cytoskeleton

Question 1

why does a cell need a cytoskeleton

Answer 1

to keep its shape and modify it in response to environmental cues

Question 2

what are microtubules for ?

Answer 2

organelle positioning

intracellular support

Question 3

what are intermediate filaments for ?

Answer 3

mechanical strength

Question 4

what are actin filaments for ?

Answer 4

cell shape
organelle shape
cell migration

Question 5

give two properties of the polymers that make up the cytoskeleton

Answer 5

• monomers are very abundant

- not covalently linked which allows the cell to go from monomer to polymer quickly and vice versa

Question 6

what happens to the filaments in a cell if there is a signal?

Answer 6

• due to the non-covalently linked polymers, disassembly of filaments and rapid diffusion of subunits occur
• reassembly of filaments at a new site helping the cell to reach the signal.

Question 7

what do accessory proteins regulate ?

Answer 7

• Site and rate of filament formation (nucleation)
• Polymerization / depolymerization
• Function

Question 8

describe the structure of actin filaments

Answer 8

• Twisted chain of units (monomers) of the protein actin (G-actin, aprox.43 KDa). This chain constitutes the filamentous form of actin(F-actin).
• Thinnest class of the cytoskeleton filaments (7 nm)
• Presents structural polarity. They have an end where monomers are added, + end, and an end where addition of monomers is less favourable, - end.
• Associated with many actin-binding proteins (ABP)
- • actin filaments dispersed throughout the cell, most likely concentrated in the cortex just beneath the plasma membrane

Question 9

what are the 3 isoforms of G-actin and where are they found?

Answer 9

• α-actin found mainly in muscle cells

* β-actin and γ-actin in non-muscle cells

Question 10

describe actin polymerisation

Answer 10

• 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).

Question 11

What are G-actin levels controlled by?

Answer 11

Mainly by 2 actin binding proteins :
• Profilin: facilitates actin polymerization. Joins monomers of actin and makes them more available to join the + end.
• Thymosin beta4: prevents the addition of actin monomers to F-actin.

Question 12

Once the actin filaments are formed how are they structured ?

Answer 12

Once filaments are formed, they can be structured in different ways :
• Actin Bundling Proteins: keep F-actin in parallel bundles (as in the microvilli observed in epithelial cells)

• Cross-linking proteins: maintain F-actin in a gel-like meshwork (as seen in the cell cortex, underneath the plasma membrane)

Question 13

which protein breaks F-actin into smaller filaments ?

Answer 13

f-actin severing proteins

Question 14

what is the role of motor proteins (myosin) in actin filaments ?

Answer 14

transport of vesicles and/or organelles through actin filaments
• Also participating in the para-crystalline array in skeletal muscle.

Question 15

give a brief description of the functions of actin filaments

Answer 15

• provide mechanical strength to the cell by forming a meshwork underneath the plasma membrane
• link transmembrane proteins to cytoplasmic proteins-form contractile ring during cytokinesis in animal cells
• participate in cell motility and migration
• interact with myosin motors to provide force in muscular contraction

Question 16

what are the functions of actin filaments in skeletal muscle

Answer 16

• Interaction with Myosin motors allow muscle contraction ( a combination of actin filaments + myosin proteins facilitate the contraction of muscle cells. )

Question 17

how are actin filaments arranged in skeletal muscles ?

Answer 17

• Arranged in a para-crystalline array integrated with different ABPs

Question 18

describe the function of actin filaments in non-muscle cells.

Answer 18

– Cell cortex : form a thin sheath beneath the plasma membrane
– Associated with myosin form a purse string ring result in cleavage of mitotic cells
• Actin filaments can participate in functions related to four aspects of the cell:
• Microvilli found in epithelial cells - facilitate the movement of the interdigital structures that increase the surface area.
• Contractile bundles – eg fibroblasts
• Lamellipodia ( big extensions) filopodia ( small extensions) – sense surrounding and determines the direction of the cell. Filopodia are thin extensions of cells formed by actin fibres. , lamellipodia are sheetlike networks of microfilaments.
• Contractile ring – during cytokinesis - involvement of an actin-myosin ring

Question 19

describe the function of actin filaments in cell migration

Answer 19

–The cell pushes out protrusions at its front (lamellipodia & filopodia ) to identify direction of migration- fillopidia – bundles lamellipoda – mesh .

