Exam 3: lecture 8 Flashcards
What are microfilaments? + functions
smallest of the filaments (7nm)
-best known for role in muscle contraction (motor proteins)
functions:
cell migration
amoeboid movement
cytoplasmic streaming
development & maintenance of cell shape (beneath the plasma membrane at the cell cortex)
structural core or microvilli
what protein is the building block of microfilaments?
actin
once synthesized, folds into a globular-shaped molecule that can bind to ATP/ADP—>
G-action: globular (free+ not attached to a microfilament)
F-actin: G-actin polymerize (strung+attached)
to add:
ATP –hydrolyzed–> GDP
How does microfilament polarity work?
S1 fragments bind + decorate the actin MFs in a distinctive arrowhead pattern
barbed end: +end
pointed end: - end
a)brief treatment w/ trypsin
split into light meromyosin +heavy meromyosin
b) further treatment
light+ heavy
heavy:
arrowhead—> S1 + S1 (use these to attach to MFs)
S2
treadmilling of actin microfilaments in Vitro
no net change in length, MTs can still add g-actin monomers at + ends and lose them at the - ends
-less dynamic than microtubules
-this occurs when the -end is not attached to cell protein
Drugs that affect polymerization of MFs: cytochalasins
fungal metabolites that PREVENT the addition of new monomers to existing MFs (keep g-actins from binding)
Drugs that affect polymerization of MFs: latrunculin A
toxin that sequesters (isolate) actin monomers + prevent their addition to MFs
Drugs that affect polymerization of MFs: phalloidin
stabilizes MFs and prevent their depolymerization (works on polymerization structures & keep them from falling apart) `
what regulates the polymerization, length, and organization of actin? + examples
actin-binding proteins
-control occurs at the nucleation, elongation, and severing of MFs and the association of MFs into networks
1.monomer binding protein (thymosin)
2. filament severing proteins (gelsolin)
3. filament bundling proteins (alpha-actinin, fimbrin)
4. filament crosslinking: filamin
5. filament capping proteins (capz)
6. filament anchoring (spectrin, erm proteins)
what are intermediate filaments?
most stable+ least soluble+ not polarized
10 nm
most abundant: keratin
can support entire cytoskeleton
tissue specific
Intermediate filament: classes I and II
Class I: acidic keratins
Class II: basic/ neutral keratins
-make up tonofilaments found in epithelial surfaces covering the body and lining its cavities
Intermediate filament: class III
includes:
-vimentin (connective tissue)
-desmin (muscle cells)
-glial fibrillary acidic proteins (GFA) (gial cells)
Intermediate filament: class IV
neurofilament (NF) proteins found in neurofilaments of nerve cells
intermediate filament: class V
nuclear lamins A, B, C that form a network along the inner surface of the nuclear membrane (most stable in cytoplasmic structure)
intermediate filament: class VI
neurofilament in the nerve cells of embryos are made of nestin
what are the structural similarities of intermediate filaments?
central rodlike domain:
4 helical segments (spring rods)
3 linker segments
n domain: amino
c terminal: carboxy
What is the order of the structure of intermediate filament assembly in vitro
a) 2 IFpolypeptide
b) the polypeptides are coiled tgt to form a dimer
c) tetramer (2 dimers layered on top of each other)
d) protofilament (bunch of tetramers on top “ropelike”)
e) intermediate filament
How do these cytoskeletons confer mechanical strength?
MTs resist bending when a cell is compressed
MFs serve as contractile elements that generate tension
IFs are elastic and can withstand tensile forces
How do IFs help w the integrated structure?
important in structural determinants in cells and tissues— thought to have a tension-bearing role (both pulling apart tension & when they’re pushed tgt)
IFs are not static structures; they are dynamically transported and remodeled
integration of cytoskeletal elements: nuclear lamina
on the inner surface of the nuclear envelope, disassemble at the onset of mitosis, and reassemble afterward
integration of cytoskeletal elements: plakins
linker proteins that connect intermediate filaments, microfilaments, and microtubules
integration of cytoskeleton elements: plectin
found at sites where intermediate filaments connect MFs and MTs
How does motor protein ( dynein & kinesin) move along the microtubule?
Kinesin:
transport vesicles to the + end
ATP —hydrolyzed—> ADP + Pi
*small balls, long chain
cytoplasmic dynein:
transport vesicles to the - end
ATP —hydrolyzed—> ADP + Pi
*big balls, short chain
(move towards the MTOC)
movement of kinesin on microtubule
structure:
stalk
springlike linker region
back foot
front foot
to start:
back foot is bind to the microtubule
front foot is in the air
ATP—>ADP + Pi
movement:
front food binds to a new B-tubulin subunit
back foot will move to front
the direction of movement of kinesin & dynein in the secretory pathway
Rough ER(+)
Golgi Complex
MTOC
Cell membrane (+)
dynein:
rough er –> golgi complex
cell membrane–> MTOC
kinesin:
golgi complex–> cell membrane
golgi complex–> rough er
what is the level of organization of skeletal muscle tissue?
largest-> smallest
muscle–> bundle of muscle fibers (muscle cells)–> individual muscle fiber (cell)–>myofibrils–>single myofibril–> portion of myofibril–> thick & thin filament
structural proteins of the sarcomere
Z line
thin filament (actin)
nebulin ( a spring)
think filament (myosin)—myomesin—-titin
-all extends til they touches alpha-actinin (CapZ)
-repeat 3 times
3 brown lines of thick filament (myosin)
what is the sliding-filament model of muscle contraction?
the sarcomere shorten as thick and thin filaments slide across each other
Thin filaments contain how many proteins?
