Cytoskeleton Flashcards
what is the cytoskeleton
network of protein filaments throughout cytoplasm, supports large volume of cytosol, dynamic and responsible for cell shape and movement
functions
mitosis cytokinesis traffick support sperm to swim muscle contarction formation of axons/dendrites cell shape
cortical actin
actin filaments found concentrated close to plasma membrane
three types of cytoskeletal filament
intermediate filaments
microtubules
actin filaments
intermediate filaments
10nm diameter
provide tensile strength
abundant in cells subject to mechanical stress
form a network throughout the cytoplasm, surround nucleus
anchored at plasma membrane at cell junctions
eg keratins
3 main classes of intermediate filaments
keratin filaments in epithelial cells
vimentin in connective tissue cells, muscle cells and supporting cells of the nervous tissue (neuroglial)
neurofilaments in nerve cells
construction of intermediate filament
from smaller protein subunits
Monomers conisist of globular N (amine) and C (carboxyl) termini, and central rod domain- long alpha helical region in between
two monomers of intermediate filament associate to form a dimer by coiling their alpha helices around each other- coiled-coil conformation
Very strong interaction between monomers because large area in contact with each other
N is start and C end of PP chain
what do two dimers align to form
staggered tetramer, N termini antiparallel
what happens to tetramers
pack together end to end, N-termini of monomers in one dimer interact with C termini of adjacent monomers in another dimer
next step
eight tetramers twisted into rope of diameter approx 10 nm
keratins
span interior of epithelial cells
indirectly connected to filaments of other cells through cell-cell junctions (desosomes)
desmosomes
cadherins directly make contact with cadherins from another cell
transmembrane proteins that span bilayer and interact with plaque proteins on cytosolic side of membranes
The plaque proteins interact with keratin
keratin-plaque-cadherin-cadherin-plaque-keratin
hence keratin cytoskeletons indirectly connected
epidermolysis bullosa simplex
intermediate filament disorder
rare genetic disorder, keratin can’t form normal filaments so skin susceptible to mechanical injury
why are cells with defective intermediate filaments more susceptible to mechanical stress
intermediate filaments provide strength and prevent cells rupturing, hold them together when sheet of cells stretched
nuclear lamina
network of intermediate filaments inside nuclear membrane
give nucleus its shape
how can you show how durable intermediate filaments are
treat with high salt or non-ionic detergent
remain intact
actin filament diameter
7 nm
monomers that make up actin
more globular
monomers (vs intermediate filament monomers which only have globular ends) associate head to tail
filaments are unstable without associated proteins
monomers that form F actin filament
G actin
Requirements for formation of F actin
presence of ATP, Mg and K
concentration of G actin must be sufficient for polymerisation
Below critical conc, actin filaments depolymerize
Polymerisation of actin
Actin monomers in the cytosol carry ATP
The ATP is hydrolysed to ADP after assembly into filaments
The ADP bound monomer is less stable in the filament
ADP can’t be exchanged for ATP until the monomer disassembles
why do actin filaments have polarity
actin monomers all have the same orientation in the filament
minus end has an exposed ATP binding cleft and plus end does not, has a dome
What proteins bind to actin and modify its properties
monomer binding proteins nucleating proteins cross linking proteins capping proteins bundling proteins motor proteins
functions of actin
mechanical strength anc cell shape
cell crawling
muscle contraction
organelle movement
Cortical actin
Actin filaments usually concentrated in a layer under the plasma membrane (cortex)
linked into a meshwork by actin binding proteins
Provide mechanical strength and cell shape
cell crawling
filopodia or lammelipodia extend a region of the plasma membrane
integrins adhere to extracellular matrix (substrate)
cells use internal contractions to pull itself forwards
Cytoskeleton underneath plasma membrane is deforming the membrane
Will grab onto a suitable substrate via integrins
filipodia
finger/needle like projections of the plasma membrane
lammelipodia
sheet like projections of the plasma membrane
cell crawling further explanation
actin polymerisation at front (plus end) of the cell leads to lamellipodium extension
Attachment to extracellular matrix (via integrins) provides an anchor point
contraction of the rear pulls the cell forward
Motor proteins
Use ATP hydrolysis
Myosin are motor proteins that can move along actin filaments
Myosin can bind to and hydrolyze ATP
This provides energy for their movement along the actin filament from minus end to the plus end
movement of myosin along actin filaments
Myosin head lacks ATP at beginning of cycle and is locked onto actin
When ATP binds the affinity for actin is reduced, myosin head released
ATP hydrolysed causes head conformational change, more upright
Then Pi leaves, confromational change and head binds to actin. Then ADP leaves and head conformational change, cocked forwards- power stroke
What happens at steady state phase of graph
rate of assembly of new subunits same as rate of disassembly
reached critical concentration
drugs that interfere with actin filament assembly
Cytochalasin D binds to plus end of F actin and prevents addition of further G actin
Phalloidin from poisonous mushroom Amonita binds F actin and prevents actin filaments from depolymerising
