Chapter 17 Flashcards
Three types of cytoskeleton filaments, widest to thinnest
Microtubules, Intermediate filaments, and Actin
Intermediate filaments function and properties
withstands mechanical stress of stretching
high tensile strength (deform but don’t break)
distribute strength among tissue cells through desmosomes
form nuclear lamina/extend through cytoplasm
Intermediate filament structure
monomers contain central rod domain and unstructured regions at both ends
rod domains (a-helical) join to form coiled coil dimer
staggered antiparallel tetramer of two dimers
lateral association of 8 tetramers add together to form filament
NO STRUCTURAL POLARITY
unstructured regions vary and are exposed on the outside of filament
cytoplasmic intermediate filaments
keratin (epithelial cells) - most diverse
vimentin/vimentin-related (connective tissue, muscle, glial cells)
neurofilaments (nerve cells)
nuclear intermediate filament
nuclear lamins form nuclear lamina
NOT rope-like, mesh-like
mutations in keratin cause what
skin more prone to blistering, even with gentle impact- epidermolysis simplex
mutations in neurofilaments cause . . .
ALS; Amyotrophic lateral sclerosis
nuclear lamina
meshwork of IF beneath nuclear envelope
attachment sites for chromatin
comprised of lamin proteins
help position chromosomes
defects in nuclear lamina cause . . .
progeria; rare class of premature aging disorders due to nuclear instability, leading to defects in cell division and chromosomal positioning
how is the nuclear lamina regulated by phosphorylation for disassembly/reassembly for cell division
phosphorylation by kinases weakens interactions between tetramers
dephosphorylation by phosphatases strengthens and rebuilds
function of nuclear lamina in connecting nucleus to cytosol and examples
Accessory proteins in membrane crosslink IFs to other cytoskeletal components outside the nucleus
Plectins: cytosolic bundling of IFs, connects nuclear lamina to cytosolic components
SUN and KASH: transmembrane proteins, link nucleus to cytoplasm, nuclear positioning
microtubules monomers
tubulin dimers made of alpha and beta tubulin, held together by noncovalent bonds
function of centrosomes
microtubule organizing center (MTOC) from which microtubules grow and extend out to the rest of the cell
main functions of microtubules
guide transport of vesicles, organelles, and other cell components
form mitotic spindle during cell division
found in flagella and cilia
which tubulin subunit is plus end/minus end
alpha = minus end
beta = plus end
tubulin monomer assembly into filaments
linear protofilament of dimers
13 protofilaments form microtubule all oriented in same direction (structural polarity)
tubulins easily add to which end of the MT filament
plus end
centrosomes structure
two centrioles surrounded by protein matrix
gamma tubulin location and function
rings found in centrosomes that serve as nucleation sites foe MT filaments
dimers add to gamma ring
which end of the MT filament is embedded in the centrosome and which ends extends into cytoplasm
minus ends embedded in centrosome
growth occurs at plus ends in cytoplasm
dynamic instability
each MT filament is constantly growing and shrinking independent of neighboring filaments due to GTP hydrolysis
requirements for a formed microtubule to persist instead of rapid disassembly
both ends protected from depolymerization
MINUS ends protected by organizing centers
PLUS ends stabilized by capping proteins
SELECTIVE STABILIZATION
when are MTs more stable/less stabel
more stable in polarized and differentiated cells (nerve cells)
less stable in dividing cells
which way to MT filaments all point to create structural polarity in neurons
plus end toward axon terminal
function of motor proteins
intracellular transport; bind to MTs and cargo (directly or via adaptors)
which direction to kinesins and dyneins move
kinesins move toward + end
dyneins move toward - end
How does ATP hydrolysis cause movement of motor proteins
ATP hydrolysis Pi release loosens attachment of rear motor head to MTs
ATP binding to front motor head changes conformation flipping rear motor head to the front
kinesins and dyneins structure
dimers of globular ATP binding head and tail
kinesins attach to which organelle
ER membrane (via receptor proteins)
pull it outward to maintain ER network
dyneins attach to which organelle
attach to Golgi and pull it inward
keeps it close to nucleus
cilia and flagella MT structure
9 + 2 array
9 dimers around outside of tube, 2 in center
