Abstract

Intracellular trafficking of organelles, driven by kinesin-1 stepping along microtubules, underpins essential processes including neuronal activity. In absence of other proteins on the microtubule surface, kinesin-1 performs micron-long runs. Under protein crowding conditions, however, kinesin-1 motility is drastically impeded. It is thus unclear how kinesin-1 acts as an efficient transporter in crowded intracellular environments. Here, we demonstrate that TRAK1 (Milton), an adaptor protein essential for mitochondrial trafficking, activates kinesin-1 and increases its robustness of stepping in protein crowding conditions. Interaction with TRAK1 i) facilitated kinesin-1 navigation around obstacles, ii) increased the probability of kinesin-1 passing through cohesive envelopes of tau and iii) increased the run length of kinesin-1 in cell lysate. We explain the enhanced motility by the observed direct interaction of TRAK1 with microtubules, providing an additional anchor for the kinesin-1-TRAK1 complex. We propose adaptor-mediated tethering as a mechanism regulating kinesin-1 motility in various cellular environments.