cytoskeleton

Question 1

How many types of polymers are there?

Answer 1

→3 types

Question 2

what are the types of polymers called?

Answer 2

→microfilaments / actin,
→microtubules
→intermediate filaments

Question 3

what are microtubules for?

Answer 3

→ organelle positioning

→ intracellular transport

Question 4

what are intermediate filaments for?

Answer 4

→ mechanical strength

Question 5

what are actin filaments for?

Answer 5

→ cell shape
→ organelle shape
→ cell migration

Question 6

give two properties of the polymers that make up the cytoskeleton

Answer 6

→ monomers are very abundant

→ not covalently linkes

Question 7

what is the structure of actin filaments and what are the isoforms?

Answer 7

→Twisted chain of units (monomers) of the protein actin (G-actin, aprox.43 KDa). This chain constitutes the filamentous form (F-actin).

→Thinnest class of the cytoskeleton filaments (7 nm)

→Presents structural polarity

→Associated with a large number of actin-binding proteins (ABP)

→There are 3 isoforms of G-actin with different isoelectric points :
α-actin found mainly in muscle cells
β-actin and γ-actin in non-muscle cells

Question 8

how can actin polymerize?

Answer 8

→Actin filaments (F-actin) can grow by addition of actin monomers (G-actin) at either end.

Question 9

what does the length of the polymerized actin filament depend on?

Answer 9

→Concentration of G-actin.

→Presence of Actin Binding proteins (ABPs)

Question 10

what are G actin levels controlled by?

Answer 10

2 actin binding proteins

Question 11

what are the two actin binding proteins that regulate G actin and what do they do?

Answer 11

→Profilin: facilitates actin polymerization.

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

Question 12

What are the two actin binding proteins that bind to F actin?

Answer 12

Actin bundling proteins
→ keep F-actin in parallel bundles (as in the microvilli observed in epithelial cells)

Cross linking proteins
→ keep actin in gel like mesh work under membrane

Question 13

what do F actin severing proteins do?

Answer 13

→break F-actin into smaller filaments

Question 14

what do motor proteins do?

Answer 14

→transport of vesicles and/or organelles along actin filaments.

Question 15

what proteins break actin apart?

Answer 15

→F actin severing proteins

Question 16

how do the severing proteins increase the surface area when breaking actin down?

Answer 16

→they work on different points like endo peptidases.

Question 17

how is actin arranged in skeletal muscle?

Answer 17

→Arranged in a para-crystalline array integrated with different ABPs
→Interaction with Myosin motors allow muscle contraction

Question 18

what is the function of actin in non-muscle cells?

Answer 18

→Cell cortex : form a thin sheath beneath the plasma membrane

→Associated with myosin form a purse string ring resulting in cleavage of mitotic cells

Question 19

what is the function of actin in cell migration?

Answer 19

1) Elongation - protrusions (lamellipodia and filopodia) pushed out
2) Adhesion - Integrins link the filaments to the extracellular matrix surrounding the cell
3) Contraction - actin and myosin interaction, contraction and retraction

Question 20

what are properties of intermediate filaments?

Answer 20

→Toughest of the cytoskeletal filaments (resistant to detergents, high salt etc).

→Ropelike with many long strands twisted together and made up of different subunits.

→Form a network:
Throughout the cytoplasm, joining up to cell-cell junctions (desmosomes).

→Withstands mechanical stress when cells are stretched.
And surrounding nucleus

→Strengthens the nuclear envelope.

Question 21

What is the structure of intermediate filaments?

Answer 21

Each unit is made of:
→N-terminal globular head
→C-terminal globular tail
→Central elongated rod-like domain
→Units form stable dimers
→Every 2 dimers form a tetramear
→Tetramers bind to each other and twist to constitute a rope-like filament

Question 22

What are the types of intermediate filaments?

Answer 22

→CYTOPLASMIC:

1) Keratins (in epithelia, protects from damage/stress)
2) Vimentin/ vimentin related (in connective tissues, muscle cells and neuroglial cells)
3) Neurofilaments (in nerve cells) NUCLEAR:
1) Nuclear lamins (in all nucleated cells)

Question 23

what do the intermediate filament binding proteins do?

Answer 23

→Mainly linkers of IF structures.

→IFBP stabilize and reinforce IF into 3D networks

Question 24

what are the intermediate filament binding proteins and their functions?

Answer 24

→Fillagrin:
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 of epithelial cells.

Question 25

what are the functions of intermediate filaments in the cytoplasm?

Answer 25

→Tensile strength: this enable the cells to withstand mechanical stress (to stretch!)

→Structural support by:
Creating a deformable 3D structural framework

Question 26

what are the functions of intermediate filaments in the nucleus?

Answer 26

→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 intermediate filaments
→process controlled by post-translational modifications (mainly phosphorylation and dephosphorylation)

Question 27

describe properties of microtubules

Answer 27

→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

Question 28

what is the MTOC?

Answer 28

Microtubule organizing centre (MTOC) are specialized protein complexes from where assembly of tubulin units starts.

Question 29

what is the MTOC in most cells?

Answer 29

Centrosome (in the perinuclear region)

Question 30

what constitutes the microtubule and what type of growth happens?

Answer 30

→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 31

what are the functions of microtubules?

Answer 31

→intracellular transport
act like railway tracks on which molecular motors run
different motors for different cargoes

→directionality of filaments is vital (each motor only moves in one direction)

→organises position of organelles
hence, provides polarisation of cells

Question 32

what are the functions of microtubules ( cilia ) ?

Answer 32

→Rhythmic beating of cilia and flagella

→Motile processes, with highly organized microtubule core.

→Bending of cilia & flagella is driven by the motor protein Dynein.

Question 33

what does a microtubule core consist of and what controls the assembly of the axoneme?

Answer 33

→Core consist of 9 pairs of microtubules around 2 central microtubule (axoneme).

→The basal body, at the base of the tubule, controls the assembly of the axoneme.