  Cytoskeletal active nematics exhibit striking non-equilibrium dynamics that
are powered by energy-consuming molecular motors. To gain insight into the
structure and mechanics of these materials, we design programmable clusters in
which kinesin motors are linked by a double-stranded DNA linker. The efficiency
by which DNA-based clusters power active nematics depends on both the stepping
dynamics of the kinesin motors and the chemical structure of the polymeric
linker. Fluorescence anisotropy measurements reveal that the motor clusters,
like filamentous microtubules, exhibit local nematic order. The properties of
the DNA linker enable the design of force-sensing clusters. When the load
across the linker exceeds a critical threshold the clusters fall apart, ceasing
to generate active stresses and slowing the system dynamics. Fluorescence
readout reveals the fraction of bound clusters that generate interfilament
sliding. In turn, this yields the average load experienced by the kinesin
motors as they step along the microtubules. DNA-motor clusters provide a
foundation for understanding the molecular mechanism by which nanoscale
molecular motors collectively generate mesoscopic active stresses, which in
turn power macroscale non-equilibrium dynamics of active nematics.
