Cytoskeleton

Question 1

cytoskeleton

Answer 1

provides cell motility, cell division, cell shape, cell strength, organelle distribution, cytokinesis, intracellular transport, made up of intermediate filaments, microtubules and actin filaments

Question 2

intermediate filaments

Answer 2

rope-like, give cells mechanical strength, alpha-helical structures form coiled-coil dimer which form staggered (anti-parallel) tetramer of coiled-coil dimers (each end is the same)

Question 3

microtubule

Answer 3

hollow stiff tubes of protein that create tracks within cell, also form mitotic spindle, has polarity (plus and minus ends of filament indicating dynamism) and enzymatic activity of subunits (tubulin, a GTPase that hydrolyzes the release of 3rd phosphate yielding GDP) and as a result is a dynamic structure

Question 4

actin filaments

Answer 4

thin, flexible, helical filaments important for cell motility and other functions, is also polar and actin is an ATPase that cleaves third phosphate off of ATP yielding ADP, dynamic

Question 5

microtubule structure

Answer 5

made up of dimers of alpha and beta tubulin which stack to form protofilaments which form hollow tube, largest of cytoskeletal filaments, undergo dynamic instability

Question 6

centrosome

Answer 6

microtubule organizing center, minus ends of filaments oriented towards centrosome and plus ends towards periphery, minus ends are typically static bc they are anchored in centrosome

Question 7

dynamic instability

Answer 7

where microtubules at their plus ends can switch rapidly between growth and disassembly (polymerization and depolymerization) when subunits are added to plus end of microtubule they are still bound to GTP, however soon after assembly, tubulin will carry out its enzymatic rxn (GTP to GDP) which induces conformational change in tubulin which makes it less favorable to stay in microtubule (but it does bc of GTP cap of newly added tubulin)

Question 8

microtubule dynamic instability is a race between:

Answer 8

1. rate of incorporation of free GTP-tubulin into filaments vs 2. rate of hydrolysis of GTP to GDP within filament; when rate of 1. proceeds faster than 2. there is growth of MT, if hydrolysis catches and there is no GTP cap, shrinking (disassembly and release) of MT occurs

Question 9

actin filament structure

Answer 9

made up of actin protein, assembly and disassembly can happen at both ends but favorability of assembly vs disassembly is different at each end and depends on free actin concentration, low [actin]- disassembly at plus and minus ends, high [actin]- assembly at both plus and minus ends, special intermediate [actin]- assembly at plus end and disassembly at minus end (“treadmilling”)

Question 10

treadmilling

Answer 10

actin monomers bound to ATP will assemble onto plus end of filament, at some point actin added will hydrolyze ATP to form ADP, ADP-bound action is not favorable to stay in filament so if it finds itself at minus end it will release; addition on one end and subtraction on other allows actin cytoskeleton to move

Question 11

molecular mechanism of dynamic instability

Answer 11

microtubules undergo rapid remodeling (allows cell to change structure and react to environment), driven by changes in tubulin dimer bc it exists in 2 conformational states: GTP-tubulin dimer is linear which increases stability and GDP-tubulin dimer is slightly out of alignment which decreases stability bc it fits less snugly into filaments, hydrolysis is driven by enzymatic activity of tubulin itself, polymerization causes tubulin to greatly increase its GTPase activity

Question 12

how does hydrolysis of GTP affect growth of microtubules?

Answer 12

it slows growth bc depolymerization would not be possible if GTP could not undergo hydrolysis and if depolymerization can’t happen filaments would not have access to disassembled units of actin

Question 13

how does the GTP cap stabilize the microtubules?

Answer 13

cap has straight protofilaments that constrain curvature of GDP portion below, when tubulin at cap end has hydrolized the constraint is released and curved protofilaments spring apart- peel off rapidly

Question 14

If MTs undergo dynamic instability then how do they form a stable “skeleton” in the cell?

Answer 14

selective stabilization via attachment to capping proteins at MT plus end (prevents filament assembly or disassembly at bound end), search and capture process of random exploration and then selective stabilization which allows cell to create highly organized system of MT tracks used to organize and/or move other components, location and amount of capping proteins is highly regulated by the cell

Question 15

filament stabilizing proteins

Answer 15

bind along length of filaments to promote stability

Question 16

filament severing proteins

Answer 16

create new filament ends for assembly or disassembly

Question 17

monomer-sequestering proteins

Answer 17

regulate availability of free subunits and therefore rate of addition of subunits to filament ends

Question 18

nucleation factors

Answer 18

enhance rate of new filament creation

Question 19

motor proteins

Answer 19

dynein (minus end directed) and kinesin (plus end directed) for microturbules, myosin family (- or +) for actin filaments, dimers with 2 globular heads and single tail, tails bind stably to cargo (vesicle or organelle), heads are enzymes with ATPase activity that drives conformational changes in head and neck region, movement driven by cycle of binding, release, rebinding to filament

Question 20

molecular mechanism of kinesin along microtubules

Answer 20

head binds to microtubule, binding induces conformational change releasing ADP from attached head, free ATP fills empty nucleotide binding site, this exchange induces conformational change triggering neck linker to zipper onto head, throws other head forward, this brings second head near next binding site and binds tightly, first head hydrolyzes its ATP which induces a conformational change causing head to lose grip on microtubule