INTRODUCTION

Unit 8 - Cytoskeleton

3 types of fibers make up the cytoskeleton

                        

                                    

                

                                           

         

Intermediate Filaments  

MicrotubulesActin

Microfilaments

 

- Microtubules interact with motor systems: dyneins and kinesins

- Microfilaments interact with myosins - Intermediate filaments do not interact with motor proteins

Intermediate Filaments

 

These supportive fibers provide for mechanical reinforcement of tissues and range in diameter from 8-10 nm

FUNCTION:“provide a supporting

framework within the cell”

Intermediate Filaments - STRUCTURE

Have great tensile strength!

Strengthens cells against mechanical stress!

monomer

dimer

tetramer

2 tetramers coiled together

8 tetramers twisted into a ropelike filament

10nm

Types of intermediate filaments

• nuclear lamins form a meshwork that stabilizes the inner membrane of the nuclear envelope;

• keratins – junctions in epithelial cells (desmosomes) and also form hair and nails;

• neurofilaments - strengthen the long axons of neurons;

What cytoskeletal element is stained green in this immunofluorescence experiment? Why?

Cell boundaries stained in blue

Intermediate filaments. They form a network that links to other cells at junctions on the plasma membrane

Intermediate Filaments

• Why can you use intermediate filaments to tell whether a cancer has spread in the body?

Microtubules

 

FUNCTION: These macromolecular assemblies are involved mainly in the movement and positioning of cell organelles.

- Minus end is attached to centrosome (or Microtubule organization Center) - Plus end is free

Microtubules - STRUCTURE

Long, hollow cylinders made of 13 protofilaments and built by the assembly of dimers of alpha tubulin and beta tubulin.

Right. Electron Microscope Image of microtubules

MICROTUBULES

- The growing end of the microtubule has subunits arranged with the beta-tubulin on the outside. The subunits in the microtubule all show a uniform polarity

Rapid Growth = plus end

Microtubule polymerization

- microtubules keep growing with freshly added tubulin dimers and GTP

- Tubulin dimers + GTP (red) bind more tightly to one another than tubulin dimers + GDP (dark green) creating a GTP cap.

Microtubule depolymerization

- the GDP-carrying subunits are less tightly bound in the polymer and readily released from the free end

- Slow microtubule growth causes the "GTP cap“ to hydrolyze GTP to GDP The GTP cap is lost

The microtubule shrinks

Centrosomes

• Organized array of microtubules that radiate outward from it through the cytoplasm.

• Contains ring-shaped structures of -tubulin.– Serves as starting point for growth of new

microtubule

Motor Proteins

• Dyneins– Travel towards

MINUS END

• Kinesins– Travel towards

PLUS END

KinesinsDyneins

--++

Actin microfilaments

  Microfilaments Interact with many types of molecules including its own class of motor proteins, the myosins

FUNCTION: support components of the cell and provide for the movement of cytoplasm and the cell surface

Bundles of Actin filaments in cells

Microvilli Contractile bundles in cytoplasm

Sheet-like and finger-like protrusions from the leading edge of a moving cell

Contractile ring during cell division

Actin in RED:

Actin - STRUCTURE

• Actin microfilaments are comprised of actin monomers that polymerize to form long, thin fibers. These are about 7nm in diameter.

Actin Polymerization

Can occur at either end– Faster rate at

PLUS end

Triphosphate (ie ATP) hydrolyzed to diphosphate (ie ADP) soon after incorporation into actin filament.– Nucleotide hydrolysis promotes depolymerization

ATP Hydrolysis

• Actin monomers in the cytosol carry ATP, which is hydrolyzed to ADP soon after assembly into growing filament.

• ADP molecules remain trapped within actin filament, until the actin monomer that carries them dissociates from filament to form a monomer again.

Drug Treatments

Colchicine: Binds free tubulin and inhibits formation of microtubules by preventing polymerization

Taxol: Stabilizes microtubules by preventing depolymerization

Cytochalasin: inhibits formation of actin by preventing polymerization

Phalloidin: stabilizes actin filaments preventing depolymerization

Practice Questions:

Here we see Phalloidin linked to a green fluorescent dye. What cytoskeletal element is being stained green? How do you know?

Nucleus stained blue

Golgi stained red

What technique was used?

What cytoskeletal element is indicated by the arrow? What are TWO other functions of this element?

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