physically binds actin, microtubules, and intermediate filaments, enabling mechanical integration e.g. if actin is pulled at one point, the force can be distributed throughout the whole network, keeping the cell structurally sound E.g. in plectin-deficient mice, this loss of co-ordination can lead to skin blistering because cells can’t stay mechanically attached, muscle weakness, because forces from actin/myosin can’t be transmitted properly

Answer 53

Small protein based structures in prokaryotes similar to actin and tubulin Actin-like (MreB) and tubulin-like (FtsZ) proteins involved in bacterial cytokinesis Evolutionary link indicates a common origin

6: Cytoskeleton Flashcards

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