Mechanisms Underlying the Active Self-Assembly of Microtubule Rings and Spools

Active self-assembly offers a powerful
route for the creation of
dynamic multiscale structures that are presently inaccessible with
standard microfabrication techniques. One such system uses the translation
of microtubule filaments by surface-tethered kinesin to actively assemble
nanocomposites with bundle, ring, and spool morphologies. Attempts
to observe mechanisms involved in this active assembly system have
been hampered by experimental difficulties with performing observation
during buffer exchange and photodamage from fluorescent excitation.
In the present work, we used a custom microfluidic device to remove
these limitations and directly study ring/spool formation, including
the earliest events (nucleation) that drive subsequent nanocomposite
assembly. Three distinct formation events were observed: pinning,
collisions, and induced curvature. Of these three, collisions accounted
for the majority of event leading to ring/spool formation, while the
rate of pinning was shown to be dependent on the amount of photodamage
in the system. We further showed that formation mechanism directly
affects the diameter and rotation direction of the resultant rings
and spools. Overall, the fundamental understanding described in this
work provides a foundation by which the properties of motor-driven,
actively assembled nanocomposites may be tailored toward specific
applications.