microtubule assembly dynamics at the nanoscale
DESCRIPTION
Microtubule Assembly Dynamics at the Nanoscale. METHODS, MEASUREMENTS, AND IMPLICATIONS FOR UNDERSTANDING MICROTUBULE DYNAMIC INSTABILITY. Henry T. Schek, III European Molecular Biology Laboratory Melissa K. Gardner David Odde University of Minnesota Jun Cheng Alan J. Hunt - PowerPoint PPT PresentationTRANSCRIPT
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Microtubule Assembly Microtubule Assembly Dynamics at the Dynamics at the
NanoscaleNanoscale
Henry T. Schek, IIIHenry T. Schek, IIIEuropean Molecular Biology LaboratoryEuropean Molecular Biology Laboratory
Melissa K. GardnerMelissa K. Gardner David OddeDavid Odde
University of Minnesota University of Minnesota
Jun ChengJun ChengAlan J. HuntAlan J. Hunt
University of MichiganUniversity of Michigan
METHODS, MEASUREMENTS, AND IMPLICATIONS FOR UNDERSTANDING MICROTUBULE DYNAMIC INSTABILITY
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BME Department
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Microtubules
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Dynamic Instability
From Fygenson et al, Phys Rev E, 1994
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BME Department
Microtubule Polymerization in Cells
• Works with actin to guide an axonal growth cone
• Kinetochore attachments
• Push on mitotic chromosomes arms
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Why Study Polymerization Under Load?
• Quantify forces
• Study microtubule dynamics with nanometer resolution
• Improve models of dynamic instability
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BME Department
Optical Tweezers
Cellular & Molecular Biomechanics Lab
Optical Tweezers Features• Extremely stable (<1 nm drift/min)• Multiple Independently Maneuverable Traps• Sub-Nanometer Detection and Manipulation• Forces from Less than one pN to greater than 100 pN• Servo-control for Force Clamp or Position Clamp
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Experimental Strategy
Tightly Focused Trapping Laser
Trapped Particle
Attached Microtubule Seed
Barrier on cover glass
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Barrier Design
• Vertex• Undercut• Laser footprint • LOR, SU-8
• No interference• No image
degradation• Constrain MT• Short MT
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Barrier Fabrication
Primary structure and undercut fabrication are independent
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Barrier Results
Scale bar=14 m Scale bar=2 m
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What an Experiment Looks Like
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Stationary Trap Results
• Microtubule growth is highly variable, and exhibits pauses over a broad range of forces, even in a single microtubule• F-V relation is complex
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Stationary trap underestimates filament displacement
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Force Clamping
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Force Clamp-MT length change
1.6 1.4 1.3 1.1 0.9 0.7Clamped Force (pN)
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Large Growth Rate VariabilityM
icro
tubu
le L
engt
hC
hang
e (n
m)
Close to zero = 3% (speed < 1 nm/s)
Growth= 55%
Shortening=41%
Growth rates{
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Variability on Longer ScalesM
icro
tubu
le L
engt
hC
hang
e (n
m)
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Nano-Shortening Events
Mic
rotu
bule
Len
gth
Cha
nge
(nm
)
9/minute, do not lead to rapid shortening
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Summary so far
• Forces greater than 1 pN - plenty to influence chromosome movements*
• Large growth rate variability
• Frequent shortening events > 20 nm
• Persistent velocities at longer times scales
*Joglekar & Hunt. Biophys. J. (2002) Marshall et al. Curr. Biol. (2001)
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GTP-Cap Hypothesis
• Grows with GTP-cap
• Occasionally loses cap
• Rapid Shortening results
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1) Bayley et al., FEBS Lett., 1989; Bayley et al., J. Cell Sci., 1990; O'Brien et al., Biochemistry, 1987; Panda et al., Biochemistry, 2002; Stewart et al., Biochemistry, 1990
2) Vandecandelaere et al., Biochemistry, 1999; Voter et al., Cell Motil. Cyto., 1991
Trouble for the GTP cap?
It has been widely argued that the GTP-cap is at most one layer thick1, or slightly larger2 (e.g. lateral cap
hypothesis)
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A microtubule that shortens sufficiently to lose more than one layer of tubulin subunits (~ 8 nm) will transition to rapid shortening.
This is contradicted by our results.
If the GTP-cap is one layer thick,
then:
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D
Can these results be explained by a mechanochemical model?
VanBuren, Cassimeris & Odde, Biophys. J. 2005
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This…
Or this?
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No
Yes
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Ave. = –9.7 nm
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Ave. = –6.4 nm < -9.7 nm (p < 10-6)
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*Nature, 2006
In contrast to the conclusions of Kerssemakers et al*, no evidence of oligomer addition
Resolution is sufficient to detect addition of individual subunits.
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Step-like events when data is processed in a manner similar to
Kerssemakers et al
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What’s a step?
+ XMAP215
Time
Dis
pla
cem
ent
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Growth rate depends more on the evolving tip structure than force.
*
*S.E. < 0.1 nm/sec
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Conclusions• At the nanoscale, microtubule growth is
highly variable• Frequent shortening events: as large as
80 nm, > 30 nm (2 layers) @ 8/min, > 40 nm @ 1/min ]
• Oligomer addition occurs rarely, if ever• Average growth rate is weakly dependent
on force, strongly dependent on tip structure
• Shortening excursions are smaller at higher forces
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Conclusions II
• Finding consistent with a physically simple mechanochemical model, which explains:– Unexpected growth-phase shortening– Smaller shortening excursions at higher force– Weak force dependence of average growth rate
• Reject other models for microtubule polymerization– Small cap induced hydrolysis models (i.e. “lateral
cap”)– “Coupled hydrolysis” models (Flyvbjerg et al, Phys.
Rev. Lett., 1994; Phys. Rev. E, 1996)
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Acknowledgements
• NSF
• Whitaker Foundation (to H. Schek)
• Burroughs Wellcome Fund