Intermediate filaments
~10nm
- Not directly involved in cell movement
- Enable cells to withstand the mechanical stress that occurs when cells are stretched
- Form network through cytoplasm, surrounding nucleus & extending into cell periphery, anchored to plasma membrane
- Indirectly connected to neighboring cells through desmosomes
- Each monomer has an extended, central α-helical domains & unstructured corboxy & amino terminal domains
- 2 monomers wrap around each other in parallel using their α-helical domains to form a coiled-coil dimer
- 2 dimers associate in a staggered & anitparallel fashion to form a tetramer. This means a mature IF will not have polarity
- 8 tetramers laterally associate & are added to growing If. No nucleation involved & IFs build onto existing IFs. IF network not dynamin
- Mature xytoplasmic IFs have rope-like structure
- Form nuclear lamina
Microtubules
~25nm diameter
- Dynamic structures that continually undergo assembly & disassembly
- Rigid, hollow rods
- Functions
1.) Separation of chromosomes in mitosis
2.) Intracellular transport of membrane-bound vesicles & organelles
3.) cell movement
- Highly dynamic. Dynamic nature dictated by ability of GTP to bind to tubulin
- β-tubulin binds GTP & this form of dimer is incorporated into MT. On GTP hydrolysis, interaction with incoming dimer is weakened. If concentration of GTP-tubulin is high, association of dimers w/ MT will be faster than rate of GTP hydrolysis & MT will grow. If GTP-tubulin concentration is low, then rate of GTP hydrolysis will exceed rate of dimer addition & MT will shrink
- Extend outward from a microtubule organising center (MTOC). (-) end of MT anchored to MTOC
- In animal cells major MTOC called centrosome, located next to nucleus
Tubulin
- Heterodimer consisting of 2 closely related proteins, α-tubulin-β-tubulin
- Dimers stack together through α-tubulin-β-tubulin interactions to form a protofilament. Gives filament polarity:
- α-tubulin exposed to (-) end
- β tubulin exposed at opposite (+) end.
- (+) end considered growing end.
- Polarity plays role in determining direction of movement along MTs
- 13 such protofilaments arrange in a tube structure to give the microtubule
Centrosome
- 2 centrioles: Unique arrangement of MT protofilaments at right angles to each other
- Perinuclear material: An amorphous collection of several proteins
- γ-tubulin ring complexes - composed of a special form of tubulin (γ-tubulin) & accessory proteins. Nucleation site for MT assembly
Kinesins
- Move towards (+) end of a microtubule
Dyneins
- Move towards (-) end of a microtubule
Actin filaments
- Actin filaments are highly concentrated at the periphery of cell, where they form a 3-dimensional network underlying plasma membrane
- Made of protein actin
- Actin exists as globular monomer (G-actin) & filamentous polymer (F-actin)
- Actin monomers (G-actin) polymerise to form actin filaments (F-actin)
- Have appearance of double-stranded helix
- Have distinct polarity at their ends ((+) & (-)).
- New monomers added to (+) end of filament
Cofilin
- Key protein for dissassembly of actin filaments
- Binds to actin filaments & increases rate of dissassociation of actin monomers from (-) end
- Remains bound to ADP-actin monomers, preventing their reassembly into filaments
Profilin
- Actin-binding protein
- Stimulate incorporation of actin monomers into filaments
- Stimulates exchange of bound ADP for ATP, resulting in formation of ATP-actin monomers
- Profilin-bound ATP-actin monomers can be re-polymerised into filaments
- Cofilin-profilin activity balance determines state of actin polymerisation
Myosin
- Bind & hydrolyse ATP, provides E for their movement along actin filaments from (-) to (+) ends of actin filaments
Myosin I
- Myosin I found in all types of cells
- Single molecules w/ 1 globular head & a tail that attaches to another molecule / organelle in cell
- Attached molecule / organelle can be moved along filament by motor activity of myosin I head
- Head domain interacts w/ actin filaments & has ATP-hydrolysing motor activity enabling it to move along filament
- Tail varies between different types of myosin I & determines what cell components will be movevd along by motor
- Can move vesicles along actin filaments towards (+) end
- Can attach plasma membrane to cortical actin filament network, pulling plasma membrane into different shape
Myosin II
- Myosin II most abundant in muscle
- Dimer w/ 2 globular heads & a tail that forms coiled-coil structure
- Associate through coiled-coil tails forming a myosin II filament
- Head domain interacts w/ actin filaments & has ATP-hydrolysing motor activity enabling it to move along filament. Myosin Ii causes actin to contract, happens in all muscle cells
