January 12, 2012

For a little protein, the cell is a big place. Many times it’s necessary for proteins to be clustered together in order to get a job done. Today’s image is from a paper describing how E-cadherin gets clustered at adherens junction sites like a flock of 12-year old teeny-bopper girls at a Twilight movie.

Sheets of epithelial cells are polarized—one side of the epithelial sheet faces the inside/lumen of an organ or tissue, while the other attaches to a supportive basement membrane. The establishment and polarization of epithelial cells depends on adherens junctions (AJs), protein complexes that serve as cell-cell junction sites. AJs are composed of a transmembrane protein called E-cadherin that connects the junctions to the cell’s actin cytoskeleton. E-cadherin must be distributed on the cell’s plasma membrane for AJ assembly, but how it is brought to the membrane and/or clustered at certain sites is not fully understood. A recent paper finds an association between E-cadherin and the exocyst protein Exo70. The exocyst is a complex that brings proteins from the Golgi apparatus (where they are sorted) to the plasma membrane. According to Xiong and colleagues, Exo70 is required for E-cadherin clustering at the plasma membrane and for maturation of newly-formed AJs. Exo70 performs this feat through its association with a kinase that can interact directly with E-cadherin (PIPKIγ, for you membrane-junkies out there). As seen in the images above, this kinase (green) and Exo70 (red) both associate at the lateral membranes of epithelial cells, where AJs form. Top row shows the cells as if we are looking down onto the cells, while bottom row shows cells as if we were looking through the plane of cells.

Xiong, X., Xu, Q., Huang, Y., Singh, R., Anderson, R., Leof, E., Hu, J., & Ling, K. (2011). An association between type I PI4P 5-kinase and Exo70 directs E-cadherin clustering and epithelial polarization Molecular Biology of the Cell, 23 (1), 87-98 DOI: 10.1091/mbc.E11-05-0449