Cytoskeleton Flashcards
What is the basic function of the cytoskeleton?
- Give cells shape
- Allows cell to organize internal components
- Movement
What are the three types of protein filaments the cytoskeleton is made of?
- Intermediate filaments
- Microtubules
- Actin filaments
What is the purpose of intermediate filaments and where are they anchored?
Helps cell withstand mechanical stress by forming a network around the cytoplasm surrounding nucleus, within nucleus and extending out to cell cortex. Anchored via desmosomes.
How are intermediate filaments generated?
- Alpha helix protein with rod domain and two unstructured domains at either end
- Rod domains of two alpha helix monomers pair to form stable coiled-coil dimers with N and C termini lined up
- Coiled-coil dimers runing in opposite directions form a staggered tetramer.
- 8 Tetramer strands pack to form a helical array to form a fiber with no polarity on either end.
What are the four types of intermediate filaments and where do they come from?
Keratin: found in cytoplasm epithelial cells as well as hair, feathers and claws
Vimentin: found in cytoplasm of connective tissue, muscle tissue and glial cells
Neurofilaments: found in cytoplasm of nerve cells
Nuclear lamins: Found in the nucleus of all animal cells
What is the purpose of keratin? What happens when keratin genes are mutated?
Purpose: Indirectly connecting adjacent epithelial cells through desmosomes
Mutation: keratin filaments are ruptured causing skin to become highly vulnerable to mechanical injury (epidermolysis bullosa)
What is the purpose of neurofilaments? What happens when neurofilaments are mutated?
Purpose: found along axons of vertebrate neurons to provide strength and stability to long axons
Mutation: Amyotrophic lateral sclerosis - accumulation of neurofilaments in the cell bodies of axons and motor neurons causing axon degeneration and muscle weakness.
What is the purpose of nuclear lamina? What occurs when there are defects?
Purpose: lines inner nuclear membrane to position chromosomes, and disassembles/reforms during mitosis
Defects: Progeria (premature aging), Impaired cell division, increased cell death, diminished capacity for tissue repair.
What is plectin’s purpose and defect results?
Purpose: Linker protein that cross-links intermediate filaments into bundles and connects them to microtubules, actin filaments, and desmosomes.
Mutations: epidermolysis bullosa, muscular dystrophy, neurodegeneration
What are SUN and KASH linker proteins?
Bridge the nucleus and cytoplasm through the nuclear envelope. Helps position nucleus within cell (KASH), and position chromatin and nuclear lamina within the nucleus (SUN)
What is the purpose of microtubules?
- Positions membrane enclosed organelles
- Creates system of tracks within the cell for transport
- Forms mitotic spindle, cilia, and flagella
What is the microtubule subunit made of?
- alpha tubulin and beta tubulin create a dimer
- Dimer stacks together through non covalent bonding to form the protofilament
- Thirteen protofilaments associate to form the wall of the hollow cylindrical microtubule.
What is the structural polarity of microtubules?
+ end: beta tubulin end (dimers add more rapidly)
- end: alpha tubulin end
What is the centrosome?
Microtubule organizing centre consisting of a pair of centrioles surrounded by a matrix of gamma tubulin proteins
What is the point of gamma tubulin proteins?
Serves as nucleation sites for the growth of microtubules where the minus end (alpha) of each microtubule is embedded in the centrosome and growth occurs at the plus end (beta).
What is the arrangement of microtubules in a fibroblast?
Single centrosome located near nucleus that gives rise to starburst array of microtubules that extends throughout the cytoplasm.
What is the arrangement of microtubules in yeast?
microtubule-organizing centre is embedded in the nuclear envelope itself
What is the arrangement of microtubules in plant cells?
Have microtubule organizing centres in the nuclear envelope and throughout the cell cortex
What is the arrangement of microtubule organizing centres in epithelial cells?
microtubules align down length of cell, PLUS ends at BASAL membrane so that proteins are transported to the appropriate domain.
What is the arrangements of microtubule organizing centers in neurons?
Support axons and dendrites that carry information to and from the neuronal cell body.
What is dynamic instability?
The switching back and forth between microtubule polymerization and depolymerization.
How does a microtubule grow?
