Figure 1. Actin colocalizes with a subset of junctional complexes in polarized MDCK cells. (a–d) Electron micrographs showing adherens junction (white arrow) and numerous interac- tions between adjacent cells along the lateral membrane (arrowheads; a). Cell–cell interac- tions are at apical (b), lateral (c), and basal membranes (d) of cells. Bars, 500 nm. (e and f) Single deconvolved optical z slices at the apical region of cells. (g and h) Projection of 35 deconvolved optical z slices spanning the apical 7 µm of cells. (i and j) Projection of the next 10 deconvolved optical z slices spanning 2 µm of the basal region of cells. (e–j) Yellow arrowheads point to areas where -catenin (blue), -catenin (red), and actin (phalloidin; green) are colocalized. White arrowheads point to areas where actin is missing from the - and -catenin puncta. Bars, 2 µm. actomyosin network to help drive movement (Solnica-Krezel, actin assembly reactions on cadherin-enriched membranes will 2006; Hammerschmidt and Wedlich, 2008; Montell, 2008). be required to bridge genetic and cell biological work to future Finally, some contacts are dissolved and internalized, requir- biochemical analysis in pure solution under defined conditions. ing a third actin organization at junctions to facilitate endo- Most of the work examining cadherin–actin interactions cytosis (Ulrich and Heisenberg, 2009). Understanding the precise has focused on developing embryos or cell culture models de- function of each of the various actin-binding proteins associated signed to mimic the initial phases of cell–cell contact and early with cadherin cell–cell junctions will ultimately require bio- steps in junctional maturation (Angres et al., 1996; Adams et al., chemical analysis, but this process will not be as straightforward 1998). Less is known regarding cadherin–actin interactions in as might have been hoped. For example, -catenin binds actin mature junctions within highly differentiated tissues. However, filaments in pure solution but fails to do so when incorporated understanding these interactions is important for human health, into junctional complexes (Yamada et al., 2005; Kwiatkowski in which subtle mutations silent during embryogenesis might et al., 2010). Therefore, complex in vitro systems that reconstitute eventually compromise junction function over time, resulting in 116 JCB • VOLUME 196 • NUMBER 1 • 2012