Lecture 6

Question 1

Describe microfilaments

Answer 1

• Monomeric actin protein subunits
• Twisted 2-standed polymer
• Provide structural support and cell motility

Question 2

Describe microtubules

Answer 2

• Hollow, tube like cylinders
• Formed from alpha-beta tubulin heterodimers
• Provide cell motility, cell polarity, structural support, Intracellular transport

Question 3

Diameter of actin filaments

Answer 3

7-9nm

Question 4

Cytoskeleton

Answer 4

Roles in cell (especially D)

Microfilaments at the leading edge - pushing it forward
Intermediate filaments surround the nucleus - positioning
Microtubules radiate out from the nucleus – transport of components

Question 5

Microfilament function

Answer 5

• Formed from actin
• G-actin converted to F-actin (reversible)
• F-actin has polarity with ATP binding site at (-) end
• Can be assembled into diverse structures

Question 6

Actin polarity

Answer 6

Polar as:
- ATP binding cleft at (-) end
- Nothing on + end

Question 7

G and F Actin and ATP

Answer 7

ATP binding to G-Actin essential for filament formation

If ATP in F-Actin cannot be hydrolysedthen the filament can’t disassemble

Question 8

What happens if nuclei added to G-actin

Answer 8

No lag phase if nuclei added at t=0

Question 9

Is growth faster at the + end or - end?

Answer 9

+ end

Question 10

What is critical concentation

Answer 10

Concentration of free ATP-G-actin at which assembly/disassembly are equal
Above Cc - Grows
Below Cc - Shrinks

Question 11

What is the Cc at + and - ends

Answer 11

0.12 microM at +
0.6 microM at -

Question 12

How can treadmilling occur?

Answer 12

Can occur because the cC is different at the two ends

So if its >cC for the +End and <cCfor –End can treadmill

ATP-actin subunits add at + end, ADP-actin subunits get removed at - end

Question 13

What do actin binding proteins do?

Answer 13

Regulate assembly and disassembly of actin-binding proteins

Question 14

Examples of actin-binding proteins

Answer 14

Profilin enhances the exchange of ADP for ATP in G-actin
Cofilin allows for the removal of ADP-actin at the - end
Thymosin beta4 allows for storage reserve of actin in case of it is needed later
Capping proteins bind to filament ends to prevent assembly and disassembly

Question 15

What protein caps actin at + end for stability?

Answer 15

CapZ

Question 16

What protein caps actin at - end for stability?

Answer 16

Tropomodulin

Question 17

What do formins do

Answer 17

Nucleate the assembly if unbranched filaments

Once Rho is in activated state (bound to GTP), associates with formin to induce a conformational change

Facilitates of formation of microfilaments

Question 18

Structure of formin

Answer 18

Formin is dimer – ring like structure

One G-actin monomer ate a time fed through the ringas the microfilament forms

Question 19

What do Arp2/3 do?

Answer 19

ARP2/3 nucleates the assembly of branched filaments

Question 20

Arp2/3

Answer 20

Bind to microfilament
New microfilament at 70o angle
Lots of branching at leading edge during cell migration

Question 21

What does phalloidin do?

Answer 21

Prevents disassembly when bound to F-actin

Question 22

Actin cross-linking proteins

Answer 22

Fimbrin - Microvilli, fidopodia
a-actinin - Stress fibers, filopdoa
Dystrophin - Links membrane proteins to actin cortex in muscle
Filamin - Filopodia, stress fibres
Spectrin - Cell cortex

Question 23

What causes Duchenne muscular dystrophy

Answer 23

Mutation of the dystrophin gene leading to defective gene
Forms adapter protein which binds to cytoskeletal components such as actin
Results in impaired muscle function and shortened life span