Organelle cargo distribution is similar in wild-type cells (left) and cells expressing nonmicrotubule-binding dynactin (right), indicating that transport remains intact.

Dynactin might bind to and organize microtubules. But it doesn't need to bind microtubules to jump-start dynein motoring, Kim et al. report on page 641.

Dynactin retrieves microtubule motors such as dynein from the cytoplasm and docks them onto their cargo. Dynactin also anchors the minus ends of microtubules on the centrosome.

One dynactin isoform that is found in human neurons lacks its microtubule-binding domain (MBD). Kim and colleagues thus supposed that dynactin might be functional even without locking onto microtubules. They found that indeed cargo transport does not rely on the MBD.

With or without a functional MBD, dynactin helped move four different physiological cargos, including vesicles and proteins, just as far along microtubules and at the same velocity and frequency.

Dynactin's MBD was previously thought to increase dynein's processivity by giving it a second hand to hang onto microtubules. But this idea was based largely on studies of the transport of dynein-coated beads. This artificial cargo, or the way dynein and dynactin were attached to it, might have altered the normal relations between dynein and dynactin observed in a cell.

Not all dynactin activities were unaffected by the loss of the MBD, however. Cells expressing the MBD-deleted form commonly arrested in prometaphase with multipolar spindles, suggesting a failure of microtubules to correctly conjoin in centrosomes and thus a role of dynactin in latching together microtubules.