We look symmetrical from the outside, but it's well known that our organs are distributed asymmetrically within our bodies. Cells also hide a distinct asymmetry, generating, for example, an apicobasal polarity by unevenly organizing cellular components. Two well-studied proteins that are implicated in apicobasal polarity in mammalian epithelial cells are atypical protein kinase C (aPKC) and PAR1, and Suzuki et al. now show that aPKC functions upstream of PAR1 in establishing and maintaining this polarity.

aPKC and PAR1 segregate along the apicobasal axis of Madin–Derby canine kidney (MDCK) cells — PAR1 localizes at the basolateral membrane, whereas aPKC is distributed in the apical membrane. Could one affect the localization of the other? Using RNA interference, the authors showed that PAR1b (one of four PAR1 homologues) was required for membrane domains to develop asymmetrically in MDCK cells. But depleting PAR1b didn't affect the development of the cell–cell junctions known as tight junctions (TJs) or the asymmetric localization of aPKC. By contrast, depleting aPKC inhibited TJ formation and prevented PAR1b accumulation.

Suzuki et al. next showed that aPKC could phosphorylate PAR1b on threonine 595 and that PAR1b was phosphorylated more in depolarized than in polarized MDCK cells. Late during the cell-polarization process, T595 seemed to become dephosphorylated. These data, with results from cell-fractionation studies and immunocytochemistry, are consistent with the need for PAR1b T595 to be dephosphorylated for its stable lateral localization — T595-phosphorylated PAR1b was mainly found in the soluble fraction.

In Drosophila melanogaster, the scaffold protein 14-3-3 binds to PAR1, and Suzuki et al. found not only that these proteins also interacted in MDCK cells, but that T595 phosphorylation enhanced this. Because 14-3-3 is thought to alter the intracellular localization of its binding partners, and because it interacted mainly with soluble, phosphorylated PAR1, the authors suggest the following model: aPKC at the membrane phosphorylates PAR1b, which then dissociates from the membrane into a soluble fraction by the actions of 14-3-3. In this way, PAR1b is excluded from the apical membrane of TJs, thereby maintaining appropriate asymmetric membrane domains. Supporting this model, a non-phosphorylatable mutant of PAR1b invaded the apical membrane of MDCK cells and caused asymmetric membrane domains to develop abnormally.