lec 11- the cytoskeleton cont. Flashcards
what do intermediate filaments (IF) do?
provide support and mechanical strength
what are IF made of?
70 different genes
how many types of IF are there?
5 based off of the cells where they are found in
how do IF’s interconnect to other cytoskeletal filaments?
via cross bridges
what are the cross bridges called?
plectin
what connects at the ends of plectin?
on one end there is a binding site for intermediate filament, and a binding site for any other cytoskeletal filament
how does labelled keratin act when in a cell with IF’s?
it gets scattered in the IF
what is keratin?
an IF that provides strength for epithelial cells, most diverse IF
what does actin filament do?
provides motility, shape, structural support, and does muscle contractions
what is the most abundant types of actin in the cell?
G-actin = globular
F-actin = filamentous
With ATP, they form into a helical filament
what do the minus and plus end of the actin filament do?
minus end: the end with an exposed binding cleft which binds to the plus end in a filament
plus end: the other end where the minus end of a G-actin binds too
what regulates polymerization and depolymerization of actin filament?
actin binding protein
what is myosin?
an actin based motor protein that has a “head or motor domain
what are the groups of myosin?
conventional and unconventional
what type of myosins are the best studied?
type II (conventional) myosins
what is an example of an unconventional myosin?
a myosin that transports vesicles and organelles
how many polypeptide chains does a myosin II molecule have?
one pair of heavy chains and two pairs of light chains
what does myosin II consist of?
- a pair of globular heads that contain the catalytic site of the molecule
- a pair of necks, each consisting of a single, uninterrupted alpha-helix and two associated light chains
- a single, long, rod-shaped tail formed by the intertwining of long a-helical sections of the two heavy chains
forms into a fiber with heads pointing out and tails pointing in
what is a muscle?
bundles of parallel muscle fibres joined by tendons to the bones that the muscle must move
what are muscle fibres?
each fiber is a multinucleate cell formed during embryogenesis and specialized for contraction
what is myofibril?
thinner cylindrical strands that make up muscle fiber
what is a sarcomere?
the contractile unit of myofibrils each which has a very specific organization
what is a sarcomere made of?
thin filaments (actin)
thick filaments (myosin)
and Z lines at the ends
what side of the actin is anchored to the Z lines?
the plus end
what does CapZ do?
caps actin to the Z end
what is nebulin?
repeating actin binding motifs that binds actin filament to Z line
what is myomesin?
bundles the myosin filaments
what is titin?
extends through the myosin filament and attaches to the Z line-helps to prevent tearing of muscle
do multiple myosins work at the same time?
no, individually
explain the contractile cycle of actin and myosin:
- ATP binds to myosin head, causing release of myosin from actin
- ATP is hydrolyzed to ADP and Pi, causing nech to straighten and head to pivot which causes a weak binding to a new actin, myosin head is cocked and ready for power stroke
- Pi releasing causes neck to go back to original conformation, tighter binding to actin, and creating a power stroke which moves thin filament
- ADP is released and new ATP can now bind to repeat cycle
what does calcium, tropomyosin and troponin do?
-tropomyosin blocks myosin binding sites on actin
-troponin forms a complex with tropomyosin
-motor neuron excitation signal, signal transduction pathway leads to calcium release from the SR, calcium binds to troponin, the troponin-tropomyosin complex moves opening binding sites, when signal stops calcium goes back to SR and muscle relaxes
what does cofilin do?
binds ADP actin and severs filaments promoting depolymerization at minus end
what does profilin do?
functions as an adenine nucleotide exchange factor, binds ADP actin at plus end changing conformation and allowing binding of ATP, binding results in dissociation of profilin, ATP actin then either joins growing filament at plus end or is bound by thymosin
what does thymosin do?
sequesters G-actin preventing polymerization, displacement of thymosin allows binding of G-actin to the plus end
are both ends of F-actin capped?
yes, it stabilizes the sarcomere by preventing plus end further growth or minus end loss of subunits
can actin be linked to other actin filaments and the cell membrane?
yes
what is the network of actin filaments and accessory proteins that underlies the plasma membrane called?
cell cortex
what promotes actin branching and growth?
Arp2/3 complex: nucleates new branches off the side of existing filaments
what activates Arps?
WASPs
what are the steps in cell motility?
1 and 2. extracellular signal recognized and signal transduction cascade initiated
3 and 4. WASP activates Arp complex and Arp nucleates new actin filaments
5. The result is branch formation at plus end
6. membrane is pushed forward
7. caps terminate elongation
8. oldest part of filament at the minus end
9. F-actin severed and depolymerized at minus end
10. profilin exchanges GDP to GTP
what does GDI do?
prevents Rho-GTPase from putting its tail into the membrane and interacting with GEF
what does GEF do?
turns the GDP on the Rho into GTP turning it “on”
how does Rho with GTP turn back into GDP?
GAP removes the phosphate on GTP, making it into GDP “off”
what does Rho-GTPase do?
regulates actin nucleators
What is tropomodulin?
Caps negative end and regulates the length of actin filament
