Lecture 26 - Cytoskeleton Part III: Intermediate Filaments & Microfilaments Flashcards
what cytoskeleton fiber has the strongest tensile strength?
intermediate filament
where are intermediate fibers the most abundant?
cells that endure physical stresses
muscle cells
neurons
epithelial cells
how do intermediate fibers connect to other components of the cytoskeleton?
with cross-linking PROs such as plectin
how many diff classes of intermediate filaments are there?
6
how many human genes code for intermediate filaments?
60
what is the biggest diff b/w intermediate filaments, MTs & microfilaments?
intermediate filaments have many diff types whereas, MTs & microfilaments don’t
describe the IF structure
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
how do IFs differ in structure from other fibers?
no polarity (not + & - ends)
new fibers can be inserted at the ends as well as in the middle
what is the function of an IF?
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
what are the 2 types of junctions that IFs form?
desmosomes - keratin IFs form junctions that hold cells together
hemidesmosomes - attach cells to the matrix
what cytoskeleton fiber is the thinnest of all fibers?
microfilaments
what are microfilaments composed of?
actin
MT are composed of _____
Microfilaments are composed of _____
IFs are composed of _____
tubulin
actin
diversity of PROs
what is required for an actin subunit to become apart of a growing filament?
ATP
do microfilaments have a polarity?
yes b/c the actin monomer has a polarity
when is ATP on a microfilament hydrolyzed?
once actin is part of the microfilament
what are the functions of actin filaments?
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
what cytoskeleton fiber is responsible for cytoplasmic streaming?
microfilaments
what cytoskeleton fiber generates locomotion in white blood cells & the amoeba?
microfilaments
what cytoskeleton fiber is reponsible for the force generate in muscle contractions?
microfilaments (interacts with myosin filaments)
myosin
motor PROs that move along actin filaments, while hydrolyzing ATP
what type of myosin is found in skeletal muscles?
myosin II
describe how microfilaments assemble
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
describe myosin II structure
2 heavy chains
Each chain has a globular motor domain that includes 2 domains for:
ATP binding site
Interaction with actin
what is the role of myosin V?
intracellular transport
myosin V structure
2 heavy chains
Longer neck region than myosin II - enables long strides
Contain cargo binding domains
rigor
ADP dissociation leaves the myosin head tightly bound to actin, in the absence of ATP this state results in muscle rigidity
when does rigor occur?
when ATP & ADP have not been produced enough
when does ATP hydrolysis occur in myosin?
everytime a myosin head moves
what does ATP hydrolysis in myosin cause?
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
Myosin V attaches to every ____ actin residue & only walks along ____ of the filament
13th
one side
what facilitates myosin V to take long strides?
the long neck region
when do the microfilaments form extensions?
when ingesting a cell during phagocytosis
cell movement
lamellipodia
growing actin filaments facilitating cell movement
what cells use lamellipodia?
cells capable of movement without cilia or flagella
what is myosin’s role in lamellipodia? & how does it do this?
pull rest of the cella long during cell movement
by attaching to other cell components
where is myosin located in a lamellipodia?
in a band behind the actin
how do lamellipodia form?
the initiation of actin polymerization by the addition of actin monomers at the PM
what controls the polymerization of lamellipodia?
actin binding PRO complexes
depolymerization occurs at the base of the lamellipodium
how is the actin network size effected by lamellipodia?
remains the same size
Dictyostelium
soil living amoeba
What occurs when Dictyostelium have a limited food reserve?
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
how does Dictyostelium seek out each other?
using chemoreceptors
one being cAMP receptor
what molecule is required for the polymerization of the actin network?
Ca2+
what are the steps in cAMP-mediated Dictyostelium movement?
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
what creates the oscillatory behavior in Dictyostelium movement?
internal cAMP concentration inactivates the receptor for external cAMP
cell fluctuates being able to detect & not detect cAMP
what causes the Dictyostelium actin network to grow?
2nd messengers or redistribution of existing messengers
leads to local rearrangements of the cytoskeleton
where does the inhibitory signal occur in Dictyostelium?
at all sites except the one stimulated with cAMP first