- Tubulin dimer has GTP bound to beta tubulin
- GTP on beta tubulin is hydrolyzed to GDP after dimer is addd to growing microtubule
- GDP remains bound to beta tubulin
What is a GTP cap?
When dimers are added faster than GTP hydrolysis can occur, the growing end of the microtubule is rich in GTP bound dimers. The GTP bound dimers bind tightly to each other compared to GDP bound dimers as they are less stable.
How does a microtubule shrink?
- Tubulin dimers at the end of a microtubule hydrolyze GTP before the next dimers are added resulting in a free end made of GDP tubulin.
- GDP dimers associate less tightly and favour dissasembly
- As they peel away from microtubule wall, dimers in cytosolic pool exchange GDP for GTP and may be added to another molecule
What are examples of how dynamic instability is regulated?
- Mitotic spindle requires rapid growth and shrinking
- Microtubules organizing the interior of the cells suppress dynamic instability by binding to proteins on either side
How are microtubules organized in an axon?
Microtubules in an axon point +ve towards axon terminals, -ve towards cell body. This is to facilitate bidirectional transport of organelles, membrane vesicles and macromolecules.
What are types of roles microtubule associated proteins play?
- Stimulation of new microtubule growth (gamma tubulin ring complex) and growth via branching (augmin)
- stabilize molecules
- Sever microtubules (katanin)
- Mediating interactions with other cell structures
- Protmote polymerization/depolymerization
What are Colchicine and nocodazole?
Drugs that bind tightly to FREE tubulin dimers, preventing polymerization.
What is taxol (pacliaxel)?
Drug that binds tightly to EXISTING microtubules to prevent depolymerization.
How do microtubule associated motor proteins drive intracellular transport?
Kinesins and Dimers have globular heads attached to microtubule and single tail holding cargo (w/ or w/o adaptor protein). They hydrolyze ATP to gain energy to make jerky saltatory movements.
What is the hand-over-hand motor protein walking process?
- ATP on head 1 (back) allows it to bind strongly to the microtubule
- Hydrolysis of ATP on head 1 loosens it
- Release of ADP and ATP binding on head 2 changes binding conformation of head and pulls head 1 forward.
Where do microtubules and motor proteins position the ER and golgi body in the cytoplasm?
Kinesin pulls ER to the edge of the cell (+ END)
Dynein pulls golgi inward towards the nucleus (- END)
What are the similarities and differences between cilia and flagella?
Similarities: composed of microtubule and dynein, and move as a result of bending when microtubules slide against each other.
Cilia: moves fluids across the cell surface by extending for a fast power stroke then a slow recovery stroke
Flagella: longer than cilia and designed to propel the entire cell
What is the structure of cilia and flagella?
- Nine outer doublet microtubules in a ring, one single doublet in the middle
- Doublets are tied to each other by linking proteins.
- Two rows of dynein are attached to the nine outer microtubules, with tails attached to one microtubule, and heads interacting with adjacent microtubules.
What is Kartagener’s syndrome?
Ciliary dynein defect that results in nonmotile sperm, lung infections, and impairment in the movement of the egg in fallopian tube.
How does dynein cause bending in flagellum and cilia?
Hydrolysis of ATP causes heads to move towards the minus end, allowing the plus end to “grow”
What is the purpose of actin filaments?
- Forms stable structures such as microvilli
- Essential for movements involving cell surface
- Forms contractile ring during cell division
- Allows muscle contraction
- Transport of molecules
What is the structure of actin filaments?
Twisted chain of identical globular actin monomers pointing in the same direction, creating plus end and minus end.
How does actin polymerize?
- Free actin monomers are bound to ATP
- ATP is hydrolyzed when it gets incorporated to the filament
- ADP in the actin filaments reduces the strength of interaction and destabilizes the filament, promoting depolymerization
How does actin grow and disassemble? What is treadmilling?
Growth: monomers are added faster than ATP hydrolysis in the plus end
Disassembly: ATP is hydrolyzed faster than monomers are added at minus end
Treadmilling: monomers move through filament form the plus end to the minus end at the same rate.
What is phalloidin, cytochalasin and latrunculin?
Phallosisin: prevents depolymerization of actin
Cytochalasin + Latrunculin: Prevents polymerization of actin
What keeps actin monomers from spontaneously polymerizing?
