Figure 1.

A conserved mechanism for the maintenance of Numb asymmetry in polarized cells. In epithelial cells, atypical protein kinase C (aPKC) is concentrated in apical junctional complexes and/or the apical cell cortex at steady state, whereas Numb is restricted to basolateral membranes. Encroachment of Numb into the apical pole of the cell results in its phosphorylation by aPKC, preventing its association with apical membranes. In mitotic Drosophila sensory organ precursor cells (pI), aPKC is concentrated in the posterior cortex. Phosphorylation-dependent displacement of Numb from the posterior pole causes Numb to concentrate anteriorly. Upon cell division, Numb is concentrated in the anterior daughter cell (pIIb), which adopts a developmental fate different than that of the posterior daughter (pIIa).