Cytoskeleton Flashcards
- Characteristics & Function - structure & function of microtubules - dynamic instability - MT - organizing center - MT - associated proteins - Structure & function of actin filaments - structure & function of intermediate filaments
cytoskeleton determines
- organization
- polarity
a cell has to:
- be correctly shaped
- be physically robust
- be internally structured
- change its place and move from place to place
3 different types of filaments that compose the cytoskeleton
- Actin filaments
- Intermediate filaments
- Microtubules
Actin filaments
- determine shape of cell
- necessary for cell locomotion
Microtubules
- determine positions of membrane-bound organelles
- direct intracellular transport
- form mitotic spindle
Intermediate filaments
- provide mechanical strength
tubulin
- protein polymers that makeup MT
- subunit that is a heterodimer (alpha and beta)
alpha-tubulin
- has a binding site that holds GTP
- GTP that is bound is never hydrolyzed or exchanged
beta-tubulin
- has a binding site that can hold either GTP or GDP
- exchangable
MT characteristics
- hollow cylindrical structures made out of protofilaments
protofilaments
- composed of alpha-beta tubular heterodimers
How do microtubules grow and shrink?
process: dynamic instability
dynamic instability
- binding and hydrolysis of GTP that occurs only in the beta subunit of tubular dimer
- addition of GTP-containing tubular to end of protofilament -> growth
Microtubules originate from a specific cellular location
microtubule organizing center (MTOC)
- e.g. in animal cells -> centrosome is major MTOC
MT function during cell division
when cell divides, MT rearrange to form a bipolar mitotic spindle
- this is responsible for aligning and segregating the chromosomes
Microtubule-associated proteins (MAPs)
- move along MT bringing transport vesicles to target organelles in the cell
- can move organelles and vesicles in the cell
- can move vesicles with pigments in the skin
kinensin
travels toward plus end
dynein
travels toward minus end
melanosomes
pigments
flagella
used to move cells in a liquid environment
- e.g. spermatozoids
cilia
move fluid above a cell
- e.g. mucus in respiratory epithelium
MT- Specific Drugs
- Taxol: binds & stabilizes MTs
- Colchicine, colcemid, vinblastine, vincristine, nocodazole: binds subunits and prevents their polymerization
Taxol (Paclitaxel)
- anticancer chemotherapy
- blocking Mts to kill cancer cells
- blocks depolimerization
Colchicin
- alkaloid of the autumn crocusses
- binds free tubular -> prevents MT polymerization and cell division
- anti-inflammatory effects
Vinca alkaloids
- binds free tubulin -> prevents MT polymerization and cell division
Actin
- form a tight, right-handed helix called filamentous actin (F-actin)
- consists of 2 parallel protofilaments
- helix
- flexible structure
Actin filament
- polymerize to grow
- depolymerize
- ATP bound actin has higher affinity for neighboring subunit and remains stable in the filament
- ADP bound actin can more easily dissociate from filament
Actin crosslink
villin, fimbrin
Stress fiber
contractile bundle
Cell Cortex
gel-like network
Filopodium
tight-parallel bundle
Actin-associated proteins & muscle contraction
- kinesin
- myosin
- dynein
tropomyosin
- blocks myosin-binding site
myosin-binding site
- exposed by Ca2+-mediated tropomyosin movement
- a lot of Ca2+ present in sarcomere
ATP + Ca2+
- needed to walk along actin filament (movement)
Phalloidin
- toxin of the death cap - toxic esp. to liver cells
- binds actin filaments & blocks depolimerisation
- stabilization
- cell not able to move or maintain homeostasis
- alpha-amanitin inhibits RNA-Polymerase II
- main toxicity of the death cap
Intermediate filaments
- no polarity (+/-)
- subunits don’t contain ATP or GTP
- don’t play a role in cell movement
- no motor proteins associated
- BUT, associated with:
- cell-cell junctions
- strengthening cells and epithelia
- giving tissues mechanical stability
- mechanical resistance: prevents separation of skin
monomer
one protein
in final intermediate filament
- there are 16 dimers of IF monomers = 32 coiled coils
- very strong structure
- network of noodles in cell
keratin filaments
- intermediate filament
- produced by keratinocytes in the epidermis
- made up of keratin (subunit)
- formation of horns, nails, hair
- anchoring of epithelial cells via desmosomes/hemidesmosomes
cytoskeleton components of MICROFILAMENTS
- 2 helical crossed ACTIN strands
- thinnest diameter
- subunit: ACTIN
- functions:
- maintains cells shape
- changes cells shape
- muscle contraction
- cell movement
- in periphery of cell, sometimes running parallel
cytoskeleton components of MICROTUBULES
- tubes of strands per MT
- thickest in diameter (has a lumen)
- subunit: alpha and beta-tubulin
- functions:
- maintains cell shape
- cell movement
- chromosome movement during cell division
- movement of organelles
- comes from organizing center (centrosomes) and spreading towards cell periphery
cytoskeleton components of INTERMEDIATE FILAMENTS
- long molecules polymers
- subunit: keratin, lain, vimentin (all proteins)
- functions:
- maintains cell shape
- mechanical shape
- formation of nuclear lamina
- distributed along whole cytoplasm of cell, building NETWORK of filaments in cytosol
