Cytoskeleton 1 Flashcards
- Discuss the concept of a cytoskeleton.
Cytoskeleton: (O&O, SSSS, LT
- It helps establish Order and Organization in the cell.
- Provides cell Shape,
- mechanical Strength,
- structures necessary for Locomotion,
- Support for the plasma membrane,
- Scaffolding for spatial organization of organelles,
- and the means for intracellular Transport.
Formed by: (MMI)
- microfilaments/actin,
- microtubules
- intermediate filaments.
- Describe microtubule and intermediate filament cytoskeleton. (their properties, their functional roles, and their protein composition).
MICROTUBULES:
Microtubules approximately 25nm
-Hollow tubular structures made up of alpha and beta tubules; they are flexible but not very resistant to stretching.
- MTs Function primarily as:
1) scaffolds for spatial organization, (ER and Golgi are anchored to MTs)
2) organelle movement,
3) movement of cilia and flagella.
- One end is attached to the centrosome (or perinuclear microtubule organizing center MTOC).
- Alpha tubulin bound to GTP and does not hydrolyze, it is a constant heterodimer component.
- Beta tubulin also bound to GTP and can hydrolyze (is exchangeable) .
-MTs arranged into hollow cylinders –> 13 parallelprotofilaments arranged alternating beta to alpha. top of beta interacts with bottom of alpha., Laterally the pritofilaments are slightly displaced, forms a spiral, but interactions are mostly a-a or b-b. These intermolecular interactions are strong within the assembled tubule , there is little exchange of dimers wthin.
-Rapid assembly and disassembly at the ends.
-GTP cap at the beta end provides stability and prevents the structure from coming undone.
-GTP-bearing beta subunits favor polymerization at the plus-end, so that’s the end that grows (+).
-The minus end is the one that undergoes disassembly.
=As GTP containing
dimers become incorporated more deeply in MTs, GTP is hydrolized to GDP, which
weakens the tubulin interaction in the protofilament.
-This results ultimately in a “treadmilling” phenomenon (MT growth at the plus-end, MT disassembly at the minus-end).
-However in the
response to a particular cellular activity, the plus-end may loose its GTP-rich cap, which causes rapid shrinkage from the plus-end until GTP-containing dimers are added back.
-This phenomenon of rapid MT dynamics is known as “dynamic instability”.
-MT associating proteins
can control MT stability by either protecting or removing GTP-cap.
-MT capping proteins that
bind to the ends of microtubules, usually increase their stability.
- Severing proteins like katanin, spastin, and fidgetin cause microtubule instability by exposing GDP-rich parts.
- Severing proteins are AAA ATPases that form hexamers that pull on ‘tails’ of the alpha-beta dimers to unravel the microtubule structure.
- This is a sequence specific interaction (so acetylation or something can prevent this).
- MTOC (aka centrosome) located near nucleus – composed of 2 centrioles–> makes the nucleation sites (consisting of rings of gamma-tubulin) that initiate MT polymerization; it is where the minus-ends are anchored.
- Centrosome duplicates in cell cycle and they move to opposite ends of cell during mitosis, and the MTs growing between them form the mitotic spindle.
- MTs also form cilia and flagella, which contain an MT core = axoneme.
INTERMEDIATE FILAMENTS
-Intermediate filaments are more complex, rope like fibrous structures.
-about 10 nm in diameter,
-prominent in cells exposed to mechanical stress;
-provide intracellular mechanical support.
-Have cytoplasmic IFs (keratins, vimentins, and neurofilaments) and nuclear lamins (filamentous proteins that help stabilize inner membrane of nuclear envelope to provide anchorage for chromosomes and nuclear pores).
-IF proteins very heterogeneous –> alpha helical domain in middle and globular domains at ends –> form coil-coil dimers.
-Dimers associate in anti-parallel fashion in a staggered way to form tetramers –> assemble into protofilaments, and eight protofilaments (8) form an IF. Lots of tensile strength.
-Keratins are part of epidermis –> mechanical strength. -Generally dimers form between 1 acidic and 1 basic chain (K18 and K8 are most common).
