Molecular motors are involved in many of the cellular
processes associated with microtubules. In nature, molecular motors
such as kinesin walk along a microtubule to deliver their cargo,
other kinesins are the driving force in spindle movement for chromosome
Microtubules in vivo are commonly 13 protofilaments. When polymerized
in vitro, microtubules may have 13, 14, 15, or 16 protofilaments.
The uncommon microtubules of 15 protofilament twist longitudinally
forming a helix This unusual property is used to examine the structure
and attachment location of motors to the microtubule surface.
In the laboratory this helix is saturated with motors using non-hydrolyzable
AMP-PNP in this way each tubulin dimer has a motor permanently
attached. This helical arrangements of attached motors
shows the motors in every possible orientation. Lengths of helical
repeats are averaged together to provide a visual representation
of the motors and microtubules.
To characterize the structure and binding of molecular motors
to microtubules we combine electron microscopy with 3-D image reconstruction
methods. The samples are imbedded in ice then examined using cryo-electron
microscopy, high magnification photographs reveal the structural
data. These images are digitized and the data is analyzed in 3-D
helical reconstruction computer programs such as Suprim or Phoelix.