week 7 Flashcards
cell movement
- involves polymerization and depolyemerization of actin
- actin is responsible for cell shape and movement of organelles under the PM
actin functions
- found in epithelial cells (in microvilli)
-cortical actin cells - contratile belt structures
- cell migration
- actin involved in moving things inside the cell
actin structures
- bundles in microvilli
- networks and bundles cortical actin
- form fillapodia, stress fibres
monomers of actin
G-actin
types of actin
alpha (muscle)
beta (cortex)
gamma (stress fibres)
- all are functionally the same
g actin
four domains
- atp binding cleft
- polarity bc unsymmetrical
polyermize G actin to make…
F actin
orientation of polymerization of g actin monomers
atp binding cleft points thhe same way , always located towards the negative end
which side polymerizes faster
the + end
double helix formation of actin filaments
- loop of 36 nm, presents a g actin every 36 nm
- g actin is polar, so f actin is therefore polar
myosin s1 experiments
- the myosin s1 that coats the actin monomers, shows a pattern of arrowheads that always point towards the + end
- this allows us to fid the - an + end
- this also stabilizes anti (cannot depolyermize)
- this can be used a a flagellar nucleus
- u can get polymerization by actin monomers above a certain cocentration
above critical concentation
polymeriation
what does teh speed of poymernzation depend on
the presence of nuclei
actin monomers must be bound to ____ in order to polyemrize
atp
- will polymerzie and the hydrolysis of ATP after polymerization NOT BEFORE
critical concentration for + end
0.12
critical concentration for - end
0.8
- needs much more actin
above both critical concentration
polymrization
below both critical coentaration
depolyermize both sides
in between the critical concentrations
- polymrerizaiton at + end for ex, and not at - end, meaning treadmilling, were ]
when does tread milling occur
- when one side is growing only
what is C+ c?
how do we regulate large stores of actin/actin polymerization
- thymosin
- profilin
- cofilin
thymosin
sequesters g actin, binds most of the g actin, removes from critical ceonctnreation so that it is not available for polymerizationn aymore
- sequesters actin and provides a reservoir
profilin
takes.g acting in the ads form to turn to g actin in atp form
cofilin
- causes depolyemizaiton of actin filaments
actin capping proteins
cap z
tropomodulin
cap z
- binds to + end
- this end does not polyermize or depolymerize when presenttr
tropomodulin
- binds to - end and does same thing as caps
where would capping proteins be found
- ## in muscle fibres
cytochalasin:
depolyermizes actin filaments,
phalloidin
stabilizes actin filamentsactin
actin disrupting drugs
drugs that can be used to stabilize/destabiilize actin filaments
formins
nucleating proteins
- regulate assembly of unbranched filaments by acting on the + end
- providing nuclei
- nucleating proteins allow polyermization to occur
how is forming activated
by a protein called rho gtp
where is rho gtp found
at the plasma membrane, can activate forming the in the gTP form, this is where polymerization willl cocur
when can polymerization occur through formin
Forman activated by rhogtp, polymerization, if g actin is above vciritcal concentration and in atp form (profilin needed)
arp23
- mediates filament branching
- allows actin to form branched filaments
how does arp23 work
needs nucleation promoting factor such as WASp or WAVE
how are WASP and WAVE activated
by cdc42 and Rac
how does listeria use this mechanism
- it has ACT protein that activates arp 23 to cause rapid branching that will allow it to burst through the PM
- no need for cdc42 or rac
what is a function of arp2/3 dependent actin assmebly (endocytosis)
in endocytosis actin polymerization can move membrane, one dnoytosis, the plasma membrane can be pulled into the cell to form a vesicle that is moving into teh ell. polymerizing pulls on the membrane (keep adding to positive end of actin)
- the vesicle moves deeper away from pm
ARP2/3 function in [haogycotsisi
- actin can pull a membrane inward, or push a membrane outward
what sorts of structures can actin form
bundles or networks
Fibrin and alpha actinin
cause bundling of parallel actin microfilaments
how are RBCs supported
the biconcave shape is due to teh actin network that is holding the Plasma membrane of the RBC
