cytoskeletal systems Flashcards
what is the cytoskeletal system?
a network of interconnected filaments and tubules extending through cytosol
what does the cytoskeleton play a role in?
plays a role in cell movement and division
what is the cytoskeleton described as?
dynamic and changeable
what are the 3 elements of the cytoskeletal system?
microtubules
microfilaments
intermediate filaments
what are microtubules composed of?
composed of tubulin subunits
what is the diameter if microtubules?
25nm
what are microfilaments composed of?
composed of actin subunits
what is the diameter of microfilaments?
7nm
what are intermediate filaments composed of?
they are variable in composition
what is the diameter of intermediate filaments?
8-12nm
where are polymer networks located?
within the cell
what are the polymer networks composed of?
composed of septics
what is the prokaryotic cytoskeletal systems?
have a polymer system that are structural and functionally similar to eukaryotic cytoskeletal elements
what are the 3 proteins in the prokaryotic cytoskeletal system?
actin like MreB
tubulin-like FtsZ
crescentin
what does actin like MreB function?
involved in DNA segregation and cell shape
what does tubular like FtsZ do?
involved in regulating cell division
what does crescentin do?
regulatory of cell shape
what are microfilaments essential components for?
muscle fibers (cellular mobility and contractabillity)
what are microtubules structural elements?
cilia and flagella (cell mobility)
what can cytoskeletal elements be viewed by?
can be viewed by microscopic techniques
what are used to perturb cytoskeletal function?
chemicals
where are cytoplasmic microtubules?
in the cytoplasm
what are cytoplasmic microtubules used for?
pervade the cytosol and are reasonable for a variety of functions
what are the cytoplasmic microtubules functions?
maintaining axons
formation of mitotic and meiotic spindles
maintaining or altering cell shape
placement and movement of vesicles
where are axonal microtubules found?
foind in motile structures
what are axonal microtubules?
organized and stable microtubules found in structures
what are the structures axonal microtubules found in?
cilia
flagella
basal bodies (cilia and flagella attach to)
what are the protein building blocks of microtubules?
tublin heterodimers
what are the characteristics of microtubules?
straight, hollow cylinders
longitudinal arrays of polymers (protofilaments)
what is the basic subunit of protofilaments?
tubulin heterodimer
what are the 2 types of tubulin heterodimers?
alfa-tubulin and beta-tubulin
how do the 2 types of tubulin bind to each other?
through non covalent bonds
what does the 2 tubules form when bonded together?
form a alfa beta heterodimer
does alfa beta heterodimers normally dissociate?
no
what are the three structures in the tubulin subunits?
- N-terminal GTP-binding domain
- central domain
- C-terminal domain
what is the central terminal domain?
where colchicine can bind
what is the C-terminal domains use?
intervals with MAPs
what are MAPs?
microtubule associated proteins
what do protofilaments inherently have due to dimer orientation?
polarity
what are the two ends of the protofilaments?
plus end and a minus end
what is the reason for protofilaments to have a plus and minus end?
because the two ends differ both chemically and structurally
what can microtubules form as?
singlets, doublets and triplets
how many protofilaments are in a singlet?
13 protofilaments
what makes axonal microtubules?
doublets and/or triplets
what is the structure of doublets and triplets like?
one contains 13 protofilamnet tubule and one or 2 incomplete rings with 10-11 protofilaments
what does reversible tubulin dimer polymerization require?
GTP and Mg2+
what do dimers aggregate into?
oligomers
what do oligomers serve as?
serves as “seeds” from which mew microtubules grow
what is it called when new microtubules grow?
nucleation
what is it called when there is addition of more subunits at either end?
elongation
why is microtubule assembly slow at first?
because the process of nucleation is slow
what is the slow nucleation called in microtubule assembly?
its called the lag phase
what phase is much faster in the assembly of microtubules?
the elongation phase
what happens when the amount of free tubulin diminishes?
the assembly is balanced by disassembly
what is the balancing of disassembly called?
its called the plateau phase
what is it called when the tubulin concentrations is exactly balanced out by disassembly?
called the critical concentration
where does the addition of tubulin dimers occurs more quickly?
more quickly at the plus end of the microtubule
when does treadmilling occur?
when free tubulin conc is about critical concentration for plus end but below for minus end
what is treadmilling?
addition of subunits at the plus end, and removal from the minus end
what happens when conc of tubulin is below on both plus end and minus end?
subunit removal at both ends
what happens when tubulin conc are above critical concentration on plus end but below on minus end?
treadmilling will occur
what happens when the tubulin conc is above critical concentration of both plus and minus ends?
addition at both ends
what contributes to the dynamic instability of microtubules?