F-actin
Tropomyosin
troponin
Troponin & tropomyosin
troponin:
composed of three polypeptides (TnT, TnC, TnI)
one troponin complex associates with each tropomyosin
tgt they constitute a calcium-sensitive switch that activates contraction in striated muscle
the process of sliding movement
myosin head (high-energy) -ADP & Pi
- cross-bridge formation; release of Pi
head only has ADP: myosin head (thick) binds to troponin complex (actin of thin)
- power stroke; ADP is released, myosin undergoes a conformational change
head pushes thin filament toward the center of the sarcomere (myosin head is low-energy)
- ATP binds myosin, causing detachment of myosin from actin; cross-linked dissociates
myosin head (low energy) with ATP
- ATP hydrolysis occurs, cocking myosin head
myosin head (high-energy) -ADP & Pi
rigormortis: myosin gets stuck in 1 place bc no ATP
what does the regulation of muscle contraction depend on
calcium
most skeletal muscles spends more time in the relaxed state than in contraction
-contraction and relaxation must be coordinated
What is the role of calcium in contraction(concentration)
tropomyosin & troponin (regulatory proteins) regulate the availability of myosin binding sites on actin filaments calcium-dependent manner
myosin binding sites on actin are normally blocked by tropomyosin, which must be moved if cross-bridges are to form
when calcium concentration is low. tropomyosin blocks the myosin binding sites on the actin filament—>preventing interaction with myosin
at high concentrations, calcium binds TnC causing tropomyosin to shift, and allowing myosin to bind
how are calcium levels regulated in skeletal muscle cells?
controlled by nerve impulses from motor neurons
muscle contraction is regulated by calcium ions in the sarcoplasm (cytosol of a muscle cell)
muscle cells have features that facilitate rapid changes in Ca2+ concentration
what happens at the Neuromuscular Junction?
neuromuscular junction: the site where a nerve contacts a muscle cells; conveying a signal to contract in form of an action potential
at the junction, axons terminals make contact with the muscle cell
-store acetylcholine, which is released in response to an action potential
What causes contraction?
in response to depolarization, the receptor channels open and release calcium in the sarcoplasm
steps
1. action potential move from the axon (neuron) to neuromuscular junction
2. depolarization of the terminal releases neurotransmitter–> bind to acetylcholine on the surface of muscle cell(starting the depolarization)
3. depolarization spread into the interior through T tubules–> calcium release in the terminal cisternae of the SR
-chemically-triggered receptors release calcium which activates a contraction; for muscles to relax, calcium levels must decrease
-the SR membrane has a calcium ATPase transporter to pump calcium back into the SR cisternae-located in the medial element of the SR
The movement of actin in crawling cells
trailing edge: back
a) contractile bundle: stress fibers (opposite directions)
-strong; increase strength to be able to contract to be pulled around
b) gel: at the cortex (all directions)
-cross-linked: creating strength to hold itself down
c)parallel bundle: filopodium (same direction
-thin, fingerlike projection
leading-edge: front
movement of lamellipodium
- the leading edge extends via the polymerization of actin at its tip
- new adhesion, anchored by actions, form on the undersurface of the lamellipodium
- the trailing edge (tail) of the cell detaches and is pulled forward by contraction of the body
what happens during attachment coupled to protrusion formation in migration nerve cell
Stationary:
polymerization and retrograde flow balanced: the myosin-driven retrograde flow of actin
moving forward:
cell attachment resists retrograde flow, resulting in extension
(attachment to substrate via integrin)
membrane protrusion and cell movement due to gelation-solation
back: gel to sol(fluid) transition
front: sol to gel transition
cell protrusion in Thyone(actin polymerization)
acrosome; actin (solubilized state)
the acrosomal process slowly comes come causing the release of actin- actin filament polymerizing
What does cytokinesis have to do with contraction
during cytokinesis, the ring is made up mostly of actin.
the contractile processes are generated by movement along actin by the motor protein myosin II
structure of a eukaryotic flagellum/ cilium
central microtubules
outer microtubule doublet: goes thru the membrane to the basal body (centriole/ MTOC) @ the neg end
what do u see at a cross section of an axoneme
central pair of microtubules
projection from central pair
radial spoke
outer doublet microtubules (A & B)
—> side arms: inner dynein & outer dynein
inter-doublet nexin connection
plasma membrane
*9:1 (9 outside: 1 inside)
axonemal dynein:
cytoplasmic motor protein:
push against microtubule axoneme
cytoplasm holds around microtubules
the mechanism of microtubule bending in cilia or flagella
dynein arm pushes the double w atp causing the sliding force–> bending
what are primary cilia
used in sensory structures
9+0 structure (lacking the central pair )
important in development; defects in them can result in disorders such as deafness and left-right asymmetry reversals
Structure of a prokaryotic bacterial flagellum and the motor responsible for its rotation
m ring
s ring
rod
hook
fiber
m-ring turn as atp is hydrolyzed
