Dynein is a poorly processive motor compared with others like kinesin. “It doesn't stay on the microtubule worth a hoot,” says King. To be efficient, dynein needs dynactin, which has its own MT-binding domains. King's group found that dynactin had not only the well-known CAP-Gly MT-binding domain, but also a neighboring basic domain that bound MTs.
When the group attached the domains to beads for single-particle tracking along MTs, the beads moved in distinctly different ways. CAP-Gly beads missing the basic sequence only swiveled around a relatively fixed point on the MT. Beads with only the basic domain or with both domains could swivel too, but they also skated the length of the MT track while still bound to it.
When the dynein motor was added to the mix, the basic domain alone or full-length dynactin increased the motor's processivity. The CAP-Gly domain alone, however, put the brakes on the motor's speed. The basic domain acts like a space-walking astronaut's tether, says King. “When dynein falls off, [the motor] can rebind and keep moving instead of floating off into the cytoplasm.”