GTP hydrolysis
how many GTP binds to a tubulin heterodimer?
2 GTP molecules
one on alfa tubulin and one on beta tubulin
what happens to the GTP that is bound to the beta tubulin when added to the microtubule?
it is hydrolyzed to GDP
what is GTP needed for in the assembly of microtubules
help promote heterodimer interactions and addition
doesn’t require hydrolysis is not required for assembly
what is the dynamic instability model?
grows by polymerization at plus end whereas another population shrinks by depolymerization
what end of the microtubule has GTP?
the plus end
what does the minus end microtubule have?
has GDP
what prevents subunit removal at the plus end?
GTPase
what happen if the GTP-tubulin is high?
added to MT quickly creating a large GTP tubulin cap
what happens if the GTP tubulin conc falls?
rate of tubulin addition decreases
what happens when GTP is sufficiently low?
rate of GTP hydrolysis exceeds rate of subunits addition and cap shrinks
what happens when the GTP cap disappears?
MT becomes unstable and loss of GDP bound subunits is favoured
what is the periods of growth and shrinkage?
catastrophy
what is it called when there is a sudden switch back to the growth phase in MT?
microtubule rescue
where does microtubules originate?
microtubule-organizing centre
how do centrosomes orientate?
they orientate at 90 degrees of eachother
in animal cells, what are centrosomes associated with?
centrioles
what are centrioles surrounded by?
pericentriolar material
what are centriole walls formed by?
nine pairs of triplet microtubules
what do the large shape protein complexes contain in centrosomes?
contains γ-tubulin
where is γ-tubulin only found in?
only in centrosomes
what do γ-tubulin ring complexes (γ-TuRCs) do?
they nucleate the assembly of new Mts away from the centrosome
what does loss of γ-TuRCs prevent?
prevents the cell from nucleating MTS
what does MTOCS do?
nucleate and anchor MTs
how do MT grow from MTOC?
grow outward with a fixed polarity
what end of the MT is anchored to the MTCO (centrosome)?
the minus end
where do the dynamic MT growth and shrinkage happen at?
at the plus end near the cell periphery
what microtubule binding proteins help with stability?
- MAPs, microtubule associated proteins
- +-TIP proteins
- microtubule destabilizing / severing proteins
what are MAPs for?
bind at regular intervals
tend to function as microtubule-stabilizing / bundling proteins
what are +-TIP proteins for?
stabilizing Mts by capturing and protecting the growing plus end
what are microtubule-destabilizing/severing proteins do?
they promote depolarization of MTs
are microfilaments the smallest cytoskeletal element?
yes
what are the functions of cytoskeletal filaments (microfilamnts)?
- muscle contraction
- cell migration, amoeboid movement
- development and maintenance of cell shape
- structural core of microvilli
what is the building block for microfilaments?
actin
what is G-actin?
globular actin
molecules that can bind ATP to ADP
what is F-actin?
actual microfilaments
(G-actin polymers)
what are the 2 types of actin?
- muscle specific (alfa actin)
- non muscle actin (β- and γ-actins)h
how are G-actin monomers polymerize?
polymerize reversibly into filaments
what phases are in polymerize?
lag phase
elongation phase
what are f-actin filaments composed of?
2 linear strands of polymerized G-actin wound into a helix
how do myosin subfragment 1(S1) monomers bind?
in arrowhead pattern
what are the ends of the arrowhead?
barbed end is the plus end
minus end is the pointed end
what do polarity of MFs reflect?
more rapid addition and loss of G-actin at plus end than minus end
what is the role of ATP in MFs?
once G-actin monomer assemble into MFs, ATP bounds them
what parts of the MF has ATP actin and what pasts of it has ADP actin?
ends have ATP actin
middle composed of ADP actin
what are stress fibres?
organized bundles allowing cells to adhere tightly to the underlying substratum
where are the gel of cross linked actin MF?
in the cell cortex just beneath the plasma membrane
where are lamellipodia and filopodia found?
in cell’s leading edge in cells that crawl
what do proteins that regulate actin monomers and their polymerization do?
if conc of ATP bound G-actin is high, will assemble until G-actin is limiting
why are large amounts of G-actin not available?
due to being bound by thymosin β4
what is profilin do?
binds to G-actin and will enable the construction of MFs
are thymosin β4 and profilin competitors for G-actin?
yes
what are the actin binding proteins that regulate the formation of MFs?
thymosin β4
profilin
what are rheumatoid arthritis proteins that regulate actin polymerization?
