Cytoskeleton

Question 1

monomers of actin are ___-actin and polymerized actin is __-actin

Answer 1

G-actin = monomer
F-actin = polymerized actin (filaments = F)

Question 2

match for an actin molecule:
barbed and pointed end
minus end and plus end

Answer 2

barbed end = + end
pointed end = - end

actin filaments have polarity

Question 3

rate limiting step of actin polymerization

Answer 3

nucleation

3 actin molecules have to come together to form a nucleus first, then polymerization process can occur (elongation, treadmilling)

Question 4

in actin polymerization, elongation occurs at ___ end

Answer 4

elongation of ATP-actin preferentially at barbed (+) end

at same time, ATP hydrolysis is occurring on (-) end where polymerization speed is slower —> steady state is achieved (ADP-actin is unstable, depolymerizes from minus, pointed end) —> treadmilling

Question 5

actin polymerization treadmilling

Answer 5

filament grows on (+)/barbed end and simultaneously dissociates from (-)/pointed end

length of actin isn’t really changing but it is moving forward

[ remember: (+) end is the leading end —> moving in (+) direction]

Question 6

this small G protein is very important for regulation of actin dynamics

Answer 6

Rho GTPase

defects in Rho GTPase can cause defect in actin polymerization —> associated with intellectual disability

Question 7

how do each of these toxins target the actin cytoskeleton:
a. phalloidin
b. cytochalasin
c. latrunculin

Answer 7

a. phalloidin: binds and stabilizes actin filaments (found in death angel mushrooms)
b. cytochalasin: binds (+) end and blocks polymerization (produced by molds)
c. latrunculin: binds actin monomers and prevents polymerization (produced by sponges)

Question 8

how do Listeria and Rickettsia (rocky mountain fever) bacteria target actin

Answer 8

hijack host actin polymerization machinery for cell to cell spreading

actin “tricked” into polymerizing at tail of bacteria, and bacteria use that force to enter other cells

can spread without detection

Question 9

subunits of microtubules and features

Answer 9

dimer of alpha and beta tubulin, arranged in tubules with 13 protofilaments

thickest cytoskeleton structure

motor proteins: kinesin and dynein

has polarity

function: intracelular transport, cell division, cell motility

binds GDP/GTP

Question 10

what cytoskeleton structure forms mitotic spindles and cilia/flagella?

Answer 10

microtubules: important for mitosis

Mitosis = Microtubules

Question 11

centrosome is the _____ organizing center

Answer 11

microtubule - radiate outwards from centrosome, which stabilizes minus end

Question 12

dynamic instability of microtubules

Answer 12

intrinsic property of microtubules

1. catastrophe: GTP hydrolysis occurring at same time as polymerization, which eventually catches up to tip of plus end, loss of GTP cap
2. rescue: rapid shrinking/depolymerization
3. growing resumes until catastrophe occurs again (cycle)

both growing and shrinking is happening at (+) end, minus end is stabilized by centrosome

Question 13

how do each of these natural toxins target microtubules:
a. paclitaxel
b. colchicine
c. vincristine

Answer 13

a. paclitaxel: binds and stabilizes microtubules
b. colchicine: binds tubulin and blocks polymerization
c. vincristine: binds tubulin and blocks polymerization

Question 14

dysfunction of this cytoskeleton structure contributes to development of Alzheimer’s

Answer 14

microtubules

intracelular neurofibrillary tangles interfere (hyper-phosphorylated tau)

Question 15

what are the three cytoskeleton motor proteins and which direction do they move

Answer 15

myosin (actin associated): moves towards + end

kinesin (tubulin associated): moves towards + end
dynein (tubulin associated): moves towards - end

Question 16

which is associated with long-range transport: actin or microtubules

Answer 16

microtubules: it’s larger (think of highway vs local road)

actin: local transport

Question 17

structure of myosin

Answer 17

actin motor, large gene family

2 heavy chains and 2+ light chains

plus-end directed

globular head or motor domain with ATPase,
tail domain is variable - contains coiled coil for dimerization and/or binds cargo

Question 18

what two things are required for movement of myosin along actin filament

Answer 18

mechanical cycle (changes in confirmation, attachment and unattachment) and chemical cycle (ATP hydrolysis)

Question 19

what cytoskeleton component is important for cytokinesis

Answer 19

actin and myosin II create contractile ring that cleaves the cell

Question 20

congenital heart disease and cardiomyopathy is associated with defective:
a. actin
b. microtubules
c. intermediate filaments

Answer 20

actin - heavily expressed in the heart muscles (for contraction)

Question 21

Griscelli syndrome Type 1 presentation and cause

Answer 21

silver hair, light skin, severe neurological effects

mutation in myosin Va, which is required for distribution of melanosomes to cell periphery

[can also be caused by mutation in melanophilin or Rab27a]

Question 22

pt. is 2yo M that presents with silvery hair, light skin, and severe neurological defects. A mutation is found that affects cytoskeleton function. What is your diagnosis and what is the mutation affecting

Answer 22

Griscelli syndrome Type 1

mutation affecting myosin - melanosomes are not able to be distributed to cell periphery

Question 23

what is the basic structure of kinesin, and how does it “walk”

Answer 23

head domain with ATPase
tail region that binds target or light chains or dimerizes

walking:
1. forward motor binds beta-tubulin, releasing ADP and binding ATP
2. conformational change causes rear head to swing forward (“walking like a drunk”)
3. new head releases ADP to bind ATP, trailing head hydrolyzes ATP
4. cycle continues

Question 24

basic structure of dynein

Answer 24

microtubule motor that is minus-end directed

head with ATPase activity
microtubule-binding domain in the stalk
stem/tail binds light chain or cargo or another microtubule

Question 25

Kartagener Syndrome

Answer 25

aka Primary Ciliary Dsykinesia

caused by mutations in axonemal dynein heavy chain (axonemal dynein important for cilia and flagella)

effect: cilia are immobile, chronic infection of respiratory tract (can’t clear infection), sterile males (sperm are immobile without functional flagella)

Question 26

A patient in your clinic presents with chronic infections of the respiratory tract. They say they have a hard time clearing mucous. Genetic testing shows their cilia are immobile, and the patient (M) is sterile. What is your diagnosis?

Answer 26

Kartagener syndrome, aka Primary Ciliary Dyskinesia

mutation in axonemal dynein heavy chain important for cilia and flagella beating

Question 27

long range axonal transport in neurons depends on _____

Answer 27

microtubules

Question 28

what are the characteristics of intermediate filaments

Answer 28

no polarity, no motors

maintain cell shape and mechanical strength (“tendons” of the cell)

assemble as antiparallel tetramers

Question 29

strongest type of cytoskeletal structure - “tendons” of the cell

Answer 29

intermediate filaments

Question 30

keratins and lamins belong to what class of cytoskeleton

Answer 30

intermediate filaments

Question 31

neurofilaments (type of intermediate filament) travel along ____

Answer 31

axonal microtubules

so problems with axonal microtubule could cause secondary problems with intermediate filaments

Question 32

variable ___ domains of kinesins allow binding to specific [forms of microtubules OR cargos and targets]

Answer 32

variable TAIL domains bind specific CARGOS and TARGETS

motor domain is highly conserved