Bidirectional transport of proteins, vesicles and organelles along polarised tracks of microtubules is mediated by kinesin and dynein. These oppositely directed molecular motor proteins often bind cargo simultaneously, raising the question of how direction of transport is regulated. Now, Ludger Santen, Stefan Diez and colleagues (Monzon et al., 2020) address this issue by performing in vitro microtubule gliding assays to systematically analyse the effect of different parameters on bidirectional transport. They first show that directionality can be regulated by changing the number of kinesin or dynein motors, and, together with computational simulations, their data suggests that kinesin activates passive dynein. The authors then demonstrate that ATP concentration, which is known to differently regulate the stepping kinetics of the two motors, affects kinesin and dynein transport velocity in unidirectional gliding assays; surprisingly, however, changing ATP concentrations does not alter the directionality of bidirectional transport. Similarly, while kinesin and dynein have different capabilities for bypassing roadblocks, bidirectional transport is not sensitive to roadblock density. Together, these findings suggest that in the molecular tug-of-war between kinesin and dynein, factors that control the force balance between the motors are key to regulating directionality of bidirectional transport, while the stepping kinetics of the motors and their ability to bypass roadblocks are less important.
