The Cytoskeleton,cillia And Undulipodia Flashcards
What is the cytoskeleton
• Fibrousnetwork–to which organelles are tethered
• Providesstructure& organisation
• Cytosol~55%totalcell volume
• 20% of the cytosol is proteins
Proteins in the cytoskeleton
~ microfilaments (actin)
~ myosin (types I & II)
~ microtubules (tubulin)
~ intermediate filaments (cytokeratins)
Role of the cytoskeleton
Mechanical support to the cell-shape(continues to remain dynamic)
Cell motility relies on cytoskeletal structures
Components of the cytoskeleton
Microfillaments
Interemediate
Microtubules
Microfilaments
• Two strands of intertwined actin – 7nm in diameter
• Cell shaper – tension bearing load
• Muscle contraction
• Cytoplasmic streaming •
Cell motility
Actin filaments
• G-actin is a 5 nm diameter globular protein that can polymerise to form F-actin which has a diameter of 7 nm
• F-actin has two intertwined polymer chains of G-actin that form a right-handed double helix with 13 actin monomers per turn
• F-actin microfilaments have +ve ends where polymerisation occurs and -ve ends where actin is lost
~ polymerisation requires hydrolysis of ATP
~ polymerisation it is controlled by capping proteins
Interactions of the actin filaments
• Fungal cytochalsins bind to +ve ends of F-actin and inhibit
polymerisation leading to cell death
• Fungal Phalloidin (Amanita toxin), from death cap mushrooms binds and stabilises F-actin preventing microfilament disassembly leading to cell death
• Mini-myosin (type I) has a globular head with ATPase activity and aAmanita phalloides short tail which can bind to other proteins
• Mini-myosin (type I) can attach to organelles including endoplasmic reticulum, Golgi and vesicles and ‘walk’ along F-actin microfilaments carrying organelles
~vesicle movements in cells, in synapses and cytoplasmic steaming in plant cells
Ameboid movement of actin and myosin
• Cell moves forward due to G-actin flowing into pseudopodia
• Actin interacts with mini-myosin causing contraction of the cell which pulls the cells trailing end
Ameboid movement
Crawling like type of movement/cell migration
Pseudopodia
Cytoplasmic rich projection used for motility
Actin, myosin & and Cytoplasmic Streaming
• A layer of cytoplasm cycles around the plant cell, moving over a carpet of parallel actin filaments (F-actin).
• Mini myosin motor proteins attach to organelles in the cytosol driving the streaming via interactions with F-actin
Actin in muscle filaments(contractions)
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Intermediate filaments
• Size~10nm
• Structural/mechanical strength of cells and tissues
• Cable-likestructure
Comprised of a variety of proteins
(no single polymers) Found only in vertebrates.
How do intermediate filaments assemble
- Singlepolypeptidechains–wind to form tetramers
Organisedinanti-parallelfashion Filamentiscomprisedofeight
protofilaments
IFs more stable – lack dynamic
movement
Phosphorylation–regulates function (nuclear envelope during mitosis) - Lamins
Intracellu;ar organisation
• Complex network in the cytoplasm
• Extends from plasma membrane to the nucleus
• Keratin/vimentin anchor the nucleus within a cell
• Integrates all aspects of the cytoskeleton (actin/microtubules)
Dmd
• Severe muscle weakness
• Loss of muscle mass (atrophy)
and function
• Disability
• Ca2+ mediated damage due to lack of dystrophin
What is dystrophin
Acts as a stabiliser during muscle contraction to prevent damage
Cell junctions
• Provide the link between neighbouring cells, tissues and organ systems
• Area of direct physical contact • Plasmodesmata in plant cells
• Connection of the cytosol from adjacent cells
• Unifies the plant as one living organism
• Transferofproteins/RNAandsolutes
Cell junctions in animals
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Tight Junctions
plasma membranes are tightly bound by proteins that form a continuous seal around cells that is water tight
Desmosomes
act like protein rivets that anchor cells together
desmosomes link to sturdy cytokeratin* fibres that project into the cytoplasm
Gap junctions
provide cytoplasmic channels between cells allowing cell functions to be linked in synchrony such as heart muscle and smooth muscle contractions
Extracellular matrix
• Glycoprotein rich network of molecules
• Collagen is the main ECM component in animal cells
• Fibrillar/non-fibrillaryformsof collagen
• Multiple types of collagen, however type I is most abundant in humans
• ECM provides a link between intra/extracellular environment
Properties of the extra cellular matrix
> Regulation
• polarity
• cell division
• adhesion
• motility
Structural support
development
Composition of the ECM
• Proteins
• Almost all of the proteins are glycoproteins
• A wide variety of collagens. Laminins. Abundant in the basal lamina of epithelia.
