Lecture 6 - The Cytoskeleton Flashcards
What is the cytoskeleton composed of?
The cytoskeleton Is composed of:
* Actin filaments - Composed of monomeric actin protein subunits assembled into a twisted two stranded polymer. They provide structural support and are important for certain types of cell motility
* Microtubules - Composed of a and b tubulin heterodimers assembled into a hollow tubelike cylinder. They provide structural support, certain types of motility and help generate cell polarity
* Intermediate filaments - formed from a family of related proteins such as keratin or Lamin. The subunits assemble to create a strong rope like polymer that depending on specific protein, may provide support for the nuclear membrane or for cell adhesion
The cytoskeleton can be controlled by external signals which can act directly or through transcription to cause the organisation and movement of organelles or changes to cell shape, movement and contraction
What is the basic building block of microfilaments?
Actin is the basic building block of microfilaments. G-actin can assemble into F-actin (filamentous) this is reversible. F-actin has polarity with the ATP binding site at the -ve end. Microfilaments can be assembles into diverse structures. F-actin spirals due to repeat every 36nm.
Actin polarity - for actin filaments polarity refers to the fact that one end is different from the other end as one side has an ATP binding cleft exposed on the terminal actin which is designated the - end.
How does actin interact with ATP
ATP-G-actin assembles on the ends of actin filaments and the ATP bound to the subunit is subsequently hydrolysed to ADP and a phosphate. The filament consists mostly of ADP-F-actin.
If ADP-F-actin is exposed at the ends then the filament will disassemble form the ends. But if ATP-G-actin is consistently added the filament will keep growing
If a mutation in actin prevents the protein from binding ATP, actin filaments will fail to assemble. If a mutation prevents actin from hydrolysing ATP the filament will be unable to disassemble normally
Describe the formation of actin filaments?
- G actin forms a nucleus
- G actin elongates the molecule in two directions, the (+) end elongates faster
- Reaches steady state when equal number of monomers are removed as added
Critical concentration is the concentration of free ATP-G-actin at which assembly/disassembly are equal at one of the ends.
Above Cc the end will grow and below Cc then ends will shrink. The Cc is different at each end.
Treadmilling - Addition at one end is equal to loss at the other end so the length remains constant. ATP-actin subunits ad to the + end and ADP-actin subunits dissociate form the - end.
Formins nucleate the assembly of unbranched filaments, formin is maintained in an inactive state, by association of its N and C termini. When Rho is in its active GTP-bound form, it will associate with formin and induce a conformational change that renders it active
ARP2/3 nucleates the assembly of branched filaments
Give some examples of actin binding protiens.
- Profilin - Enhances exchange of ADP for ATP on G-actin
- Cofilin - Enhances the loss of ADP-actin from the - end
- Thymosin beta4 - binds G-actin to provide reserve actin when needed
- Capping proteins - Bind to filament ends preventing assembly and disassembly
○ CapZ binds to and stabilizes + end
○ Tropomodulin binds and stabilizes the - end
Toxins such as phalloidin binds to F-actin preventing disassembly - attached to a fluorescent tag can be used to stain actin
What cross linking proteins are associated with actin
Actin structures are organised by cross-linking proteins.
* Fimbrin - for microvilli
* Spectrin for the cell cortex
* Filamin for filopodia
* Dystrophin for muscle cell cortex
What disease is associated with a defective adapter protein?
Duchenne muscular dystrophy
Dystrophin gene is defective, it is an adapter protein that binds to cytoskeletal components such as actin. It leads impaired muscle function and shortened lifespan.
