The cytoskeleton and mechanobiology of the cells

Question 1

What are the functions of the cytoskeleton?(4 functions)

Answer 1

Functions:

• Organazation of the cell
• Mechenical interaction with the ECM
• Determining the polarity of the cell (making a distinction between up/down/left/right within a cell)
• It constantly builds, degrades and remodels

Question 2

Accecory proteins

Answer 2

These proteins regulate and organize filaments, convert signals in to cytoskeletal action and helps maintaining an organized but flexible internal structure. They bind to filaments or subunits and determine spatial distribution and dynamic behavior of the filaments. For actin this would be ABPs.

Question 3

Intermediate filaments

Answer 3

A filament that is bound by noncovalent forces and grows in a symmetrical manner.

Question 4

Microtubules

Answer 4

A filament that is bound by noncovelent forces, grows asymmetricly and consists of 13 protofilaments of tubulin that form a hollow cylinder. It adds mechenical strength and is easy to remodel.

Question 5

Microfilaments

Answer 5

Filaments that are made of actin. They are bound by noncovalent forces, grow asymmetrical and use atin bining proteins(ABPs) as accessory proteins. They need a nucleus to start growing.

Question 6

Actin

Answer 6

Actin is the chemical that makes up microfilaments. Actin is made out of globular actin subunits (G-actin). 3 different isoforms exists with a slightly different amino acid sequence and so function. G-actin assemblies head-to-tail where the + end is the fast growing end (preferred end). A ‘nucleus’ is needed to form strands of actin, to start the elongation phase which is followed by an equilibrium (steady-state) phase, which uses a system called threadmilling.

Question 7

Threadmilling

Answer 7

Actin filaments maintaining a constant length, even though subunits are bound and released continuously.

Question 8

What are proteins and chemicals that affect the polymerization of actin?(7 molecules)

Answer 8

Toxic chemical:
-     Cytochalasins
-	Phalloidins
Other molecules:
-	Thymosin
-	Profilin
-	Formins 
-	Nucleating proteins + Arp 2/3 complexes
-	ABPs (actin filament binding proteins)

Question 9

Cell migration

Answer 9

Cell migration is heavily dependent on the actin-rich cortex under the cell membrane and happens in three steps: protrusion, attachment and traction by motor proteins. Polarization is important for this process

Question 10

Rho family proteins

Answer 10

This family of proteins controls the polarization of a cel by controlling the structural organization of the cytoskeleton.
The familyconsists of GTPases. The 3 most important:
- Cdc42: organizes membrane protrusions; bundles actin fibers that push against the cell membrane.
- Rac: organizes lamellipodia; crosslinked small and thin actin fibers which form a network.
- Rho: organizes stress fibers; large and thick bundles of actin responsible for cell support.

Question 11

Cytochalasins

Answer 11

A toxic chemical that prevents active polymerization by binding to the + end of actin.

Question 12

Phalloidins

Answer 12

A toxic chemical that stabilizes the actin filaments by binding to the sides. Also used for staining for microscopy experiments (red)

Question 13

Thymosin

Answer 13

A protein that organizes actin filament assembly by inhibiting + end growth

Question 14

Profilin

Answer 14

A protein that organizes actin filament assembly by inducing rapid + end growth. Profilin competes with Thymosin for binding to actin monomers.

Question 15

Formins

Answer 15

This molecules facilitates straight, non-branched actin growth.

Question 16

Nucleating proteins + Arp 2/3 complexes

Answer 16

These molecules create nucleation: elongation of the + end of actin filaments

Question 17

ABPs (actin filament binding proteins)

Answer 17

These regulate actin filament behavior. There are 2 major groups of ABPs: side-binding and end-binding proteins.

• Bundling proteins influence actin structure by forming networks of actin filaments. Fimbrin forms a tight packed parallel bundle with actin filaments, and α- actinin forms a loose packed contractile bundle (myosin can enter bundle due to loose packing)
• Not all ABPs are straight, there are also flexible or stiff bent connections possible by Filamin or Spectrin for example.

Question 18

Tropomyosin

Answer 18

This is a side-binding ABP.

Question 19

Tropomodulin

Answer 19

This is an end-binding ABP, a capping protein (- end)

Question 20

What is the cytoskeleton made off?

Answer 20

It is made of three types of filaments:
- 	Actin (microfilaments)
-	Intermediate filaments
-	Microtubules
Each of these filaments has a different fuction in the cell and is made up of different subunits, all thee of them are helical and self-associating.

Question 21

Symmetric filaments

Answer 21

Filaments that grow and dissasembel on both ends

Question 22

Asymmetric filaments

Answer 22

Filaments that grow grow faster from one and then from the other

Question 23

Polarazation

Answer 23

The distinction between front and back of the cell. Rho family proteins control polarization by controlling the structural organization of the cytoskeleton.