Understanding kinesin and dynein mechanochemistry underlying bidirectional transport

Bidirectional cargo transport in neurons involves competing activity of kinesin and dynein motors attached to a shared cargo, but the mechanisms by which these antagonistic motors coordinate and compete are not well understood. We are investigating these motor coordination mechanisms using reconstituted pairs of kinesin family and dynein-dynactin-BicD2 (DDB) complexes, which are visualized by total internal reflection fluorescence microscopy. To interpret the underlying mechanochemical mechanisms, we developed stochastic models of motor stepping that include load-dependent stepping and detachment rates obtained from optical tweezer experiments. The models are tuned by comparing the distribution of instantaneous velocities between the model and experiments. These efforts suggest that both kinesin and dynein motors have load-insensitive or even catch-bond characteristics in this geometry. Moreover, the data are best explained by the motors reattaching very rapidly folliowng any detachment. In other experiments we are investigating the catch-bond behavior of these motors in geometries where vertical loads inherent into traditional one-bead optical tweezers experiments are eliminated. These experimental and computational studies reveal new features of kinesin and dynein motors that play key roles in bidirectional transport.