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Kinesin Motor Transport is Altered by Macromolecular Crowding and Transiently Associated Microtubule-Associated Proteins

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.