The spatial organization of epithelial cells is dependent on the length and stability of adherens junctions. Bellaïche and colleagues demonstrate that the lipid phosphatase and tumour suppressor PTEN, known to mediate cell packing through undefined mechanisms, is a critical regulator of junction length during Drosophila melanogaster wing development (Dev. Cell 25, 534–546; 2013).

Expanding previous characterizations of pten mutants, the authors found defects in hexagonal cell packing that were independent of apical–basal polarity disruption. A theoretical model in combination with laser ablation of junctions predicted that the 'cobblestone' phenotype arises from a heterogeneity in junction adhesion and contractility. Indeed, analysis of myosin II localization and E-cadherin–GFP behaviour agreed with this idea. The analysis showed that PTEN loss prevents junction elongation, leading to short unstable junctions that continue rearranging — another prediction of the model.

Analysis of GFP-tagged proteins demonstrated that myosin II and its upstream regulator Rho kinase are enriched at newly formed junctions, and decrease during junction elongation in wild-type, but not in pten mutant, tissue. Furthermore, loss of Rho kinase suppressed the loss of cell packing in pten mutants, suggesting that PTEN reduces Rho-kinase-mediated myosin II contractility at junctions — most probably through PtdIns(3,4,5)P3, given that PtdIns(3,4,5)P3 junctional levels fail to decrease in the mutants. Further simulations suggested that local maintenance of tension is sufficient to cause the cell-packing phenotype of pten mutants. The authors also characterized the contribution of PTEN to the global tissue architecture of the Drosophila wing.