ADP/cofilin
formins
what are ADP/confilin?
known to bind ADP-G-actin and F-actin and is thought to increase turnover of ADP actin at the minus end of MFs (destruction of MFs)
what are formins?
associate with the fast growing end of actin filaments (plus end)
what are most formins?
are Rho-GTPase effector proteins
what are the actin binding proteins that regulate length?
proteins that serve actin filaments
proteins that cap actin filaments
what is the protein the severs actin filaments?
gelsolin
what does gelsolin do?
breaks actin MFs and caps, preventing further polymerization
what are the proteins that cap actin filaments?
cap Z
tropomodulins
what is cap Z?
binds to plus end to prevent addition of subunits there
what is tropomodulins?
bind to minus ends, preventing loss of subunits there
what are the actin binding proteins that regulate organization?
proteins that crosslink actin filaments
proteins that bundle actin filaments
what is the protein that crosslink actin filaments?
filamin
what is filamin?
acts to splice, joining 2 MFs together where they intersect
what is the protein that bundle actin filaments?
alpha Actinin
what is alpha Actinin?
protein that is prominent in such structures
what are proteins that crosslink actin filaments for?
actin networks often from as loose networks of cross linked filaments
what are proteins that bundle actin filaments?
focal contracts or focal adhesions
what does microvilli consist of?
tight bundle of actin microfilaments with ends points toward the tip
how are MFs connected to the plasma membrane?
by crosslinks made of myosin I and calmodulin
how are MFs in bundles are tightly bound together by?
crosslinking proteins fimbrin and villin
what are fimbrin and villin do?
bounds the MF in place
what is the network of filaments at the base of microvillus
called is terminal web
what is the filament of the terminal web mainly composed of?
myosin II and spectrin
where do the myosin II and spectrin connects to?
connect the MFs to each other
what can proteins link actin to?
membranes
what do MFs do to the cell membrane?
exert force on it during cell movement or cytokinesis
what are some proteins exert force on the PM during cell movements?
ankyrin and spectrin
what kind of network can actin form?
dendritic (treelike) network
what are the Aro2/3 complex do?
nucleates new branches on the side of filaments
how are Arp2/3 branching activated by?
family of proteins that include WASP and WAVE/Scar
what can form instead of branched networks?
long actin filaments
what is actin polymerization is regulated independently of the Arp2/3 complex through?
through formins
how do formins promote polymerization?
they move along the end of the growing filaments
what regulate where and when actin based structures assemble?
phospholipids
what do phosphatidylinositol-4,5-bisphosphate (PIP2)
can bind to various proteins to the membrane and regulates their interactions with actin
what does RHO family GTPase regulate
regulates where an when actin based structures assemble
what are the tree monomeric G proteins called Rhode Island GTPase?
- Rhode Island activation
- Cdc42 activation
what is Rhode Island activation?
results in formation of stress fibres
what is Rac activation?
results in extension of lamllipodia
what is Cdc42 activation?
results in the formation of filopodia
what are Pho GTPase stimulated by?
guanine-nucleotide exchange factors (GEFs) through the exchange of bound GDP for GTP
what are GTPase activating proteins (GAPs) inactivate?
it inactivated Rhode Island GTPases by causing them to hydrolyze their bound GTPs to GDP
what does guanine nucleotide dissociation inhibitors (GDIs) inactivate?
Rho GTPase in the cytosol
where are intermediate filaments abundant in?
animal cells
what is the most abundant microfilament?
keratin
why is keratin important?
component of structures that grow from skin in animals
what is the most stable and least soluble component of the cytoskeleton?
intermediate filaments
what do intermediate filaments likely do for the entire cytoskeleton?
likely support it
what are intermediate filaments assembled from?
fibrous subunits
what is the fundamental subunits of intermediate proteins?
dimers
are intermediate filaments fibrous or globular?
fibrous
what flanks the central helical domain of intermediate filaments?
N and C-terminal domains that differ greatly among intermediate filament proteins
what is the basic structural units consist of in intermediate filaments?
2 intermediate filaments intertwined into coiled-coil aligned in parallel
what do two intermediate filament dimers form?
tetrameric
protofilament
what do protofilaments overlap to build?
build up filamentous structure about 8 protofilaments thick
what are intermediate filaments thought to play a role in?
tension-bearing role
what are intermediate filaments less susceptible to than microtubules and microfilaments?
less susceptible to chemical attack
what are spectraplakins?
linker proteins that connect intermediate filaments, microfilaments and microtubules
what is one spectraplakins called that is found at sites where intermediate filaments connect to microfilaments and microtubules
plectin
