Chemistry Reference
In-Depth Information
Figure 3.9 Processive movement of individual
brain myosin V molecules along actin filaments.
A, successive video images at 0.5-s intervals.
A light spot at the left-hand side ismoving toward
the stationary light spot at the actin-filament end.
Scale bar: 2
becomes brighter with time due to accumulated
motors. Single myosin V molecules produce
moving spots on the filament. Scale bar: 2 mm. c,
a video image showing bright spots at the barbed
end of filaments after 1 min of incubation with
the myosin V. From Ref. [62].
m. b, video images at successive
2-s intervals. The barbed end of the actin filament
m
hand-over-hand mechanism in which both heads can bind to actin and a step is
accomplished by the trailing headmoving past its partner to become the new leading
head (Figure 3.10A). When the arm containing the
fluorescent probe is the stepping
one, its displacement is the larger value and when the other armmoves, the probe is
displaced by a smaller value.
In later studies adding angular detection to the lateral motion detection of FIONA,
the three classes of molecule were determined to give alternating 64-10-64, 50-25-50
and 44-30-44 nm step displacements of the probe (DOPI: [56]; partial polTIRF: [73]).
These values are just what would be expected if the molecule moves 36
-
37 nm per
step and the labeled CaM exchanges into the second, fourth and sixth positions away
from the motor domain (Figure 3.10A). A distance of 36 nm is the half-pitch of actin
(Figure 3.8), allowing the molecule to walk relatively straight along the actin axis,
rather than twirl in the tight helical path de
ned by the actin monomers.
When a GFP
fluorophore was engineered into the head of myosin V, the steps
alternated between 74 nm and virtually zero, also as expected [74]. With two different
colored probes simultaneously exchanged into various pairs of locations inmyosin V,
the two arms are shown directly to swap between leading and trailing positions along
the actin axis (Figure 3.10B, also [75, 76]). These FIONA experiments provide strong
