  Intracellular transport of vesicular cargos, organelles, and other
macromolecules is an essential process to move large items through a crowded,
and inhomogeneous cellular environment. In an effort to dissect the fundamental
effects of crowding and an increasingly complex cellular environment on the
transport of individual motor proteins, we have performed in vitro
reconstitution experiments with single kinesin-1 motors walking on microtubules
in the presence of crowding agents and transient microtubule-associated
proteins that more closely emulate the cellular environment. Macromolecular
crowding due to inert polymers caused enhanced run lengths of motors, but
displayed an increased tendency for non-specific motor association and
diffusion, most likely due to depletion interactions. We found that transiently
bound associated proteins slowed forward motion, but did not drastically affect
the association times, in opposition to previously reported obstacle properties
of stably associated microtubule-associated proteins, such as the neuronal
protein tau. Such studies of the transport properties of molecular motors in
increasingly complex reconstituted environments are important to illuminate the
fundamental biophysical principles underlying the essential process of
intracellular cargo transport.