ankyrin
actin network binding proteins that support teh cell memrbane
ezrin
- bundles of actin
- link the bundles to the Plasmam membrane using ezrin
dystrophin
- link plasma membrane of muscle cells so that dystrophin can pull when muscle contracts
muscular dystrophy
- repeated damage to muscle, leading to dysfunctional dystrophin
- cannot regenerate muscle anymore
myosin
- actin’s motor protein
what is the most abundant myosin
myosin 2
myosin heavy chainchain
- heavy chain with head, neck and tail
- ehad binds actin, and uses atp to move to plus end by doing binding and atp hydrolysis
- neck domain bends
- tail domain attaches to cargo
what is myosin composed of
heavy chain, and light chains\
- the head is an atpASE
- NECK BINDS LIGTH CHAIN
- tail binds cargo
light chains of actin
- involved in binding to teh neck region
imporance of neck region in light chain
- the neck region is involved in determining how fast myosin moves
- it is regulated/faciliated by the binding of light chaisns to the neck region, regulates how fast or slow the myosin moves
myosin classes
myosin 1, 2, 5
myosin 1
- heavy chain acts as a monomer does not form dimer
- neck binds light chains to regulate meovement, tail binds to phospholipids in membranes
- involved in endocytosis
- hols actin at the plasma membrane and supports it
- can hold on to membrane or just support
myosin 2
- is found in muscle, involve din muscle contraciton
- forms structure with overlapping tails and heads found on either side
myosin 5
- dimer of two heavy chains
involved in vesicular transport - the tail region has a variable domain
- can bind to different types of vesicles
- doesn’t use light cahins
- transport vesicles towards the plus end of atin
- long neck region and lots of light cahins
- takes the longest steps bc long neck
- can bind PM like myo 1 either for transport or just for support
length of myosin 5 steps
72 nm
what determines the rate of movement myosin 2
length of neck
sliding-filament assay used to detect myosin powered movement
- taking fragments of what needs to be studied from the myosin and digesting it in different ways
- once added to glass cover slip, should stick to it
- add flureoscently labelled actin, that can be visualized under fluorescent mciroscope
- add different things to see what is required for movement (i.e. add ATP,
what is the point of teh sliding filament assay
this is how we understand that the neck is important for velocity, which is regulated by the light chains. longer neck = faster velocity, since more light chains are allowed to bind.
rigger state
myosin in ADP form is bound to actin, will not move until ATP is added.
power stroke
myosin head pushes teh actin )-) end towards the left, now the Pi leaves
- atp binds, let go, hydrolysis, power stroke, back to rigorous state `
sarcomere
- distance between z discs, and in-between is teh a band and I band ( the a band is dense) and the I band (less dense
z discs
hold the actin mcirofialemnts that are extending into the middle of teh sarcomere
middle of sarcoma
myosin thick filaments,
a band
two myosin heavy chains, thick filaments that have their heads between the actin microfilaments,
what happens to the z discs/sarcomere during muscle contraction
-
are the actin microfilaments stable
- they must be stable
(dont want actin and sarcomeres to depolymerize) - caps at plus end at z disc
- tropomodulin caps the - ends
- nebulin coating also helps in stability
- titin
titin
holds the myosin thick filaments in the centre.
- these molecules are squished when contractionn occurs
- relax, titin expands a bit wihich helps in pushing the sarcomere apart
sarcolemma
- plasma membrane surrounding a muscle cell
transverse tubtules
projections of the memraben that reach deep into the cell
- they are in close contact with the SR
SR
stores calcium
- surrounds sarcomeres
voluntary muscle contraction leads to
- nerve impulses, (electrical changes in PM)
- the nerve impulse comes down, hits the pM, EXTENSION OF T TUBULE,
- opening of calcium channels, release of Ca from Sr
- this helps facilitate muscle contraction
troponin and tropmyosin
coat actin in a way where biding nd inhibiting the myosin binding sites.
how is movement of T and T triggered
by calcium release. opening of binding site, and myosin can take a step
