Phosphatidylinositol 3,4,5-trisphosphate (PIP3), the product of phosphatidylinositol 3-kinase (PI3K), contributes to the establishment of cell polarity. Horiguchi et al. provide evidence that PIP3 not only is produced by local activation of PI3K at the plasma membrane but also may be produced at internal membranes and transported by PIP3-containing vesicles on microtubules to the growing tips of neuronal projections. First, they determined that GAKIN (guanylate kinase-associated kinesin, also known as KIF13B) interacted with PIP3 binding protein (PIP3BP, also known as centaurin-α) through the GAKIN FHA domain. In vitro GAKIN and PIP3BP mediated the movement of PIP3 liposomes on microtubules. In PC12 (pheochromocytoma 12) cells and culture hippocampal neurons, tagged GAKIN, tagged PIP3BP, and a marker for PIP3 were colocalized at the tips of neurites; in hippocampal cells, these three molecules were most abundant in the longest neurite, the axon. Overexpression of a dominant-negative form of GAKIN (with the motor domain deleted) in PC12 cells decreased the abundance of PIP3 at neurite tips. In hippocampal neurons, overexpression of wild-type GAKIN or dominant-negative GAKIN disrupted the formation of morphologically distinct axon-dendrite structure and produced cells with multiple, highly branched neurites. The authors suggest that PIP3 produced at internal membranes, for example at internalized receptors, or PIP3 produced at the cell body may contribute to cell polarity through transport along microtubules to the growth cone.