Cytoskeleton Flashcards
What is one of the most abundant cellular proteins?
Actin 1-5% non muscle cells
How does actin convert to filament form?
- Globular actin monomers associate to form double helical filament actin
What is the rate limiting step for actin and MT kinetics?
- Nucleation, not elongation
- Elongates till it reaches critical concentration of monomers
What are the T and D forms of actin
- A(T)P form more likely to associate to filament
- Over time converts to D form to add timer for each actin monomer
- Creates treadmilling
How do actin T and D forms create differences between both ends?
- Due to continuous addition of T at one end and hydrolysis to D on other end
- Create barbed at T and pointed at D end
- Barbed end desocciates slower than pointed end
What are the main classes of actin nucleators?
- ARP2/3 attaches to actin and mimics it to form a new branch
- Spire proteins provide scaffold for stable linear formation
- Formins act like arms help add monomers to linear branches, recruited by Rho
How is Arp2/3 activated?
- By WASP
What do profilin and cofilin do?
- Cofilin induces depolymerization and fragmentation (which increases concentration of free ends)
- Cofilin has a higher affinity for D form
- Profilin binds to T form monomers sequestering them
What are the roles of Myosin II and V?
- Myosin II is a long protein with 2 heavy and light chains
- Myosin V walks hand over hand and is continuously attached
How does a cell move forward?
- First makes an extension of lameillipodium or filopodium
- Weakens posterior focal contacts and forms anterior ones
- myosin II used to pull/squeeze cell contents forwards
How can cells solve mazes using chemotaxis?
- Rather than use direct gradients which can be weak
- Self generated gradients formed by breaking down attractant allows cell to increase difference and thereby strengthen gradient
- Further sources are better able to replenish gradient, so degree of replenishment
How do Rho-like GTPases cause actin structures/polarization in cells?
- Rho Activates Rock and Formins
- cdc42 activates formins and ARP2/3
- Rac activates ARP2/3
- Rock induces myosin to form stress fibers
- Formins induce the formation of filopodia
- ARP2/3 leads to formation of lamellipodia
How are MTs polar?
- slower growing - and faster growing +
How does MT dynamic instability work?
- Most of the tubule is GDP tubulin which prefers a curved conformation
- Polymerization requires GTP tubulin and forms a straightening cap
- GTP to GDP hydrolysis energy stored in mechanical stress
- Release of energy causes catastrophe where the lines burst open and need to be rescued
How is rate of growth in MTs controlled?
- Rate of growth, shrinkage, frequency of catasrophe and rescue
What are the proposed models for MT growth?
- Growing with straight protofilaments that are connected
- Growing sheet like structures which roll up into a tube
- Grow as curved protofilaments which straighten by thermal fluctuations
Where does MT nucleation occur?
- y tubulin/y TuRCs is the core nucleation factor
> yTuRCs create snake like spiral of y tubulin creating rings of 13 MT protofilaments - Occures at MT organising centers (MTOCs), generally centrosome but also things like golgi
How does the centrosome provide a coordinate system for the cell?
- Is outside nucleus, only interacts when nucleus dissolved
- Exerts differential pressures to different structures/regions
- Kind of like charge source that creates differential source, like multipole?
How are centrosomes structured?
- Mature mother centriole and immature daughter centriole
- Distal appendages/protofilaments and subdistal appendages extend the centriole to point in many directions
What do kinesin 3 and 4 do?
+
- connects spindle MTs to chromosome
What does kinesin 5 do?
+
- Crosslinks MTs sliding antiparallel ones outwards
Role of kinesin 13?
- Depolymerizes at spindle poles and regulates stability
Role of kinesin 14?
- Slides MTs to poles
Role of Dynein in replication?
(-)
- Pulls MTs to poles to create pulling force
- Helps link kinetochore and MTs
Role of Kinesin 4 and 10?
+
- Attaches chromosome arms to spindle and moves to center
Role of kinesin 7?
+
- Slides unattached kinetochores towards spindle center
Role of kinesin 8?
+
- Dampens kinetochore oscillations?
Role of kinesin 13
- Depolymerises kinetochore MTs
How do flagella cause oscillations?
- Ring of MT doublets each with dyneins between eachother and the central doublet
- MTs on left or right side try to walk whilst other side inhibited
- Central dyneins keep in place, instead up or down pull push from each dynein cause up or down bending somehow
How do sperm swim?
- Not just side to side but also rotating flagella which from one perspective averages out to just side to side
- Provides better swimming through thick media
How do antagonism between kinesin 13 and MAPs control aster MT dynamics during M phase?
- MAPs stability, 13 depolymerises
- cdk inhibits MAPs for M-phase
- Astral force becomes less, so coordinate centers move more to edges
How do MTs selfassemble near DNA?
- DNA coated by beads recruiting y-TURCs
- kinesin 5 guides to kinetochore
- dynein focuses to poles
- chromokinesin(4,10) aligns DNA
How is chromatin dependent MT nucleation caused?
- RanGEF binds to chromatin
- RanGAF cytoplasmic
- Creates gradient of Ran which releases TPX2
- TPX2 recruits y-TuRC
- Augmin attaches to existing MTs, allowing y-TuRC to attach and pulls it towards poles
How is kinetochore MT dynamics regulated?
- Kinesin 7 walks along MT to keep it in place
- Dynein pulls along MT
- Kinesin 13 depolymerises MT
- So holding on to it but pulling more rope inwards
- Kind of like pulling rope with one hand and holding in place with other
What is the contractile ring made of?
- F actin and myosin
- Made where two spindles pass eachother
How is the centralspindlin formed?
- Consists of kinesin 6 and RacGAP which recruit proteins for cleavage
- Moved to ends of spindles
- Inhibited until anaphase
- Enriched at overlaping spindle ends
- Activates RhoGEF to create ring of Rho(T) contractile ring
How does RhoT fasciliated assembly and contraction of actomyosin ring?
- Recruits formin to form actin ring
- Recruits Rock to activate myosin