mechanism of cilia and flagella movement
created by bending microtubule
ciliary dynein attached to adjacent microtubules to generate sliding force (ATP)
flexible protein links convert sliding motion to bending motion
when does the growing MT begin to shrink
when GTP hydrolysis catches up to growing end (GTP cap lost) and GDP bound tubulin has lesser affinity for binding so filament rapidly disassembles
what type of tubulin is added to growing end of MT
GTP bound tubulin added to growing MT, high affinity for one another
actin filaments main function
modify cell shape during division
form contractile ring
cell movements of protists/neutrophils
Actin filament location
found in bundles more than individual filaments; throughout cytoplasm
concentrated in cell cortex
Actin structure and characteristics
long thin and flexible
2 twisting strands of actin globular monomer
contains plus and minus end (POLARITY)
actin treadmilling
ATP-acting added to plus end and ADP-actin falls off minus end at same rate
actin-monomer binding proteins example
formin, ARP complex, monomer sequestering protein
actin filament binding proteins examples
severing protein, cross-linking (cortex), capping, side-binding (tropomyosin), motor protein, bundling (in filopodia)
the leading edge of cell movement process
driven by actin polymerization
lamellipodia and filopodia stretch forward with plus ends pointing towards PM
lamellipodia
flat sheet-like dense meshwork of actin
filopodia
thin stiff loose bundle of actin
where is the nucleation complex for actin filaments
growing edge (plus end) of filaments
function of ARPs in formation of lamellipodia
branched actin filaments grow from ARPs on existing filaments; plus ends protected by capping filaments
function of formin
promotes formation of unbranched filaments - filopodia
3 steps of cell movement forward
protrusion
attachment
traction
protrusion
actin polymerization at leading edge pushes PM forward and forms new actin cortex
attachment
new anchorage points made between actin and surface on which cell is crawling
-integrins anchoring proteins
traction
contraction at rear of cell draws body forward, old anchorage points at back released
-myosin motor proteins
Actin binding proteins respond to extracelullar signals and control actin filaments through . . .
surface receptors that activate signaling pathways which converge on Rho GTPases (molecular switches by GTP hydrolyzation)
myosin I
actin dependent motor protein
found in all cell types
one head domain (actin) and one tail (varies)
ATP driven
travels minus to plus end
myosin II
actin dependent motor protein
mainly in muscle cells
2 heads and coiled coil tail
tails associate to form filaments
bipolar-heads extend in dif directions
travels minus to plus
which part of myosin II interacts with actin to slide actin filaments over each other
heads; this causes muscle contraction and contractile ring during cell division
skeletal muscles makeup
striated appearance
made of numerous, very long multinucleated cells (aka muscle fibers) that contain numerous myofibrils
myofibril structure
chain of sarcomeres
sarcomere structure
two Z discs attached to actin filments (thin) at plus end
actin filaments attached to central specialized myosin II filaments (thick)
why do muscles contract
synchronized sarcomere shortening
actin and myosin slide past each other because of myosin heads walking toward plus ends
why do muscle cells relax
myosin heads release actin filaments and sarcomeres lengthen again
how do muscle cells respond to signals to contract
action potential spreads to myofibrils via T tubules (extensions of the PM into the cell)
T tubules open voltage-gated Ca+2 channels, which also mechanically opens sarcoplasmic reticulum Ca+2 channels
increased Ca+2 within the cell binds to troponin and induces conformational change
troponin change causes tropomyosin to not block myosin binding sites on actin anymore and initiate contraction
what happens to the Ca+2 after elctrical signal passes to relax cells
Ca+2 pumped back into sarcoplasmic reticulum
process of myosin moving along actin
- myosin w/no ATP or ADP bound tightly to actin
- ATP binds and myosin detaches from actin
- ATP hydrolysis causes a conformational change in myosin so displaced along actin filament (ADP and Pi attached to myosin still
- ADP-bound myosin weakly binds to new actin site, causing release of Pi and return to original conformation
- myosin loses ADP and tightly binds to new region of actin