-Keratins can be a huge diagnostic marker.
-Keratin mutations may interfere with filament assembly. The resulting epidermis is highly
sensitive to mechanical stress and blisters easily, causing a severe disorder called epidermolysis
bullosa simplex. Mutations in some of the associated proteins (e.g., those that anchor the filaments in desmosomes) may result in similar clinical syndromes.
-Neurofilaments are in axons–> control axon diameter. Mutations in the light chain may interfere with axonal transport of neurofilament subunits and cause a peripheral neuropathy called Charcot-Marie-Tooth syndrome.
Abnormal neurofilament assembly seems to be involved in the neurodegenerative disease,
amyotrophic lateral sclerosis (ALS or Lou Gehrig’s Disease).
-Glial fibrillary acidic protein (GFAP) –> astrocytes.
-Abundant in inflammatory and neurodegenerative diseases.
-Lamins:
Mutations in lamins can result in nuclear instability. Lamin mutations are linked to various
progeria syndroms.
- Discuss cytoskeletal dynamics and the role of certain proteins and drugs in tubulin polymerization/depolymerization.
Drugs that modify MT polymerization have been isolated from plants:
- Colchicine –> inhibits MT polymerization
- Vinblastime and Vincristine also block MT polymerization
-Paclitaxel (Taxol) binds MTs and stabilizes them –> causes tubule aggregates.
These can block mitosis and so are interesting for cancer treatment.
- Explain the concept of molecular motor. Explain the mechanisms of tubulin-based movement and intracellular transport.
- MT network forms ‘tracks’ that work with motor proteins like kinesins and dyneins to allow intracellular transport.
- Head domain binds the MTs and tail domain binds cargo.
- When motor proteins bind ATP, they’re bound to MTs –> hydrolysis allows them to walk; about a 50/50 ratio.
- Kinesins go towards the + end
- Dyneins move towards the – end (towards nucleus).
- This is intense with axonal transport because may have to move things very far –> mutations can cause intense disease (neuropathies).
- Mutation in kinesin = cargo not getting to target.
- Mutation in dynein = cargo not getting back to the body to be degraded.
- NGF helps things get back by forming a tract complex?
- Discuss the role of microtubules in mitosis.
Mitotic spindle formed by MTs and helps segregate replicated chromosomes in mitosis.
3 types of MTs:
-Kinetochore MTs –> attached to kinetochore
-Astral MTs that radiate out from centrosomes
-Overlap MTs - interdigitate at equator of spindle
-MT plus-ends point away from centrosomes;
kinesin-like motor proteins bind to overlap MTs from opposite poles –> cause spindle to grow and centrosomes to become more distance (astral MTs contribute a pulling force).
-Minus end motors + kinetochore MT shortening separate daughter chromosomes.
- Defects here would be quite bad, but can also use as a chemotherapy to destabilize microtubule network.
- MT toxins can block mitosis, so important in treating cancer. BUT these may affect other MT functions, like axoplasmic transport –> can cause serious side effects (like peripheral neuropathy).
- Discuss the cytoskeleton in the context of disease processes.
*Under the category of IF
Keratin diseases:
-Keratin mutations interfere with filament assembly –> epidermis is sensitive to mechanical stress and blisters easily –>Epidermolysis Bullosa simplex.
Neurofilament Diseases:
- Mutations in light chain of neurofilaments may interfere with axonal transport –> peripheral neuropathies like Charcot-Marie-Tooth syndrome.
- Abnormal neurofilament assembly may also be involved in ALS.
Lamin diseases:
-Lamins, which line the nucleus, can result in nuclear instability if mutated. Have been implicated in myopathies and progeria syndromes (premature aging; mutation near CAAX?).
*Under the catergory of microtubules:
-Mutations in dynein motor forms a class of ‘primary ciliary dyskinesia syndromes’ in which outer dynein arms of cilia and flagella are missing.
Monocilia (photoreceptors and chemosensors) –> not functioning properly –> doesn’t allow for proper left-right identity.
