Intracellular trafficking of organelles, driven by kinesin-1 stepping along microtubules, underpins essential cellular processes. In absence of other proteins on the microtubule surface, kinesin-1 performs micron-long runs. Under crowding conditions, however, kinesin-1 motility is drastically impeded. It is thus unclear how kinesin-1 acts as an efficient transporter in intracellular environments. Here, we demonstrate that TRAK1 (Milton), an adaptor protein essential for mitochondrial trafficking, activates kinesin-1 and increases robustness of kinesin-1 stepping on crowded microtubule surfaces. Interaction with TRAK1 i) facilitates kinesin-1 navigation around obstacles, ii) increases the probability of kinesin-1 passing through cohesive islands of tau and iii) increases 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. Furthermore, TRAK1 enables mitochondrial transport in vitro. We propose adaptor-mediated tethering as a mechanism regulating kinesin-1 motility in various cellular environments.

B CUBE Center of Molecular Bioengineering and Cluster of Excellence Physics of Life Technische Universität Dresden 01307 Dresden Sachsen Germany Max Planck Institute of Molecular Cell Biology and Genetics 01307 Dresden Sachsen Germany

Institute of Biotechnology of the Czech Academy of Sciences BIOCEV Vestec 25250 Prague West Czech Republic

Institute of Biotechnology of the Czech Academy of Sciences BIOCEV Vestec 25250 Prague West Czech Republic Faculty of Science Charles University Prague 12800 Prague Czech Republic

Institute of Biotechnology of the Czech Academy of Sciences BIOCEV Vestec 25250 Prague West Czech Republic School of Medical Science Griffith University Southport QLD 4222 Australia

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