Chemistry Reference
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mechanical steps up to forces of 7
-
8 pN [155]. FIONAexperiments with quantumdots
bound to the dynein tail gave mostly 8-nm steps [148, 155], but when quantum dots
were bound to the head at the C-terminus, they showed predominantly 16-nm and
larger steps [148]. These results suggest an alternating leading
-
trailingmechanism as
with the other motors. The dynein rings (13
-
15 nm diameter) are larger than the
tubulin dimers (8.3 nm) however, making direct binding of the two heads to adjacent
dimers along a proto
lament unlikely. Flexibility of the stalk and the tendency to take
variable sideways and backward steps resemble the motions of myosin VI.
Organelle motion in cells is bidirectional with single particles often reversing
their direction. The destinations of vesicular cargos vary with physiological stimuli.
Thus the relationship between molecular motors directed toward the cell periphery,
e.g. kinesin and myosin V, and those mainly moving toward the center, such as
dynein and myosin VI is an important question in cell biology. Inhibition of dynein
in cells disrupts both directions of transport [59, 156, 157], suggesting a close
cooperation or interaction between dynein activity and plus-end directed kinesin
transport. This interaction might be mediated by the dynactin accessory
complex [158].
When the dynactin complex was made
fluorescent in a transgenic mouse, the
puri
ed dynein
-
dynactin complex showed strong ATP-dependent bidirectional
motion along microtubules (Figure 3.14). In an equivalent processivity assay,
kinesin moved unidirectionally with a regular stepping rate (Figure 3.14). Thus,
Figure 3.14 Traces of motion exhibited by
dynein
micrograph of the rhodamine-labeled
microtubule. Scale bar: 4
GFP as visualized by TIRF.
Top row: video images at 200 ms intervals,
10 mM ATP. The filled arrowhead indicates the
starting position of the complex. The open
arrowhead indicates the location at each time
point. The far right image is an epifluorescence
—
dynactin
—
m
m. Lower row a-e,
kymographs fromdynein
GFP time-
series show reversal of direction with long
processive motion in each direction. f, kinesin-
GFP motility is much steadier. From Ref. [159].
—
dynactin
—
