Thesis-PDF - IAP/TU Wien

4.15), their ends are termed the (-) and the (+) end, and there exist protein
families that can move along the tubes in either of these directions. The two
fascinating protein antagonists are kinesins and dyneins (see Fig. 4.15). These
proteins are capable of moving along a microtubule and, since the tubule has a
defined direction, kinesins always move in the (+) direction and dyneins usually
move in the (-) direction. Even more remarkable is that they are able to move
attached masses. Alone they can move single vesicles, but when acting in unison
it is possible to move direct whole structures!
Together with their motor proteins MTs are essential in molecular cell architecture
for providing transport highways and a mechanism generating relative motion.
Mechanical properties of MT have been described by measuring bending stiffness
with Laser Tweezers, AFM, hydrodynamic flow and buckling in vesicles. Measured
values correspond to Young’s moduli between 1 MP a (rubber) and 7 MP a
(plexiglass). ([102])
Figure 4.14: Schematic drawing of microtubule architecture and how it is
assembled into a flagellar structure (9 doublets of microtubuli forming the
outer wall, a pair is inside). This kind of structure is not only found in alga
but also e.g. as motory flagellum of human sperm. The single microtubule
would have its (-) end on top, its (+) end at the bottom, depending on the
orientation of the heterodimers building the tubule. α−tubulins always face
the (-) end, β−tubulins the (+) end. Image adapted from [102].
Conventional kinesin (or simply ‘kinesin’) is the best known member of the
KinNs (one of the three known families of kinesins)
• Kinesin walks on MT to the (+) end in steps of 8.3 nm per hydrolyzed ATP
(see below). Each second such a motor can complete 100 ATP turnovers and
walk 800nm. A single powerstroke of the motor can generate a force of 6pN.
([103])
• Kinesin is a processive motor, i.e. it does not dissociate from MTs during
walking.
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