• Fibronectin: Binds cells to the ECM.
• Elastins: Provide flexibility to skin, arteries, and lungs. (These are not glycosylated.)
• Proteoglycans
• Proteoglycans are glycoproteins but consist of much more carbohydrate than protein
• The protein backbone of proteoglycans is synthesized, like other secreted proteins, in the endoplasmic reticulum.
• Several sugars are incorporated in proteoglycans. The most abundant one is N- acetylglucosamine (NAG)
• This glycosylation occurs in the Golgi apparatus.
ECM and stem cells
• ECManchorsstemcellniches
• Anchoring important for mitotic spindle orientation
• Essential in stem cell self-renewal
• PropertiesoftheECM–stiffness, may play a role in cell-fate determination
Mictrotubule structure
• Alphatubulin(purple)
• Betatubulin(blue)
• Thirteenpro-filaments, comprised of tubulin dimers, arrange to form cylindrical microtubule
• Dynamicprocessofgrowth & shrinkage
• Mediated by GTP hydrolysis
Microtubule structure
• Microtubules can have three structures:
O singlets in cytoplasm & mitotic spindles
OO doublets in cilia & flagella
OOO triplets in centrioles & basal bodies
Centrioles and the Centrosome
• In many cells, microtubules grow out from a centrosome near the nucleus
• The centrosome is a “microtubule- organizing center”
• In animal cells, the centrosome has a pair of centrioles, each with nine triplets of microtubules arranged in a ring
Role of mixctrotubules in cell cycle
• Mitosis–partitionofreplicated
chromosomes
• Involvestheassemblyand disassembly of a key microtubule structure – mitotic apparatus or mitotic spindle
Centrioles
• Centrioles have 27 stable microtubules organised into 9 + 0 triplets surrounded by a protein matrix
• Centriole pairs are organising centres that form microtubule spindles during mitosis
Microtubule Arrangement
Do it
Mitotic Spindle
-an apparatus of microtubules that controls chromosome movement during mitosis
-• During prophase, assembly of spindle microtubules begins in the centrosome, the microtubule organizing center
• The centrosome replicates, forming two centrosomes that migrate to opposite ends of the cell, as spindle microtubules grow out from them
• An aster (a radial array of short microtubules) extends from each centrosome
• The spindle includes the centrosomes, the spindle microtubules, and the asters
• During prometaphase, some spindle microtubules attach to the kinetochores of chromosomes and begin to move the chromosomes
• In metaphase, the chromosomes are all lined up at the metaphase plate, the midway point between the spindle’s two poles
• In anaphase, sister chromatids separate and move along the kinetochore microtubules toward opposite ends of the cell
• The microtubules shorten by depolymerising at their kinetochore ends
• Microtubules are dismantled by the kinetochore to release tubulin subunits
Microtubules – organelle movement
• Axonal transport of vesicles along microtubules in nerve axons involves kinesin and cytosolic dynein motor proteins:
➢ vesicle can be transported 250 – 400 mm/day
• Kinesin /dynein head proteins have ATPase activity and tails that bind to vesicles:
➢ kinesin mediates anterograde movement of vesicles towards synapses along singlet microtubules
➢ cytosolic dynein mediates retrograde movement to the cell body for recycling along singlet microtubules
• In synaptic endings motor proteins can transport neurotransmitter vesicles along actin microfilaments
Kinesin
anterograde (forwards)
• Spindle length, movement during mitosis & depolymerisation
Dynein
retrograde (backwards)
• Transport of cellular cargo
• Cytoplasmic dynein: organelle function & integration
• Axonemal dynein: cilia/flagella movement
Cilia
-short hair like appendages extending from the surface of a living cell
-short
-rotational motion like a motor and are very fast moving
-found in eukaryotes
Flagella
-long threadlike appendages on the surface of a living cell
-longer than cillia
-wave like motion
-found in eukaryotic and prokaryotic cells
Similarities of flagella and cillia
• A core of microtubules sheathed by the plasma membrane
• A basal body that anchors the cilium or flagellum
• A motor protein called dynein, which drives the bending movements of a cilium or flagellum
Dynein walking and its contribution to cilia and flagella walking
−Dynein arms alternately grab, move, and release the outer microtubules
• Protein cross-links limit sliding
• Forces exerted by dynein arms cause doublets to curve, bending the cilium or flagellum
• Dynein arms can ‘walk’ along microtubules towards the basal body (-ve end) bending and rotating the cilia / flagella
