Figure 4. Model for establishment of hair bundle polarity by microtubule-mediated hair cell-intrinsic machinery.

Left, en face diagram of an individual hair cell showing the distribution of proteins implicated in basal body positioning, colored according to the key. The nascent hair bundle is colored pink with the basal body (gray circle) at its vertex. Centriolar microtubules are depicted as blue lines. Right, magnified view of the boxed area on the left. During development, cortical LGN/Gαi recruits LIS1/dynein to exert force on centriolar microtubules, pulling the kinocilium/basal body toward the lateral hair cell cortex. Interaction of microtubule plus-ends with the hair cell cortex stimulates Rac-PAK signaling, likely through delivery of a kinesin-II cargo. In turn, localized Rac-PAK activity stabilizes microtubule cortical attachment to position the basal body appropriately. Plus and minus signs indicate the plus-and minus-end of the microtubule, respectively.