W2 Cytoskeleton

Question 1

Cytoskeleton

Answer 1

Keep shape + modify to environmental clues
Dynamic structure
Monomers not covalently linked

Question 2

Network of polymers + function

Answer 2

Microtubules, actin/intermediate filaments

For → shape of cell, intracellular movement of organelles + cell movement

Question 3

Accessory proteins regulate…?

Answer 3

Site/rate of filament formation = nucleation (polymerisation or assembly reactions where the first steps are energetically less favoured than the continuation of growth)
Polymerization/depolymerization
Function
Cross linking of filaments = forming higher order structures
Signalling of intra/extra w/cytoskeleton assembly modules

Question 4

What are actin filaments

Answer 4

Helical polymers made of actin
Flexible, organised into 2D networks + 3D gels
Cell/organelle shape
Cell migration
Organisation/regulation = ABP (actin binding proteins)

Question 5

Actin filaments structure

Answer 5

Twisted chain of units (monomers) of the protein actin (G-actin, approx.43 KDa kilodaltons). This chain constitutes the filamentous form (F-actin)
Helical repeat every 37 nm
Thinnest class of the cytoskeleton filaments (7 nm)
Presents structural polarity = binding of myosin subfragment (S1) to filament causes barbed +ve and pointed -ve ends on filament
Associated with a large number of actin-binding proteins (ABP) = variety of organization and function
There are 3 isoforms (different form of a protein) of G-actin with different isoelectric point (pH were molecule carries no net electrical charge):
α-actin found mainly in muscle cells
β-actin and γ-actin in nonmuscle cells

Question 6

Actin filaments polymerization

Answer 6

Actin filaments (F-actin) can grow by addition of actin monomers (G-actin) at either end.
The length of the filament is determined by:
Concentration of G-actin.
Presence of Actin Binding proteins (ABPs)
Capping proteins = control access to free barbed ends of actin filaments as have high affinity for barbed ends + micromolar conc in CT ensures most barbed ends capped

Question 7

Regulation of G actin levels

Answer 7

Profilin: facilitates actin polymerization

Thymosin β4: prevents the addition of actin monomers to F-actin

Question 8

Actin bundling proteins

Answer 8

Keep F-actin in parallel bundles (as in the microvilli observed in epithelial cells)

Question 9

Cross-linking proteins

Answer 9

Maintain F-actin in a gel-like meshwork (as seen in the cell cortex, underneath the plasma membrane)

Question 10

Lamellipodia

Answer 10

Thin protein sheet on actin (propels cell across a substrate)

Question 11

Filopodia

Answer 11

Thin, tubular protrusions formed at leading edge of motile cells that are composed of linear bundles of actin filaments

Question 12

Actin in skeletal muscle

Answer 12

Arranged in a paracrystalline array integrated with different ABPs
Interaction with myosin motors allow muscle contraction

Question 13

Actin in non muscle

Answer 13

Cell cortex = form a thin sheath beneath the plasma membrane

Associated with myosin to form a purse string ring result in cleavage of mitotic cells

Question 14

Cytokinesis

Answer 14

Actin-myosin ring called contractile forms around equator of cell tightens to form cleavage furrow

Question 15

Cell migration w/actin

Answer 15

The cell pushes out protrusions at its front (lamellipodia & filopodia)
Actin polymerization
These protrusions adhere to the surface
Integrins (link the actin filaments to the extracellular matrix surrounding the cell)
Cell contraction and retraction of the rear part of the cell
Interaction between actin filaments and myosin

Question 16

Intermediate filaments structure

Answer 16

Absent in fungi/plants
Toughest of the cytoskeletal filaments (resistant to detergents, high salt etc).
Ropelike with many long strands twisted together and made up of different subunits
Intermediate size (8-12 nm) between actin and microtubules
IF formation starts w/folding of IF proteins into conserved alpha-helical rod shape then series of polymerization + annealing = filaments w/described structure above
Lack polarity as not all interact w/each other
More stable + do not bind nucleotides

Question 17

Intermediate filament function

Answer 17

Form a network:
Throughout the cytoplasm, joining up to cell-cell junctions (desmosomes).
Withstands mechanical stress when cells are stretched
Surrounds nucleus + strengthens the nuclear envelope

Question 18

Intermediate filament polymerization

Answer 18

Each unit is made of:
N-terminal globular head
C-terminal globular tail
Central elongated rod-like domain

Units (w/alpha helical region) form stable dimers (called coiled-coil dimer)
Every 2 dimer forms a tetramer
Tetramers bind to each other and twist to constitute a rope-like filament

Question 19

Intermediate filament binding protein

Answer 19

Mainly linkers of IF structure

IFBP stabilize and reinforce IF into 3D networks

Question 20

Examples of IF binding protein

Answer 20

Filaggrin:
binds keratin filaments into bundles
Synamin and Plectin:
bind desmin and vimentin
Link IF to the other cytoskeleton compounds (i.e. actin and microtubules) as well as to cell-cell contact structures (desmosomes).
Plakins:
Keep the contact between desmosomes (structure by which two adjacent cells attached) of epithelial cells

Question 21

Intermediate filament function in CT

Answer 21

(Keratins in epithelia, vimentins + neurofilaments)
Tensile strength: this enable the cells to withstand mechanical stress (to stretch)
Structural support by:
Creating a deformable 3D structural framework
Reinforcing cell shape and fix organelle localization

Question 22

Intermediate filament function in nucleus

Answer 22

Present in all nucleated eukaryotic cells
Form mesh rather than “rope-like” structure
Line in the inner face of the nuclear envelope to:
strengthen it
provide attachment sites for chromatin
Disassemble and reform at each cell division as nuclear envelope disintegrates
i.e. very different from the stable cytoplasmic IFs
process controlled by post-translational modifications (mainly phosphorylation and dephosphorylation)

(Lamins) Anchor for chromatin

Question 23

Microtubules structure

Answer 23

Hollow tubes made up from the protein tubulin
Relatively stiff (25nm), is the thicker of the filaments
Each filament is polarized (i.e. has direction – head/tail or +/-)
It is a dynamic structure
Assemble and disassemble in response to cell needs
tubulin in cell is roughly 50:50 as free or in filament
i.e. very different from the stable cytoplasmic intermediate filaments

Question 24

Microtubules polymerization

Answer 24

Microtubule organizing centre (MTOC) are specialized protein complexes from where assembly of tubulin units starts
Centrosome (in the perinuclear region) is the MTOC in most of the cells
Contains γ-tubulin ring that initiates the microtubule growth
Heterodimers of α and β tubulin constitute the microtubule
It is a polarized growth (i.e. there is an end that grows faster (+end) than the other (- end)

Question 25

Microtubules function

Answer 25

Intracellular transport
Act like railway tracks on which molecular motors run
Different motors for different cargos
Directionality of filaments is vital (each motor only moves in one direction)
Dynein - moves cargo towards minus ends of MT
Kinesin - Typically moves cargo towards plus ends of MT
Organises position of organelles
Provides polarisation of cells
Directionality of filaments is vital

Question 26

Microtubules in rythmic beating of cilia/flagella

Answer 26

Motile processes, with highly organized microtubule core.
Core consist of 9 pairs of microtubules around 2 central microtubule (axoneme)
Bending of cilia & flagella is driven by the motor protein Dynein
The basal body, at the base of the tubule, controls the assembly of the axoneme
Cilia in the respiratory tract, sweeping mucus and debris from lungs
Flagella on spermatozoa