cell cytoskeleton Flashcards
Explain the importance of the cytoskeleton.
Network of protein filaments
- Very dynamic, responds to environment quick
- Important in cell shape, interior organisation and movement
- Support cell
- Maintain shapes
- Holds organelles in position
- Movement of cytoplasm
- Interacts with extracellular structures, anchors cell in place
Compare and contrast microfilaments, microtubules, and intermediate filaments in terms of structure and function. MICROFILAMENTS
microfilaments
- smallest in diameter
- made of globular actin proteins
- actin proteins bind to form helical polymers
- two helical polymers form microfilament
- twisted chain diameter is 7nm
- long, thin and very flexible
- polarized structures
- have +/- end, grow quick at + end
- allow actin monomers to interact with each other to form helical chains
Microfilaments polymerization and depolymerization
- have +/- end, grow at + end, lost from minus end
- polymerization of actin into microfilaments is reversible
- hydrolysis of ATP determines polymerization and depolymerization
- when bound to ATP, actin has high affinity for other actin molecules and binds tightly to them
- when hydrolyzed to adp, affinity decreases
- microfilaments break down into monomers of free actin
high concentration –> growth of filament, rate of addition higher than loss
intermediate concentration (tread milling) –> equal growth and loss, filament stays the same length
- loss from - add, add app come back on + end, actin filament is moving in one direction, direction of movement
Microfilaments and actin-binding proteins
- actin binding proteins control organization of actin filaments
- can exist as, single, linear bundles, 2D networks, 3D gels
stability
- stabilizes microfilament from depolymerizing
- side-binding, prevents branching
organization
- bundling and cross linking proteins involved organization
- nucleating protein helps polymerization
function and movement
- motor proteins, can walk on microfilaments MYOSIN, vesicle transport and muscle contraction
Explain the cellular processes in which each filament is involved: MICROFILAMENTS
- highly concentrated in cortex beneath cell membrane, in contact with plasma membrane
2 major roles:
- cell stability and shape
- cell movement
in non muscle cells, associated with cell shape
- microvilli
- contractile bundles in cytoplasm
- fingerlike filopodia in moving cell
- contractile ring during cell division
cell shape- cell division
- pinch contraction of middle of cell, 2 daughter cells into 1, through myosin proteins
cell movement
- form cellular extensions called filopodia pseudopodia
Cellular process: CELL SHAPE AND MICROFILAMENTS
- epithelial cells
- line intestine to increase absorption have microvilli to increase surface area
- microvilli supported by microfilaments
-cross linking actin binding proteins form netlike structure of microfilaments
- interact with intermediate filaments at base of each microvilli which helps stabilize microvilli
Cellular process: VESICLE MOVEMENT AND MICROFILAMENTS
movement of vesicle inside cell:
- microfilament attached to plasma membrane
- myosin motor proteins walk on microfilament carrying vesicles
- bring it to plasma membrane
contraction of membrane
- myosin motor protein attached to membrane
- one side attached to membrane, other side to microfilament
- movement of protein takes microfilament (and all other microfilaments associated with it) in one direction
- pinching during cell division
Cellular process: MUSCLE CONTRACTION AND MICROFILAMENTS
- different class of myosin (II)
- moment of myosin on microfilaments brings them together
- actin filaments slide against myosin during muscle contraction
- acetylcholine detected, calcium increases in cytoplasm, protein that hide binding site removed through changes in conformation, now myosin can walk on actin and bring myosin filaments closer together
Cellular process: CELL MOVEMENT AND MICROFILAMENTS
SEE DIAGRAM
- dynamic properties + motor proteins
- cell receives signal, in response changes cytoskeleton to move in direction of source of signal
extension
- at leading edge, actin polymerization pushes plasma membrane forward (protrusion) and forms new region of actin cortex
adhesion
- new anchors made between bottom of cell and surface, substratum, on which its crawling (attachment)
translocation
- depolymerization and contraction at the other end of the cell due to myosin moving along actin filaments draws cell body forward
de-adhesion
- new anchor points made at front, old released at back
- cycle is repeated
Compare and contrast microfilaments, microtubules, and intermediate filaments in terms of structure and function. INTERMEDIATE FILAMENTS
intermediate filaments
- tough and rope like
- very flexible
- tensile strength
- made of FIBROUS proteins
- resist mechanical stress (skin cells)
- more permanent structures, do not rupture under stress but deform
- more structural support than movement
structure
- long twisted strands of protein
- monomer is alpha helical
- 2 monomers is dimer
- 2 dimer bind laterally to make tetramer
- 8 tetramer interact laterally and now are ready to be added to growing filament
- make rope like structure
4 major classes
- not different in amino acid but terminal sites, bind different proteins
cytoplasmic
- KERATIN: epithelial cells
- VITEMIN: connective tissue, muscle, glial cells
- NEUROFILAMENT: nerve cells
nuclear
- NUCLEAR LAMINS: in all animal cells
Cellular process: INTERMEDIATE FILAMENTS AND MECHANICAL STRENGTH AND CELL SUPPORT
- create strong durable network in cytoplasm
some types:
- form meshwork called nuclear lamina beneath inner nuclear membrane to support and strengthen nuclear envelope
other types:
- extend across cytoplasm
- bind 2 parts of the cell, desmosomes are where 2 neighbouring cells bind each other (from one junction to another)
- intermediate filaments bound to plasma membrane at level of desmosomes
- collection of intermediate filaments connect cell together allows it to stretch
- give mechanical strength to cells and distribute mechanical stress in epithelial tissues
Compare and contrast microfilaments, microtubules, and intermediate filaments in terms of structure and function. MICROTUBULES
- rigid hollow tubes
- need organization centre to grow from, centrosomes
- made of tubulin dimer which has 2 GLOBULAR subunit tubulin proteins: alpha and beta
- 13 protofilaments of tubulin
- several micrometers long
- more rigid, ruptured when stretched
Microtubule polarization, centrosomes and dynamic instability
polarization:
- have +/- end
- addition of tubulin (alpha beta dimer) to + end
centrosomes:
- need organizing centre to grow from
- centrosomes have rings to which alpha and beta tubulin proteins are added
- minus end is inside centrosome connected to ring
- + end extends into cytosplasm
- microtubules grow and shrink fast INDEPENDENT of one another
dynamic instability:
- switching back and forth between polymerization and depolymerization
growing
- subunits of microtubules bind gtp have high affinity for one another
- grow towards positive side
shrinkage
- if addition of tubulin is slow
- hydrolyze gtp and lose affinity for each other
- not tightly packed together
- depolarization from + end happens
very fast
- once gtp is lost, nothing prevents microtubule from depolarization
- add gtp quick to regrow
Microtubules and microtubule binding proteins
- plus end initially free but binds to specific protein which help stabilizes it
- minus end protected by centrosome
Explain the Cellular processes: STRUCTURAL AND FUNCTIONAL ROLES
- form rigid internal skeleton
- framework along which motor proteins can move structures within cell
