Cytoskeleton A dynamic structural and functional framework Three types of filaments: A. Intermediate B. Microtubules C. Microfilaments Cellular distribution of intermediate filaments and microtubules is similar
Polimerization: an example Three phases: 1. Lag phase: nucleation 2. Elongation 3. Equilibrium
Equilibrium 1. Dynamic equilibrium 2. Dynamic unstability: slow elongation followed by rapid (catastrophic) depolymerisation 3. ‘Tread-milling’
- Intrinsic flexibility -Thermal (entropy) flexibility (persistence length) A = persistence length F Z = end-to-end distance L c = contour length Polymer mechanics Bending stiffness: F Longitudinal stiffness: F Torsion: F Mechanism: The direction of force:
Functions of Microfilaments Actin filaments are concentrated beneath the plasma membrane (cell cortex) and give the cell mechanical strength. Assembly of actin filaments can determine cell shape and cause cell movement. Association of actin filaments with myosin can form contractile structures.
What kind of molecular motions are responsible for cell locomotion?
Movement Subcellular, cellular levels Requires ATP (energy conservation) Cytoskeleton-mediated Assembly and disassembly of cytoskeletal fibers (microfilaments and microtubules) Motor proteins use cytoskeletal fibers (microfilaments and microtubules) as tracks
~25nm thick, tube shape 13 protofilaments Right hand, short helix Left hand, long helix Stiff polymer chain (persistence length: a few mm!) Structural polarization: + end: rapid polymerization, - end: slow polymerization GTP-cap see ‘search and capture’
The monomer is not globular, a fiber! Tissue specific IF types Nuclear laminsA, B, C lamins (65-75kDa) Vimentin typeVimentin (54kDa) Desmin (53kDa) Peripherin (66kDa) KeratinsType I (acidic) (40-70kDa) Type II (neutral/basic) (40-70kDa) Neuronal IFneurofilament proteins (60-130kDa)
The subunit of filaments: „coiled-coil” dimer Vimentin dimer
Polymerisation of IF protofilamentum filamentum Polymerised in cell lack of dynamic equilibrium Central rods ( -helix) hydrofob-hydrofob interactions -> colied-coil dimer 2 dimer -> tetramer (antiparallel structure) Tetramers connected longitudinally -> protofilaments 8 protofilaments -> filament
Cytoskeleton associated proteins Many families of proteins which can bind specifically to actin A. According to filaments 1. Actin-associated (e.g. myosin) 2. MT- associated (e.g. Tau protein) 3. IF- associated B. According to the binding site 1. End binding proteins (nucleation, capping, pl. Arp2/3, gelsolin) 2. Side binding proteins (pl. tropomyosin) C. According to function 1. Cross-linkers a. Gel formation (pl. filamin, spectrin) b. Bundling (pl. alpha-aktinin, fimbrin, villin) 2. Polymerization effects a. Induce depolymerization („severing”, pl. gelsolin) b. Stabilizing (pl. profilin, tropomiozin) 3. Motor proteins
Polystyrene beads of different diameters (0.5, 1, 3µm) have been functionalized with N-WASP and placed in a reconstitued motility medium containing actin, Arp2/3 complex, ADF/Cofilin, gelsolin (or any capping protein) and profilin. In vitro model
Formins (Manuelle Quinoud) A proposed mechanism from S. Zigmond.
1.They can bind to specific filament types 2. They can travel along filaments 3. They hydrolyze ATP Motor proteins
1. Actin-based: myosins Conventional (miozin II) and nonconventional myosins Myosin families: myosin I-XVIII 2. Microtubule based motors a. Dynein Flagellar and cytoplasmic dyneins. MW~500kDa They move towards the minus end of MT b. Kinesin Cytoskeletal kinesins Neurons, cargo transport along the axons Kinesin family: conventional kinesins + isoforms. MW~110 kDa They move towards the minus end of MT 3. Nucleic acid based DNA and RNA polymerases They move along a DNA and produce force Types of motor proteins
Motor proteins “Walk” or slide along cytoskeletal fibers Myosin on microfilaments Kinesin and dynein on microtubules Use energy from ATP hydrolysis Cytoskeletal fibers: Serve as tracks to carry organelles or vesicles Slide past each other
1. Structure N-terminal globular head: motor domain, nucleotide binding and hydrolysis specific binding sites for the corresponding filaments C-terminal: structural and functional role (e.g. myosins) 2. Mechanical properties, function In principle: cyclic function and work Motor -> binding to a filament -> force -> dissociation -> relaxation 1 cycle requires 1 ATP hydrolysis They can either move (isotonic conditions) or produce force (isometric conditions) Common properties N C
The working cycle of motor proteins Duty ratio: In vitro sliding velocity: Cycle time:Attached time: attached on detached off ATP cycle power stroke back stroke attachment detachment = working distance =working distance (or step size); V=ATPase activity; v=In vitro sliding velocity
Processivity and the duty ratio Processive motor: r->1 pl. kinesin, DNA-, RNA-polimerase the motor is attached to the track in most of the working cycle Nonprocessive motor: r->0 pl. conventional myosin A motor protein can produce force in the pN range. =working distance or step size V=ATPase activity v=in vitro motility velocity