Two motors, one transmission

Kinesin II binds to a variety of organelles and transports them toward the plus ends of microtubules—but how are the organelles linked to the motor? On page 297, Deacon et al. provide the surprising answer that kinesin II uses the dynactin complex, an attachment system that is also used by the minus end–directed motor protein dynein. The results provide the first direct physical evidence that anterograde and retrograde organelle transport are coordinated and identify dynactin as a possible master integrator for both systems.

Using Xenopus melanophores, the authors studied the bidirectional transport of melanosomes, pigment-containing organelles that are loaded with both kinesin II and dynein motors. Biochemical analysis of this system shows that the XKAP subunit of kinesin II interacts with the p150^Glued subunit of dynactin. p150^Glued cannot bind to both dynein and kinesin II at the same time, suggesting that the two motors compete for attachment to the organelle. This competition might be a novel mechanism ensuring that each melanosome recruits equal amounts of each motor, and differential phosphorylation of these motors could then determine whether anterograde or retrograde movement is dominant. Dynactin could therefore mediate organelle capture while coordinating motor selection, binding, and processivity. ▪