Kekkon6 and Kekkon3 - Novel Insights into the Kekkon Family

Transepithelial barriers represent important mechanisms by which epithelial cells delimit tissue compartments and maintain distinct extracellular environments. Such cellular barriers are key in regulating organ and tissue homeostasis and their dysregulation leads to a wide variety of pathologies. Novel tight junctions termed tricellular junctions (TCJs) appear to provide this barrier activity at the molecular level. Despite their proposed key role in barrier function, our understanding of these junctions is limited, with only a few molecules localized to tricellular junctions having been reported. Here we add to this understanding by identifying a LIG family member, Kek6, in Drosophila that represents the first example of a molecule uniquely excluded from TCJs. LIGs represent transmembrane molecules with Leucine-rich repeats and Immunoglobulin domains whose expression is often enriched in the developing nervous system. Data on Kek6 confirms this nervous system expression. Investigation into the mechanism which controls Kek6€™s unique exclusion from TCJs has proved that it is not solely mediated by the C-terminal intracellular PDZ domain-binding site. Although PDZ domain-binding sites of various proteins have been implicated as important for protein localization, it is thought that it is the extracellular domain of Kek6 that is the part of the protein which is responsible for its unique localization pattern. Shown here, it is believed that Kek6 participates in a stabilizing homophilic interaction which may support the hypothesis that the extracellular domain is required for localization. Kek6 expression in one cell is not sufficient for expression in the bicellular junctions. Adjacent cells must both express Kek6 in order for Kek6 to be stably localized to the bicellular junction. Studies on the potential relationship between Kek6 and Gliotactin, the Drosophila protein which localizes to TCJs, revealed that there is no direct relationship between these two proteins but does not eliminate the potential of unidentified shared interactors. Further investigation of Kek6 will allow for the elucidation of the role of Kek6 in TCJs which will help further develop the junction biology field. In addition to the information provided on Kek6, this study reports the first localization and functional knowledge of Kek3. Misexpression of Kek3 leads to cross vein defects and reduction/loss of bristles revealing that Kek3 may be a modulator of BMP signaling. Although family member Kek5 has been previously identified as a modulator of BMP signaling, the mechanism of this function is still under investigation but it is believed that Kek3 is acting through a different mechanism