Lecture 16 - Cytoskeleton & CM: Microfilaments Flashcards
microfilaments are the _______ of the cytoskeletal filaments
smallest
what roles to microfilaments play?
- muscle contraction
- cell migration
- amoeboid movement
- cytoplasmic streaming
def: highly conserved globular protein, can be divided into muscle-specific or non-muscle streams
actin
muscle-specific actins are ___ actins and non-muscle specific actins are ____ and ____ actins
alpha, beta and gamma
what happens to actin once its been synthesized?
folds into globular U-shaped molecule that can bind to ATP and ADP
G-actin polymerize to form __________
microfilaments = F-actin
microfilaments hydrolyze ____ while microtubules hydrolyze ____
ATP, GTP
T or F: G-actin can polymerize reversibly into filaments with a lag phase and elongation phase
true
def: composed of two linear strands go polymerized G-actin, wound into a helix
F-actin
T or F: all the actin monomers in the filament have the same orientation
true
where are microfilaments concentrated?
beneath the plasma membrane
the polarity of microfilaments is reflected in the….
more rapid addition of G-actin at the (+) end than the (-) end
after G-actin monomers assemble onto a microfilament, the ATP bound to them is _____ _______
slowly hydrolyzed, hydrolysis is not required for polymerization
what do the growing ends of the microfilament have on them vs. the rest of the microfilament
growing ends have ATP-actin, rest has ADP-actin
cells that crawl have _______ and _______ at their leading edge
lamellipodia and filopodia
cells that adhere tightly to the underlying substratum have organized bundles called _____ _____
stress fibers
def: organization and behaviour of microfilaments inside a cell are regulated by these
actin-binding proteins
def: binds to ATP-actin monomers and prevents them from polymerizing
thymosin beta-4
def: binds to ADP-G-actin and catalyzes the exchange of ADP for ATP, promoting polymerization
profilin
def: filament-severing protein, that facilitates depolymerization
cofilin
def: bind the ends of a filament, to prevent further loss or addition of subunits, this can stabilize microfilaments
capping proteins
def: binds to (+) ends to prevent addition of subunits there
CapZ
def: bind to (-) ends, preventing loss of subunits there
tropomodulins
def: breaks actin MFs and caps the newly exposed (+) ends, preventing further polymerization
Gelsolin
_______ control the assembly of actin filaments in vivo
formins
def: large multi-domain proteins that directly nucleate polymerization of unbranched actin filaments
formins
the formins remain associated with the fast growing ___ end, enabling rapid insertion of actin subunits while protecting the end from capping proteins
(+)
def: act to join two microfilaments together where they intersect
filamin
def: formed by actin when bundled in tightly organized arrays
focal contacts or focal adhesions
def: bundles microfilaments in microvilli
fimbrin
indirect connections to the membrane requires one or more ________ __________ _________
peripheral membrane proteins
an example of microfilament to membrane linkage is through _______ and _________
spectrin and ankyrin
what is the slowest step in the microfilament formation
nucleation
when activated by WASP, the ______ ________ nucleates new branches on the sides of MF
Arp2/3 complex
what does Arp2/3?
creates a template to which actin monomers can be added
actin can form a ______ _____ network prominent in Lamellipodia
tree-like
which cytoskeletal component is the most stable, the least soluble, and are not polarized?
intermediate filaments
what is an abundant intermediate filament?
keratin
what do intermediate filaments do?
support the cytoskeleton by acting as a scaffold
what is different about intermediate filaments?
they are fibrous rather than globular
def: all have a central rod-shaped alpha helix domain that is conserved in size, secondary structure, and to some extent, in sequence
intermediate filaments assembling to form fibrous subunits
flanking the central helical domain are __-_________ and __-_________ domains that differ greatly among intermediate filaments proteins
N-terminal and C-terminal
IF proteins are primarily fibrous molecules, with a _________ domain on each end
globular
what happens when two fibrous polypeptides spontaneously interact?
- wrap around each other creating a 45 nm rope-like dimer
- the two dimers assemble together in a staggered C- to N-terminal orientation creating an anti-parallel tetramer with no polarity
________ tetramers pack laterally together to for a filament that is one “unit” long
eight
where are units of the IF removed from?
the interior rather than the ends
does IF assembly require energy?
no
microtubules __________ _________ when a cell is compressed
resist bending
microfilaments serve as contractile elements that ____________ __________
generate tension
intermediate filaments are elastic and can ________________ _____________ __________
withstand tensile forces
are intermediate filaments static structures that can be dynamically remodelled?
yes
def: insoluble fibrous network that helps maintain the shape of the nucleus and serves as attachment site for chromatin
nuclear matrix
def: thin dense meshwork of fibres lining the inner surface of the inner nuclear membrane
nuclear lamina
what is the nuclear lamina composed of?
intermediate filaments composed of lamins
kinesin is an_________ MT motor
anterograde
dynein is an _________ MT motor
retrograde
