The distinction was noted while visualizing a constitutive version of the Kinesin-1 motor, which in mature neurons is found only in the axon. Because the plus ends of axonal microtubules point away from the cell body, whereas those in dendrites face both directions, Kinesin-1's plus end–directed activity could explain this specific accumulation. But the new results show that Kinesin-1 is found in a subset of young neurites, often in just one, at a time when the microtubules in all neurites are still similarly oriented.
The accumulation of Kinesin-1 in a given young neurite was transient—it periodically switched homes until the time at which the axon was specified. With each switch, only one or two neurites contained the motor at a time. To explain the preference of Kinesin-1 for a given neurite, Banker suggests, “the population of microtubules [in that neurite] is distinct. But I'm pretty mystified as to what that difference is.”
One of his theories suggests that tubulin is transiently modified posttranslationally in certain neurites. Because Kinesin-1 has a relatively low affinity for microtubules but travels far with each binding, localized modifications that increase its affinity even slightly might strongly favor its accumulation in that neurite.
Stable accumulation of truncated Kinesin-I within a neurite coincided with that neurite's specification as the axon and is now the earliest marker of this event. But, says Banker, “every time you see one of these molecular distinctions, it takes the question one step higher, to ‘what led to that?’” This new question will now haunt neurobiologists, at least for the near future.