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Ch
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functions requiring
movement
⁃ referred to as cytoskeleton
⁃ Cytoskeleton is relatively simple in its construction:
⁃ microtubules: made of dimers (pairs of proteins) of alpha & beta tubulin
⁃ microfilaments: made up of polymers of protein actin
⁃ intermediate filaments: made up of dozens of diff
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proteins)
⁃ Functions of cytoskeleton: just a few
⁃ Gives the cell shape & mechanical resistance to deformation
⁃ association w/ extracellular connective tissue & other cells it stabilizes whole
tissues
⁃ Microtubules:
⁃ have a polarity (have minus & plus end)
⁃ originate from microtubule organizing center (MTOC) & grow outward
⁃ centrosome: primary MTOC (major organizing center)
⁃ 13 tubulin in each dimer makes its structure
⁃ have a natural dynamic instability (molecules are constantly growing & shrinking)
⁃ they’re always growing & shrinking which gives it a dynamic instability
⁃ actively expanding & contracting allowing cell to move
⁃ Dynamic Instability: in part allows microtubules to “pull” things apart directly
⁃ “search & capture” model target of discovery
⁃ tubulin dimers can bind 2 molecules of GTP
⁃ GTP bound alpha tubulin is stable but GTP bound beta tubulin isn’t stable
b/c GTP is hydrolyzed
⁃ catalytic activity w/ beta tubulin will cause it to cleave a phosphate group
& make GTP become GDP
⁃ GDP-bound tubulin is less stable than GTP thus it’s more likely to fall off
⁃ GTP bound tubulin stabilizes growing microtubule, but if hydrolysis catches up
rapid depolymerization begins (falling apart)
⁃ catastrophe: switch from growing to shrinking tubules
⁃ rescue: when GTP bound tubulin binds again & stabilizes microtubule
⁃ microtubules grow & shrink so they won’t take up too much space in cell
⁃ it will grow & shrink until it hits something it wants to transport
⁃ it is a way of scanning the cell for things that need to be transported
⁃ motor proteins associate w/ microtubules to move cargo around cell

⁃ Dynesin: moves cargo from plus to minus end of microtubules
⁃ Kinesin: moves cargo from minus end to plus end
has a head, stalk, & tail
has 2 identical motor heads
head has microtubule binding domain & ATP binding area
ADP molecule in the motor head is swapped w/ ATP
ATP is then hydrolyzed which pulls the microtubule structure along
binding of ATP changes it shape which thrust the other motor head forward
when ATP is hydrolyzed it changes interaction b/t kinesin head & microtubule (which is
why the head lets go of microtubule)
⁃ Microfilaments: the smallest of the cytoskeletal elements
⁃ exist in cytoskeleton as a double helix of filaments made of actin
polymers
⁃ monomer forms of actin are referred to as G-actin & the polymerized
form that makes up filaments is referred to as F-actin
there is an ATP binding domain in actin
⁃ actin is one of the most abundant proteins in eukaryotic cells
⁃ also has a plus & minus end (impacts the directionality of things being
move by actin)
⁃ actin is found beneath the cell cortex to support specialized cell shapes
helps maintain structural stability of cell
⁃ Cell Motility via Actin Polymerization:
⁃ a lot of cell motility is caused by actin polymerization
⁃ cell is attached to integral membrane protein
started with G actin filaments & through polymerization other filaments are added
this network of actin is polymerized & stretched forward to attach to another integral
membrane protein
in order for cell to move forward the attachment point at the back of the actin must be
depolymerize to detach
⁃ Myosin as a Motor Protein:
⁃ head domain binds the F-acatin & uses ATP hydrolysis to generate force
& to walk along the filament towards the plus end
⁃ neck domain acts as a linker & as a lever arm for transducing force
generated by the catalytic motor domain
⁃ the tail domain generally mediates interaction w/ cargo molecules &
other myosin subunits
ATP binds a myosin head (this isn’t bound to actin)
ATP is hydrolyzed to ADP + Phos
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bound myosin binds to actin
binding to actin dislodges the Phosphate group from myosin
this changes confirmation of the myosin neck causing a force that propels the myosin
forward
ATP then binds the myosin head which causes it to detach from the actin
repeat- hydrolysis of ATP





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