Lecture 26 - Cytoskeleton Part III: Intermediate Filaments & Microfilaments

Question 1

what cytoskeleton fiber has the strongest tensile strength?

Answer 1

intermediate filament

Question 2

where are intermediate fibers the most abundant?

Answer 2

cells that endure physical stresses

muscle cells

neurons

epithelial cells

Question 3

how do intermediate fibers connect to other components of the cytoskeleton?

Answer 3

with cross-linking PROs such as plectin

Question 4

how many diff classes of intermediate filaments are there?

Answer 4

6

Question 5

how many human genes code for intermediate filaments?

Answer 5

60

Question 6

what is the biggest diff b/w intermediate filaments, MTs & microfilaments?

Answer 6

intermediate filaments have many diff types whereas, MTs & microfilaments don’t

Question 7

describe the IF structure

Answer 7

IF have a monomer composed of an alpha helical domain with a globular domain at each end (one at the amino end & one at the carboxyl end)

2 monomers come to together where their subunits are parallel to each other

Dimers come together & are aligned in opp orientations

Tetramers aggregate together to form IFs

Question 8

how do IFs differ in structure from other fibers?

Answer 8

no polarity (not + & - ends)

new fibers can be inserted at the ends as well as in the middle

Question 9

what is the function of an IF?

Answer 9

Main function: mechanical support or strength

Hair, fingernails & skin are composed of keratin IFs

Nuclear lamina is composed of IFs, line the inside of the nuclear envelope
Important for the integrity of the nucleus
Progeria has an irregular shaped nucleus b/w of the defect in nuclear lamina PROs

Neurons composed of neurofilaments

Cancer diagnostics – keratin have
subtypes that are unique to diff epithelial cells or subtypes for diff cell types
Antibodies bind to keratin
Determines origin of tumour

Question 10

what are the 2 types of junctions that IFs form?

Answer 10

desmosomes - keratin IFs form junctions that hold cells together

hemidesmosomes - attach cells to the matrix

Question 11

what cytoskeleton fiber is the thinnest of all fibers?

Answer 11

microfilaments

Question 12

what are microfilaments composed of?

Answer 12

actin

Question 13

MT are composed of _____

Microfilaments are composed of _____

IFs are composed of _____

Answer 13

tubulin

actin

diversity of PROs

Question 14

what is required for an actin subunit to become apart of a growing filament?

Answer 14

ATP

Question 15

do microfilaments have a polarity?

Answer 15

yes b/c the actin monomer has a polarity

Question 16

when is ATP on a microfilament hydrolyzed?

Answer 16

once actin is part of the microfilament

Question 17

what are the functions of actin filaments?

Answer 17

form a band just beneath the PM to provide mechanical strength

Links transmembrane PROs to cytoplasmic PROs, repositions receptors on PM

Anchors centrosomes to poles

divides the cell during cytokinesis

Question 18

what cytoskeleton fiber is responsible for cytoplasmic streaming?

Answer 18

microfilaments

Question 19

what cytoskeleton fiber generates locomotion in white blood cells & the amoeba?

Answer 19

microfilaments

Question 20

what cytoskeleton fiber is reponsible for the force generate in muscle contractions?

Answer 20

microfilaments (interacts with myosin filaments)

Question 21

myosin

Answer 21

motor PROs that move along actin filaments, while hydrolyzing ATP

Question 22

what type of myosin is found in skeletal muscles?

Answer 22

myosin II

Question 23

describe how microfilaments assemble

Answer 23

actin subunits are added to growing filaments, actin subunits require a bound ATP in order to polymerize with the chain. Once polymerized the ATP is hydrolyzed

Added to ends of the growing chain

Question 24

describe myosin II structure

Answer 24

2 heavy chains

Each chain has a globular motor domain that includes 2 domains for:
ATP binding site
Interaction with actin

Question 25

what is the role of myosin V?

Answer 25

intracellular transport

Question 26

myosin V structure

Answer 26

2 heavy chains Longer neck region than myosin II - enables long strides Contain cargo binding domains

Question 27

rigor

Answer 27

ADP dissociation leaves the myosin head tightly bound to actin, in the absence of ATP this state results in muscle rigidity

Question 28

when does rigor occur?

Answer 28

when ATP & ADP have not been produced enough

Question 29

when does ATP hydrolysis occur in myosin?

Answer 29

everytime a myosin head moves

Question 30

what does ATP hydrolysis in myosin cause?

Answer 30

movement of the neck region, to get the heads to walk ATP helps secure a spot on the actin & its hydrolysis causes the head to move

Question 31

Myosin V attaches to every ____ actin residue & only walks along ____ of the filament

Answer 31

13th one side

Question 32

what facilitates myosin V to take long strides?

Answer 32

the long neck region

Question 33

when do the microfilaments form extensions?

Answer 33

when ingesting a cell during phagocytosis cell movement

Question 34

lamellipodia

Answer 34

growing actin filaments facilitating cell movement

Question 35

what cells use lamellipodia?

Answer 35

cells capable of movement without cilia or flagella

Question 36

what is myosin's role in lamellipodia? & how does it do this?

Answer 36

pull rest of the cella long during cell movement by attaching to other cell components

Question 37

where is myosin located in a lamellipodia?

Answer 37

in a band behind the actin

Question 38

how do lamellipodia form?

Answer 38

the initiation of actin polymerization by the addition of actin monomers at the PM

Question 39

what controls the polymerization of lamellipodia?

Answer 39

actin binding PRO complexes depolymerization occurs at the base of the lamellipodium

Question 40

how is the actin network size effected by lamellipodia?

Answer 40

remains the same size

Question 41

Dictyostelium

Answer 41

soil living amoeba

Question 42

What occurs when Dictyostelium have a limited food reserve?

Answer 42

will aggregate to form a multicellular assembly called a pseudoplasmodium or slug they move together on mass & go to the top of the soil where they create a stalk they form a FB to produce spores where new spores are carried off to find new food reserves & the cells in the stalk die

Question 43

how does Dictyostelium seek out each other?

Answer 43

using chemoreceptors one being cAMP receptor

Question 44

what molecule is required for the polymerization of the actin network?

Answer 44

Ca2+

Question 45

what are the steps in cAMP-mediated Dictyostelium movement?

Answer 45

cAMP reception at the PM activates GPRO G PRO stimulates adenylyl cyclase cAMP diffuses out of the cell into the medium Internal cAMP inactivates the external receptor (to prevent it from following its own path) A diff G PRO stimulates phospholipase C (producing IP3) IP3 induces Ca2+ ion release Ca2+ ions act on cytoskeleton to induce the extension of the pseudopodia

Question 46

what creates the oscillatory behavior in Dictyostelium movement?

Answer 46

internal cAMP concentration inactivates the receptor for external cAMP cell fluctuates being able to detect & not detect cAMP

Question 47

what causes the Dictyostelium actin network to grow?

Answer 47

2nd messengers or redistribution of existing messengers leads to local rearrangements of the cytoskeleton

Question 48

where does the inhibitory signal occur in Dictyostelium?

Answer 48

at all sites except the one stimulated with cAMP first