How does Intracellular MolecularMotor-Driven Transport Work?
Collaboration between the groups of
Clare Yu2 and Steve Gross1,2
1Department of Developmental and Cell Biology
&
2Department of Physics
University of California, Irvine
Intracellular Traffic
 How is intracellular
transport regulated?
© Scientific American
Filaments
Actin filament
• 10 nm diameter
• 2.77 nm rise
• 26 subunits/74 nm repeat
- end
+ end
+ end
Microtubule
 25 nm diameter
 13 protofilaments
- end
Motor proteins
Myosin
Kinesin
Motor
proteins
move cargo
along
filaments
Molecular Biology of the Cell, 3rd Ed, 1994
Biochemistry, 4ht Ed, 1995
Microtubules (MT)
are like freeways and
actin filaments are
like local surface
streets.
How does the cell regulate the transport of vesicles?
Model System:
Melanophores (Pigment granule cells)
• Cells
change color by
Dispersing or Aggregating
pigment granules
• granules move bi-directionally along
microtubules
(Kinesin-II and Dynein)
•Granules also move along actin filaments
(Myosin-V)
Dispersion:
MTActin
Aggregation:
ActinMT
Experiment: Cargos traveling solely on actin filaments
go farther during dispersion than during aggregation.
Do cargos go faster during dispersion? No, the velocity of
the cargos is the same for aggregation and dispersion.
(Explanation lies in collective motion rather than in
understanding individual molecular motor operation.)
Why do cargos go
farther during
dispersion than
during aggregation?
• During dispersion cargos go “straight” to the end of the
filament and do not turn at intersections with other
filaments. This is good for spreading out pigment
granules uniformly.
• During aggregation cargos have a 50-50 chance of
switching to another filament at each intersection. So
they don’t go as far. Frequent switching is a good way
to find a microtubule.
Two Types of Theoretical Modeling
Confirm This Scenario
• Langevin solution interpolates between
short time ballistic (straight-line) motion
and long time diffusive motion.
• Computer simulations of cargos moving
along actin filaments also confirms this
picture.
Solution to Langevin Equation
t /

r (t )  D t   1  e 
2
• Langevin solution interpolates between
short time straight line motion and long
time diffusive motion.
• Fitting displacement data yields D and 
which can be used to obtain the mean free
path ℓ (distance traveled before turning).
• The mean free path is given by
 D / 2
Langevin Fits Yield Mean Free Path
• Dispersion: <ℓ> = 810 ± 8 nm
• Aggregation: <ℓ> = 225 ± 4 nm
Compare Langevin with Electron
Micrographs of Actin Filaments
Dispersion: Langevin ℓ > L/2 where L≈1300 nm is a typical
filament length implying cargos go to end of filament
Aggregation: Langevin ℓ ≈ 1.5 d where d≈160 nm is the
typical distance between filament intersections consistent
with cargos switching with 50% probability at intersections
Electron Micrographs of Actin Filaments
The actin filaments appear denser during aggregation
which would encourage frequent switching from one
filament to another. (Not enough EMs to confirm this.)
Simulations of Cargos Moving on
Actin Filaments
• Density of filaments taken from EMs
• Distribution of filament lengths taken from
EMs
• For aggregration, switching probability is
50% at filament intersections
• For dispersion, cargos go to end of filament
and then attach to a new filament
• Result: Average mean free paths agree with
EMs and Langevin, confirming scenario
Simulations of Cargos Moving
Along Actin Filament Networks
Trajectories more localized
Trajectories more spread out
Cargo displacement after 30 sec from
simulations
•Cargos are more
localized during
aggregation
•Cargos are more
evenly spread out
during dispersion
SUMMARY:
We have explained how and why
cargos go farther during dispersion
than during aggregation
• During dispersion cargos go “straight” to the end of the
filament and do not turn at intersections with other
filaments. This is good for spreading out pigment
granules uniformly.
• During aggregation cargos have a 50-50 chance of
switching to another filament at each intersection. So
they don’t go as far. Frequent switching is a good way
to find a microtubule.
Possible Way that the Switching
Probability Is Regulated
• During aggregation, there are about 60
motors per cargo, but only one active motor
pulls a cargo along a filament. Another
motor can attach to a nearby filament and
cause a switch to the new filament.
• During dispersion, there are about 90
motors per cargo, but only 2 active motors
pull a cargo. Another motor may try to
attach to another filament but it is not strong
enough to cause a switch to a new filament.
Collaborators
• Joseph Snider (Physics Dept., U.C. Irvine)
(Langevin and simulations)
• Francis Lin (Physics Dept., UC Irvine) (data
analysis)
• Neda Zahedi (King’s College London and U.
Conn. Health Sci. Ctr.) (experiments)
• Vladimir Rodionov (U. Conn. Health Sci.
Ctr.) (experiments)
• Website: http://bioweb.bio.uci.edu/sgross/
Quantification of motion
• Particle tracking: 8nm resolution, 30 Hz
• Analysis: Displacement vs. time R (t)
random motion