The physiological function of an ion transport protein is determined, in part, by its subcellular localization and by the cellular mechanisms that modulate its activity. We are interested in the molecular signals and interactions that control the sorting and regulation of ion transport proteins in polarized cells. Recently we have focused our attention upon members of the tetraspan family of interacting polypeptides. The tetraspan superfamily constitutes a large and growing collection of membrane proteins that appear to play a role in organizing membrane domains. As their name implies, these polypeptides span the membrane four times and share limited sequence homology. Tetraspan proteins form heteromeric complexes with one another and participate in a wide variety of interactions with other membrane proteins. We have recently found that several ion transport proteins engage in specific interactions with “subcellular domain-appropriate” tetraspans, both in heterologous expression systems and in situ. We have also found that these interactions can regulate the distribution and physiologic function of several critical ion transport systems. In addition, yeast two hybrid screens have revealed several novel partners for the Na,K-ATPase. The list of these partners includes phosphatases, transmembrane receptors and regulators of signaling complexes. Our data suggest that these interactions modulate the activity and trafficking of the Na,K-ATPase in heterologous expression systems as well as in situ. Our studies of membrane protein trafficking extend to the polypeptides associated with autosomal dominant polycystic kidney disease (ADPKD). ADPKD is caused by mutations in the PKD1 or PKD2 genes, which encode the polycystin-1 and polycystin-2 proteins, respectively. Polycystin-1 is a plasma membrane protein that may be involved in signaling from sites of cell-cell contact, while polycystin-2 is a transmembrane protein that shares homology with some members of calcium channel families. Genetic and biochemical evidence suggests that these two proteins participate in the same signaling pathway. Physical interaction between both proteins has been demonstrated and a mutation in either of these two genes leads to the same phenotype. Nevertheless, the functions of these proteins and their common transduction pathway are largely unknown. A new signaling paradigm known as regulated intramembrane proteolysis (RIP) has been recently described. In this model, the intracytoplasmic portion of the transmembrane receptor is released after ligand interaction and enters the nucleus, where it directly acts as a modulator of gene expression, bypassing adaptor proteins and kinase cascades. We have found that polycystin-1 undergoes a RIP-like proteolytic cleavage that releases its C-terminal tail (CTT), which enters the nucleus and initiates signaling processes. The cleavage occurs in vivo in association with alterations in mechanical stimuli. Polycystin-2 modulates the signaling properties of the polycystin-1 CTT, and appears to serve as a cytoplasmic buffer that modulates the quantity of CTT that is available to enter the nucleus. In order to explore further the role that this cleavage plays in the normal functioning of the polycystin proteins and in the pathogenesis of ADPKD we are working to identify the enzyme responsible for the release of the polycystin-1 C terminal tail and to identify the stimuli and signaling pathways that induce or prevent the cleavage. In addition, we are conducting studies designed to identify the protein partners with which the C terminal tail fragment interacts in association with its nuclear translocation. We find that the CTT interacts directly with β-catenin. This interaction does not occur with CTT construct that lacks the putative nuclear localization sequence. Furthermore, expression of the CTT decreases the capacity of β-catenin to participate in WNT signaling, possibly by altering the ability of β-catenin to interact with the TCF transcription factor.