Characterization of septate junction biogenesis during embryogenesis in Drosophila. Sonia Hall, Jennifer Mendez, Sam Long, Robert Ward. Molecular Biosciences, University of Kansas, Lawrence, KS.

   Polarized epithelial cells form tissues that provide a protective barrier from the outside environment and allow for the compartmentalization of internal organs. The ability of an epithelium to act as a barrier is a universal requirement for all multicellular organisms, and is mediated by the formation of cellular junctions along the lateral membranes between epithelial cells. Tight junctions serve this function in vertebrate organisms, whereas septate junctions (SJ) are used in invertebrate organisms. Although tight and septate junctions are ultrastructurally distinct, they share several molecular components including proteins of the claudin and MAGUK families. Genetic studies in Drosophila have identified more than 20 genes that function in the assembly or maintenance of SJ, and recent studies have begun to shed light on the process of biogenesis. Most SJ genes are zygotically expressed and initially trafficked to the basolateral membrane. Many of these proteins are subsequently endocytosed and retargeted to the apical lateral region. Mature SJ are composed of highly crosslinked protein complexes that show very little mobility in the plane of the membrane. Interestingly, SJ proteins exhibit an interdependence, in which the loss of one core protein results in the mislocalization and increased mobility of all other SJ proteins along the lateral membrane. We noticed that the degree of mislocalization of certain SJ proteins is dependent upon which core component is missing. We are therefore extending these studies by examining the localization of ~10 SJ proteins in more than 20 different mutant backgrounds. In addition, we are using immunohistochemistry to track the colocalization of many SJ proteins as they are trafficked to the SJ during its biogenesis in wild type cells. These analyses should allow us to make inferences about the substructure of SJs during their biogenesis and maintenance.