–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 to facilitate the movement.

–Cell contraction and retraction of the rear part of the cell to facilitate the movement.
•contraction involves the Interaction between actin filaments and myosin

Question 20

describe the structure of intermediate filaments

Answer 20

• Rope like with many long strands twisted together and made up of different subunits.
• Intermediate size (8-12nm) between actin and microtubules.
• One type of intermediate filament forms a meshwork called the nuclear lamina beneath the inner nuclear membrane.
• In epithelial tissue, span the cytoplasm from one cell junction to another, strengthening the epithelium .
• Form a network:
• Throughout the cytoplasm, joining up to cell-cell junctions (desmosomes).:
Withstands mechanical stress when cells are stretched. Ie mechanical strength
• And surrounding nucleus:
Strengthens the nuclear envelope.

Question 21

describe the polymerisation of intermediate filaments

Answer 21

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

Question 22

what are the functions of intermediate filament binding proteins

Answer 22

• Mainly linkers of IF structures.

* IFBP stabilize and reinforce IF into 3D networks

Question 23

give 3 examples of intermediate filament binding proteins with their functions.

Answer 23

• 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 24

list the cytoplasmic intermediate filaments and where they are localised

Answer 24

keratins - in epithelia

vimentin and vimentin-related: in connective tissue, muscle cells and neuroglial cells.

neurofilaments - in nerve cells

Question 25

list the nucleur intermediate filament and where they are localised

Answer 25

nuclear lamins, in all nucleated cells.

Question 26

what are the functions of intermediate filaments in the cytoplasm

Answer 26

• In the cytoplasm they provide: • 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 27

what are the functions of intermediate filaments in the nucleus

Answer 27

* present in all nucleated eukaryotic cells, inner part of the nucleus there are intermediate filaments called lamins. * form meshwork * line in the inner face of the nuclear envelope to: * strengthen it * provide attachment sites for chromatin * lamins 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)

Question 28

which IFPB is responsible for binding keratin filaments into bundles ?

Answer 28

fillagrin

Question 29

describe the structure of microtubules

Answer 29

• 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 • one end attached to MTOC

Question 30

how do microtubules polymerise ?

Answer 30

• Microtubule organizing centre (MTOC) are specialized protein complexes from where assembly of tubulin units starts. • MTOC is localised in the Centrosome (in the perinuclear region) in most of the cells. There is one type of protein that constitutes the microsomes, this can exist in 3 isoforms, alpha, beta and gamma tubulin. - Contains GAMMA-tubulin ring that initiates the microtubule growth. Gamma tubulin + accessory proteins in the gamma tublin complex constitutes the MTOC • From there, Heterodimers of alpha and beta 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

outline the main functions of microtubules

Answer 31

-Involved in intracellular trafficking in collaboration with protein motors that use ATP .-Determine the positions of membrane-enclosed organelles. -Coordinate the migration of chromosomes during mitosis and meiosis by forming the mitotic spindle .-Participate in motility of cilia and flagella in collaboration with motor proteins

Question 32

describe in detail the functions of microtubules

Answer 32

• intracellular transport : with protein motors that use ATP - 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) : - moves towards periphery of the cell ( + end ) – kinesin, moves towards the MTOC, or the centrosome – dynein. • organises position of organelles - hence, provides polarisation of cells - directionality of filaments is vital * coordinate the migration of chromosomes during mitosis and meisosis by forming the mitotic spindle. * Participiate in the Rhythmic beating of cilia and flagella - 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. - Examples : cilia in the respiratory tract, sweeping mucus and debris from the lungs and flagella on spermatozoa.

Question 33

what is the MTOC

Answer 33

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

Question 34

what is the MTOC in most cells?

Answer 34

Centrosome (in the perinuclear region)

Question 35

what constitutes the microtubule and what type of growth happens?

Answer 35

→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 36

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

Answer 36

→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

Question 37

which IFB keep the contact between desmosomes of epithelial cells?

Answer 37

plakins

Question 38

what are the three polymers that form the cytoskeleton

Answer 38

microtubules, intermediate filaments, actin filaments

Question 39

which cytoskeletal polymer has a hollow tube like structure?

Answer 39

microtubules

Question 40

which cytoskeletal polymer is made up of a heterogenous group of filamentous protein ?

Answer 40

intermediate filament