Lecture 28 Cytoskeleton 1

monomers that make up microtubules and actin filaments bind to and hydrolyze ____

nucleotides

critical concentration Cc

rate of subunit addition is equal to the rate of subunit loss

konC

number of subunits that add to the polymer is proportional to the concentration of the free subunit

koff

number of subunits that leave polymer at a constant rate

the fast growing end of a filament is called ___ and the slow growing end call ___

plus end ; minus end

the different rates of the plus and minus end is due to what

different conformation as it enters the polymer

actin binds and hydrolyzes ___

tubular binds and hydrolyzes __

ATP

GTP

hydrolysis of the bound nucleotide reduces the binding affinity of the subunit for the neighboring subunits, making it more likely ___

to dissociate

T form adds to the filaments and the D form leaves

lag phase corresponds to

time taken for nucleation

growth phase occurs when

subunits add to the exposed ends of the growing filaments, causing elongation

equilibrium phase (steady state) occurs when

the growth of the polymer due to monomer addition is precisely balanced to the shrinkage due to disassembly back to monomers

three major families of cytoskeleton filaments

microtubules

actin

intermediate filaments

which filament is the largest and most rigid

microtubules

• rupture when stretched

• polar

which filament is the smaller, more flexible structure

actin / microfilaments

• polar

which filament is made of rope like fibers and allow cell to distribute tensile forces to prevent rupturing

intermediate filaments

• coil coil tetramer aka strong as balls

• non polar

2 kinds of intermediate filaments

cytoplasmic

• keratin filaments in epithelial cells

Nuclear lamins

• in all animals cells

what is the purpose of nuclear lamina

support and strength nuclear envelope to protect our DNA

what promotes nuclear envelope disassembly during mitosis

phosphorylation of the nuclear lamina (nuclear intermediate filaments)

• can reform due to nuclear import proteins being imported after every cycle of cell division

what is progeria

a disease the results from deformed nuclear lamina where the nuclear envelope does not have enough support which leads to impaired cell division, increase cell death

• result → premature aging

how does mutant keratin lead to epidermolysis bullosa

normal keratin filaments create a structural network that provides resilience to mechanical stress.

• if mutant and pressure is applied to the skin, it can redistribute that force and then skin blisters

what is the microtubule organizing center

centrosome containing structure organizes the microtubules

describe the minus and plus ends of a microtubule

minus end contain alpha subunit

plus ends contains beta subunit

• microtubules grow from minus to plus end

why do microtubules have 13 protofilaments

allows microtubules to be thermally stable and still be dynamic

in microtubules, which tubular subunit never hydrolyzes GTP

alpha subunit

• GTP in beta is hydrolyzed/exchanged to control whether or not we will add or lose subunits

describe how GTP hydrolysis controls dynamic instability

GTP tubulin enters polymer and hydrolyzes GTP to GDP

• when GDP bound it has lower affinity for polymer

Polymer only grows when GTP tubulin is added faster than it is hydrolyzed

• GTP cap forms to protect end (think of the wax thing that protects the end of shoe laces)