Vesicles move into the axon on a unique set of microtubules (green).

If you've seen one microtubule, you've seen them all. Not true, according to Nakata and Hirokawa, who demonstrate on page 1045 that a unique set of micro- tubules is crucial for protein targeting in neurons.

In neurons, vesicle trafficking must be unusually precise to move proteins efficiently to the axonal plasma membrane, because the entrance to the axon is so narrow. This precision is now shown to be directed by a set of densely packed microtubules extending from the cell body into the proximal portion of the axon. These microtubules preferentially bind KIF5, the motor that carries axon-bound post-Golgi vesicles.

The unique chemical nature of the microtubules that attracts KIF5 is not yet clear. In addition to an unusually high turnover rate (the inhibition of which redistributed axon-directed transport), the population had higher levels of the tip-binding protein EB1. Although it is possible that EB1 somehow directs KIF5 binding, the authors believe this is not the case. They suggest instead that posttranslational modification of tubulin or cross-linking proteins may be involved. Although the KIF5-binding microtubule population was noted only in neurons, other cell types (e.g., motile fibroblasts or activated T cells) may use different microtubule/kinesin pairs to regulate polarized vesicle trafficking. ▪