Intracellular transport

Transcription

1 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

2 Smith, Gross, Enquist, PNAS 98: 3466 (2001)

3 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

4 Transport physics on different length scales diffusion vs. directed transport x Dt x vt typical values in cells: v ~ 1 µm/s D ~ 1 µm 2 /s (for proteins, less for larger cargoes, more for small molecules) over 10 µm: diffusion takes 100 s, directed transport 10 s

5 Transport in cells microtubules actin filaments nucleus myosins kinesins, dyneins image source: wikipedia

6 Cytoskeletal motors fuel: ATP forces: few pn speeds: ~µm/s motor moves in defined direction [Vale & Milligan 2000] kinesin + dynein - on microtubules - + myosin V + myosin VI - on actin [J. Beeg]

7 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 [Ashkin et al. Nature (1990)] [Hendricks et al. Curr. Biol. (2010)]

8 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 [Ashkin et al. Nature (1990)] how are the motors coordinated? (tug-of-war vs. coordination complex )?

9 Tug-of-war model N minus motors n - bound N + plus motors n + bound - + stochastic binding/unbinding of motors deterministic movement of cargo force-dependent rates velocity from force balance tug-of-war: force btw. the two motor species Müller, Klumpp, Lipowsky, PNAS 105, 4609 (2008)

10 Tug-of-war model stochastic binding/unbinding of motors d dt P(n +, n ) = ε + (n +, n )+ε (n +, n ) ( )P(n +, n ) deterministic movement of cargo + π + (n + 1, n )P(n + 1, n )+ π (n +, n 1)P(n +, n 1) 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)

11 Two force scales force-dependence of velocity and unbinding rate " v(f) = v 0 $ 1 F # F s % ' & ε(f) = ε 0 e F/F d stall force F s detachment force F d unbinding in tug-of-war: ε + (n +, n ) ~ exp[~ F s / F d+ n + ] key parameter: F s /F d

12 weak motors (low F s /F d ): little movement typically n - =n + strong motors (high F s /F d ): 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)

13 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 : F s >F d

14 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)

15 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)]

16 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)

17 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)

18 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?

19 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

20 Synthetic transport systems detection/diagnostics with small concentrations typically inverted geometry by geometrically or chemically defined roads van den Heuvel & Dekker Science 317, 333 (2007) Fischer et al. Nat Nanotech 4, 162 (2009)

21 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