- Actin binding proteins
- formins (promote polymerization)
-thymosin and profilin (sequester/isolate monomers)
What is the process of cell crawling?
- Polymerization of cytoplasmic actin at the leading edge creates lamellipodia (crevice) and filopodia (protrusion)
- Lamellipodia and filopodia make attachments to the substratum
- myosin motor protein draws the body of the cell forward.
What is the actin’s response to extracellular signals?
- Signal molecules trigger activation of Rho GTPases
- Rho GTPase triggers bundling of actin filaments and activation of formin proteins
- Activation of formin and bundling of actin promotes the formation of lamellipodia.
What is myosin?
The motor protein of actin that uses energy hydrolysis to move along the actin filament. Includes myosin-I and myosin-II subfamilies.
What is the difference between myosin I and myosin II?
Myosin I: single globular head and tail attached to cargo. May also bind to actin filament in cell cortex to pull plasma membrane into new shape.
Myosin II: Many double-headed molecules form myosin filaments which slide two actin filaments of opposite orientation past each other. Found in bundles of muscle contraction and contractile ring in cell division.
what are muscle fibers structure?
- Multinucleated cells formed by the fusion of many smaller cells.
- Cytoplasm of muscle cells contain myofibrils (contractile elements) made of sarcomeres (contracile units).
- Z disc are where the plus ends are anchored, while minus ends overlap with the ends of myosin filaments.
What is the process of muscle contraction?
Myosin head binds and hydrolyze one molecule of ATP resulting in a conformational change that propels the myosin unidirectionally along the actin filament
How does calcium trigger muscle contrcation?
- Tropomyosin binds to actin helix, blocking myosin heads from attaching to actin.
- Nerve stimulation and electrical signal causes Ca++ release in sacroplasmic reticulum
- Increase in cytosolic Ca++ binds to troponin which causes a shape change that shifts tropomyosin to allow myosin heads to bind to actin and initiates muscle contraction.
What is the state of extracellular matrix in animal connective tissue?
Extracellular matrix is abundant and carries the mechanical load, including tensile strength (via collagen) and elasticity (via elastin)
What are fibroblasts and osteoblasts?
Fibroblast: connective tissue in skin and tendon
Osteoblast: connective tissue in bone
How is collagen formed?
Individual polypeptide chains assemble into triple stranded collagen molecules, then to fibrils and finally procollagen fibers. Procollagen fibers are cleaved to form mature collagen outside the cell.
What are integrins’ role in cytoskeleton?
Proteins that indirectly interact with collagen fibers in the extracellular matrix via fibronectin to the cytoskeleton inside.
How are integrins activated?
- Intracellular signalling molecule activates integrin from cytosolic side of membrane, causing it to reach out and grab an extracellular structure
- Binding to external structure can switch on various intracellular pathways by activating protein kinases associated with the intracellular end of the integrin.
What is the basal lamina?
Thin tough sheet of extracellular matrix composed of type IV collagen and laminin.
What are tight junctions?
Tight seals made from proteins claudin and occludin that prevent the leakage of molecules across the epithelium through gaps between its cells, and apico-basal polarity of epithelial cells.
What are adherens junctions?
Cell junction of epithelial cells made of extracellular transmembrane cadherin attached to intracellular actin, allowing the epithelium to bend
What are desmosomes?
Cell junction made of extracellular transmembrane cadherin attached to cytosolic keratin intermediate filaments
What are hemidesmosomes?
Cell junction made of cytosolic keratin intermediate filaments attached to integrins on the basal lamina.
What is a gap junction?
Closeable channel made of connexons aligned end to end that create a cytosolic bridge allowing inorganic ions and hydrophilic molecules to move directly from the cytosol of one cell to the cytosol of another.
What makes plant cells rigid?
Lack of tension-bearing intermediate filaments and the development of an expanding primary cell wall and a rigid secondary cell wall.
What are the two types of cell walls in a plant?
Epidermal cells of a leaf: waxy waterproof walls
Xylem cells of the stem: hard thick woody walls
What is the plant cell wall made of?
Cellulose microfibril is made from bundles of cellulose molecules interwoven with other polysaccharides and structural proteins, allowing the plant cell wall tensile strength.
What is plasmodesmata?
Small channels that connect the cytoplasm of adjacent plant cells, can span the cell walls and are lined with plasma membrane.
