Cytoskeleton I &II Flashcards
cytoskeleton
Dynamic intracellular structure - does lots of things for cell
provides:
o Cell shape
o Mechanical strength
o Structures needed for locomotion
o Support for the plasma membrane
o The scaffold for the spatial organization of organelles
o The means for intracellular transport of organelles and other cargo.
The three types of cytoskeletal element
- microfilaments
- microtubules
- intermediate filaments
microtubule structure/properties
tubular structures, one end attached to centrosome
GTP favors microtubule growth
microtubule predominant proteins
tubulin: alpha, beta, gamma
typically building blocks of heterodimers of tubulin protein (alpha and beta
microtubule function
- Movement of flagella and cilia
- cellular cytoskeleton (scaffold)
- intracellular transport
- cell division
Intermediate filament structure/properties
complex ropelike filament made from 8 tetramers
Intermediate filament predominant proteins
vimentin
keratin
neurofilaments
GFAP
Intermediate filament function
mechanical stability
How does GTP affect microtubule growth?
GTP favors microtubule growth of the plus end (beta subunit)
How do microtubules confer polarity?
they have an alpha tubulin on one end (minus), and a beta tubulin (plus) on the other
describe treadmilling
growth at plus end, disassembly at minus end due to GTP hydrolyzing to GDP,
describe dynamic instability of microtubules
MT capping proteins can bind to ends to increase stability
MT severing proteins can expose GDP rich regions and increase instability
2 examples of severing proteins
Spastin and Katanin
plant Drugs that inhibit MT polymerization
the “ines”
cholchicine
vinblastine
vincristine
- prevents MT growth = bad for cell
- blocks mitosis
Drug that stabilize polymerization
paclitaxel (taxol)
causes tubulin aggregates = bad for cell
-blocks mitosis
kinesins
dynesins
which direction is cargo being moved?
molecular motors - prime sites of regulation
kinesins: move cargo towards plus end
- (need adaptors since they do not directly bind to cargo)
dynesins: move cargo towards minus-end
- only GDP bound
Mechanism of tubilin based movement and intracellular transport
Coordinated with ATP hydrolysis - when either head is bound to ATP it binds to MT.
o MT binding o Conformational change o MT release o Conformational relaxation o MT re-binding
This process moves the motor protein and the cargo along the microtubule in a stepwise fashion.
Different types of microtubules (MT) associated with mitosis
- astral MT
- Kinetochore MT
- Overlap MT
What is the mitotic spindle made of?
MTs and associated proteins
What is the only case where 2 centrosomes can be seen?
A cell about to undergo mitosis
before cytokinesis
Role of microtubule (MT) in mitosis (cell division)
- microtubule plus ends point away from the centrosome (mitotic spindles)
- The motors, astral MT, (and other proteins) cause spindle to grow and centrosomes to become more distant.
- minus end directed motors separate daughter chromosomes and move them along MT to centrosome
Of the 3 types of cytoskeletal elements, which ones are polarized and which one is not?
polarized: MT, MF
Nonpolar: NF
-no plus or minus end
axoneme
- what is it?
- what allows it to bend?
microtubule core of cilia and flagella
- the highly coordinated action of dynein activity
Intermediate filament disease associated with Keratins
interferes with filament assembly
- epidermis highly sensitive to mechanical stress/blisters
= Epidermolysis bullosa simplex
Intermediate filament disease associated with Neurofilaments
interferes with axonal transport of neurofilaments
=charcot marie tooth
=Amyotrophic lateral sclerosis (ALS)
Intermediate filament disease associated with Lamins
results in nuclear instability
=progeria syndromes (weird alien head)
Disease associated with mutation in dynein motor
outer cilia/flagella are missing
=primary ciliary dyskinesia syndrome
What is another name for centrosome? Where is it located?
perinuclear microtubule organizing center (MTOC)
located near the nucleus
What is another name for microfilaments?
actin cytoskeleton
microfilament structure/properties
helical filaments
ATP promotes actin polymerization
(remember tubulins bind GTP)
microfilament predominant protein
G-actin monomers
microfilament function
cell shape
movement
polarity
cell division
2 steps in microfilament formation
- nucleation
2. extention/retraction of actin filament
describe role of proteins in actin filament nucleation
actin nucleation (new actin production) involves:
- Actin related proteins (ARP2/3) which leads to branched filaments.
or - FH2 which leads to parallel filaments
describe role of proteins in actin extention/retraction
polymerization/depolymerization
Preventing polymerization:
- profillin binds to actin
- capping proteins cap the plus/minus end
Induce depolymerization
1. cofilin severs actin filaments and induces depolymerization
actin cytoskeleton role in epithelial cell polarity
name what disease is associated with loss in function
actin anchors PROTEINS involved in tight junctions and adherens junctions that hold epithelial cells together
loss of cell adhesion = increase in cell motility = epithelial to mesenchymal transition = cancer
what are molecular motors? example?
proteins that can transform energy from ATP hydrolysis into motion.
myosin: actin associated moter proteins
Myosin structure
2 heads - 1 binds actin - 1 binds ATP 1 Tail - binds cargo
Myosin 3 main classes and function
Myosin II
- muscle contraction: myosin filaments walk along actin thin filaments towards plus end during muscle contraction
Myosin I + V:
- unconventional myosin and binds to organelles/vesicles thru tails and moves them along actin filaments
Concept of cell movement (motility)
cells commonly translocate by amoeboid movement. Amoeboid movement is governed largely by changes in actin cytoskeleton, induced by extracellular signals.
Key steps of cell movement (motility/amoeboid locomotion)
think of picture
- protrusion: signal from Rac and WASp causes actin to polymerize (catalyzed by Arp 2/3) and form projection called lamellipodia and Filopodia
- attachment: lamellipodia protrusions attach with substratum ahead
- traction: cell is pulled forward by tension created by anchored actin protrusions and myosin molecular motors
- detachment: adhesions behind new sites of anchorage are released to allow for translocation
Filopodia
result from polymerization catalyzed by formins
have attached receptors for ‘sampling’ the environment in front of the cell during movement
role of actomyosin ring in cell division
actomyosin ring: ring structure with associated myosins that form around a dividing cell
it contracts through action of myosin heads and ATP hydrolysis during cytokinesis to separate daughter cells.
mechanism regulating the establishment and activation of actomyosin ring
the site and timing of contraction of the actomyosin ring regulates the symmetry of cell division
(could result in assymetric cell division)
Rho-GTP activates 1 of 2 pathways for contraction of ring
- activates formin
- activates Rock kinase
examples of asymmetric cell division
- red blood cells: nucleus is separated from remainder of cell in cytokinesis
- platelet formation: cell duplicates its genetic material before dividing, and later divides assymetrically many times to give rise to many cell fragment (megakaryotypes)
- sperm development: cells can be left connected by canals by persistence of actomyosin ring following division until ready to be released and cleavage occurs
- division of epithelial cells ensures cells maintain a monolayer distribution (not stacked)
