Download Intracellular transport

Transport in cells
Intracellular transport
•  introduction: transport in cells, molecular players etc.
•  cooperation of motors, forces good and bad
•  transport regulation, traffic issues, …
Stefan Klumpp
image source: wikipedia
Smith, Gross, Enquist, PNAS 98: 3466 (2001)
Transport physics on different length scales
macroscopic
microscopic
intracellular
objects: cm – m
transport over. m – km
objects: µm – mm
transport over: mm – cm
objects: nm – µm
transport over µm-cm
inertia
turbulent flow
friction, viscosity
Stokes flow
fluctuations,
Brownian motion
image source: wikipedia
Transport physics on different length scales
diffusion vs. directed transport
x ≈ Dt
x ≈ vt
typical values in cells: v ~ 1 µm/s
D ~ 1 µm2/s (for proteins, less for larger cargoes,
more for small molecules)
over 10 µm: diffusion takes 100 s, directed transport 10 s
Transport in cells
microtubules
actin filaments
nucleus
myosins
kinesins, dyneins
image source: wikipedia
Cytoskeletal motors
•  fuel: ATP
•  forces: few pN
•  speeds: ~µm/s
•  motor moves in defined
direction
–  kinesin +
–  dynein –  myosin V +
–  myosin VI -
on microtubules
[Vale & Milligan 2000]
-
+
on actin
[J. Beeg]
Transport by motor teams
•  Transport along filaments of the cytoskeleton
well characterized at single-molecule level
(forces, speeds, chemomechanical coupling)
•  In cells, large cargoes often transported by
several motors
•  Bidirectional transport: different types of
motors
[Hendricks et al. Curr. Biol. (2010)]
[Ashkin et al. Nature (1990)]
Transport by motor teams
•  Transport along filaments of the cytoskeleton
well characterized at single-molecule level
(forces, speeds, chemomechanical coupling)
•  In cells, large cargoes often transported by
several motors
•  Bidirectional transport: different types of
motors
→ how are the motors coordinated ?
(tug-of-war vs. “coordination complex”)
[Ashkin et al. Nature (1990)]
?
Tug-of-war model
N+ plus motors
n+ bound
N– minus motors
n- bound
-
+
•  stochastic binding/unbinding of motors
→ force-dependent rates
•  deterministic movement of cargo
→ velocity from force balance
tug-of-war: force btw. the two motor species
Müller, Klumpp, Lipowsky, PNAS 105, 4609 (2008)
Tug-of-war model
•  stochastic binding/unbinding of motors
d
P(n+, n− ) = − (ε+ (n+, n− ) + ε− (n+, n− )) P(n+, n− )
dt
+ π + (n+ −1, n− )P(n+ −1, n− ) + π − (n+, n− −1)P(n+, n− −1)
•  deterministic movement of cargo
tug-of-war: force btw. the two motor species
→ force-dependent rates
→ force balance (both motor types move with same velocity)
Müller, Klumpp, Lipowsky, PNAS 105, 4609 (2008)
Two force scales
force-dependence of velocity and unbinding rate
" F%
v(F) = v0 $1− '
# Fs &
stall force Fs
unbinding in tug-of-war:
ε (F) = ε 0 e F/Fd
detachment force Fd
ε+ (n+, n− ) ~ exp[~ Fs / Fd+ n+ ]
→ key parameter: Fs/Fd
weak motors (low Fs/Fd):
•  little movement
•  typically n-=n+
strong motors (high Fs/Fd):
•  switching between fast plus and fast
minus movement
•  typically only plus or minus motors
bound (n-=0 or n+=0)
Müller, Klumpp, Lipowsky, PNAS 105, 4609 (2008),
Biophys. J. 98, 2610 (2010)
Tug-of-war instability
Why not only blockade?
slight predominance of plus motors
→ minus motors experience larger force
than plus motors,
are more likely to unbind
remaining minus motors experience
even larger force
Cascade of unbinding until only
plus motors left
motors must be strong enough to pull other motors off : Fs>Fd
Experimental evidence for a tug-of-war
endosomes in Dictyostelium cells:
endosome elongates during slow phase
Soppina et al., PNAS (2009)
but also: some observations point to additional biochem. regulation
e.g. lipid droplets likely to continue moving in same direction after forced
unbinding (Leidel et al., Biophys J 2012)
Strain forces between motors of one team?
motor stepping is stochastic
→ distances between motors fluctuate
→ stretching elastic elements of the motors
negative effect of forces between motors ?
new experimental systems: synthetic motor complexes
•  defined number, type and geom. arrangement
•  defined coupling
[Rogers et al. PCCP (2009), Derr et al. Science (2012)]
Strain forces between motors of one team?
explicit theoretical description of
stepping:
different interference effects with
different motor types
kinesins – enhanced unbinding
myosins – reduced velocity
Berger et al. Phys Rev Lett (2012), Cell Molec Bioeng (2013)
in agreement with experiments
Rogers et al. PCCP (2009), Lu et al. J Biol Chem (2012)
Traffic control in the cell
- without signs and traffic lights (?)
coordination without a coordinator
random bidirectional transport
effective diffusion
(but more rapid, D~v Δx)
circumvent obstacles
can be biased or steered
(modification of microtubules, MAPs)
Verhey & Hammond,
Nature Rev Mol Cell
Biol 10, 765 (2009)
self-organized traffic
limited control:
•  localized loading of motors/ activation
•  plus steering by filament modification
big question: logistics
how many motors needed? recycling of motors? motors
needed for recycling?
Traffic systems
dense traffic:
>10 years of theory (traffic jams, formation
of lanes etc.)
systematic experiments recent
[Leduc et al PNAS 109:6100 (2012)]
main difference: unbinding + diffusion
Synthetic transport
systems
detection/diagnostics
with small concentrations
typically inverted geometry
van den Heuvel & Dekker
Science 317, 333 (2007)
by geometrically or
chemically defined ‘roads’
Fischer et al. Nat Nanotech 4, 162 (2009)
Summary
•  Traffic in cells, based on molecular motors moving along
cytoskeletal filament
•  bidirectional motion: coordination by mechanics (and
biochem signaling?)
•  traffic issues: jamming etc
•  self-organized traffic, minimal external control
Thanks to
Melanie Müller, Yan Chai, Reinhard Lipowsky,
Florian Berger, Corina Keller