Chemistry Reference
In-Depth Information
Figure 3.13 Neck linker positions in dimeric
kinesin-1 with both heads bound to a
microtubule. A, In this model, the neck linker in
the leading head points rearward and the one in
the trailing head, docked onto themotor domain,
points forward. Cys
215
and Cys
342
, labeled in one
chain of the dimer (red spheres), are expected to
be farther from each other in the leading head
than in the trailing head. B, time traces of donor
(Cy3, blue) and acceptor (Cy5, red) fluorescence
intensities (arbitrary units), calculated FRET
efficiency (E
FRET
, green) and axial displacement
of the fluorophore (purple), moving along
axonemes at 0.5 or 1
M ATP. Image sampling
rate: 15 frames s
1
. Black lines show running
averages or linear fits. Vertical dotted lines mark
anticorrelated FRET changes. The red arrow
indicates photobleaching of the acceptor dye;
black arrow indicates photobleaching of the
donor dye or detachment from the axoneme.
From Ref. [123].
m
processive motility. PolTIRF measurements of the angle of the neck linker and its
rotational mobility also showed variations during motility consistent with the neck
linker model [124]. The free energy liberated when this short peptide binds to the rest
of the head, however, is insuf
cient to account for the
6 pN)
mechanical work kinesin can perform per ATPase cycle [125] and other steps such
as binding of the free headmust contribute to generating force andmotion. Evidence
for another interpretation of the data, that ATP binding causes increased mobility of
the attached head rather than docking of the neck linker, is summarized in [126].
As discussed for myosins V and VI, the high processivity of kinesin requires a
mechanism to gate the biochemical reactions of the two heads andmaintain themout
of phase. As in myosin, intramolecular strain between the two heads of kinesin
bound to the microtubule alters the af
nity of the heads for ADP [127] and prevents
premature ATP binding to the leading head [128
-
131]. These characteristics are
similar to the strain-dependent gating of myosin VI [115].
48 zJ (8 nm
3.3.5
Other Kinesins
The other kinesin family members have diverse and interesting mechanisms
and cellular roles. As mentioned, in conventional kinesins, the motor domain is
