Lecture 6 Flashcards
Describe microfilaments
- Monomeric actin protein subunits
- Twisted 2-standed polymer
- Provide structural support and cell motility
Describe microtubules
- Hollow, tube like cylinders
- Formed from alpha-beta tubulin heterodimers
- Provide cell motility, cell polarity, structural support, Intracellular transport
Diameter of actin filaments
7-9nm
Cytoskeleton
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
Microfilament function
- 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
Actin polarity
Polar as:
- ATP binding cleft at (-) end
- Nothing on + end
G and F Actin and ATP
ATP binding to G-Actin essential for filament formation
If ATP in F-Actin cannot be hydrolysedthen the filament can’t disassemble
What happens if nuclei added to G-actin
No lag phase if nuclei added at t=0
Is growth faster at the + end or - end?
+ end
What is critical concentation
Concentration of free ATP-G-actin at which assembly/disassembly are equal
Above Cc - Grows
Below Cc - Shrinks
What is the Cc at + and - ends
0.12 microM at +
0.6 microM at -
How can treadmilling occur?
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
What do actin binding proteins do?
Regulate assembly and disassembly of actin-binding proteins
Examples of actin-binding proteins
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
What protein caps actin at + end for stability?
CapZ
What protein caps actin at - end for stability?
Tropomodulin
What do formins do
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
Structure of formin
Formin is dimer – ring like structure
One G-actin monomer ate a time fed through the ringas the microfilament forms
What do Arp2/3 do?
ARP2/3 nucleates the assembly of branched filaments
Arp2/3
Bind to microfilament
New microfilament at 70o angle
Lots of branching at leading edge during cell migration
What does phalloidin do?
Prevents disassembly when bound to F-actin
Actin cross-linking proteins
Fimbrin - Microvilli, fidopodia
a-actinin - Stress fibers, filopdoa
Dystrophin - Links membrane proteins to actin cortex in muscle
Filamin - Filopodia, stress fibres
Spectrin - Cell cortex
What causes Duchenne muscular dystrophy
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
