Microtubule (MT)-associated proteins (MAPs, such as tau and MAP9) regulate kinesin- and dynein-driven intracellular transport on MT tracks. The molecular mechanisms of this MAP-dependent regulation remained elusive because MAP-motor contacts are transient, heterogeneous, and often mediated by intrinsically disordered segments. Here, we combine all-atom molecular dynamics (MD) simulations with cryo-EM and functional assays to address this question. We show that tau inhibits kinesin-1 motility by overlapping with its tubulin binding site. In comparison, tau and dynein can bind to the same tubulin, but the proximity of tau to the dynein binding site destabilizes electrostatic contacts required for strong binding of dynein to MTs. Unlike tau, MAP9 does not overlap with the kinesin binding site, yet it distinguishes between kinesin-1 and kinesin-3 through a divergent loop-8 of the kinesin motor domain. Simulations showed favorable electrostatic and hydrophobic interactions of MAP9 with loop-8 of kinesin-3 but repulsive interactions with that of kinesin-1. Loop-swap experiments confirmed loop-8 as the determinant of MAP9 selectivity, restoring kinesin-1 motility on MAP9-decorated MTs. By resolving transient interactions between MAPs and motors on the MT, MD simulations provide a mechanistic link between MAP binding modes and emergent transport behaviors.
Mert Gür (Sun,) studied this question.