Gamblin et al. reveal how two proteins grapple to control polarity in epithelial cells.
During Drosophila development, the protein Yurt initially settles on the sides of an epithelial cell, preventing the cell’s lateral membranes from acquiring apical traits. Later in development, the protein relocates to the apical end of the cell and stops this part of the membrane from growing too large and disrupting tissues. As Yurt moves, it sheds some of its phosphates, suggesting that a kinase helps hold it in place. A likely candidate for this task is aPKC, which enables the apical membrane to retain its characteristics.
Gamblin et al. found that Yurt and aPKC bind to one another and that aPKC phosphorylates Yurt at multiple sites.
To determine the developmental effects of the proteins’ interaction, the team tracked fly embryos that manufactured excess amounts of Yurt or that produced a nonphosphorylatable version of the protein. Embryos that overproduced Yurt sported large holes in their ventral epidermis. In cells of embryos that carried the phosphorylation-resistant version of Yurt, levels of aPKC plunged and Yurt spread all around the cell membrane. For these embryos, the results were disastrous—their outer covering crumbled and they died.
Thus, the two proteins oppose one another. By phosphorylating its rival, aPKC confines Yurt to the lateral portions of the cell and allows polarization. Meanwhile, Yurt suppresses aPKC, checking the spread of the apical membrane.
Text by Mitch Leslie