Autosomal dominant polycystic kidney disease is caused by mutations in PKD1 or PKD2. The respective gene products, polycystin-1 (PC1) and polycystin-2 (TRPP2) interact and are thought to function in a common signaling pathway. It is not known how the polycystin complex is activated through extracellular signals and which downstream effectors regulate morphogenetic programs controlling the shape of epithelial tubes. The goal of this project is to identify and characterize novel components in the PC1 protein complex to gain a better understanding of the polycystin signaling network. We aim at answering two fundamental questions: (i) How do polycystins regulate formation and maintenance of epithelial tubes in three-dimensional tissues and (ii) what causes cyst formation if polycystins are mutated? We hypothesize that extracellular ligands bind to PC1 to regulate the activity of the ion channel TRPP2. This triggers cytosolic Ca2+ signals, which activate yet to be identified downstream effector proteins controlling morphogenesis. In this project we are proposing to dissect the function of PC1 using biochemical and genetic approaches. To identify novel components in the polycystin complex, including putative PC1 ligands, we have performed interaction proteomic screens using PC1 as bait. We have identified a set of novel candidate proteins in the PC1 complex. After validation of complex members, the molecular and cellular functions of these proteins will be studied to understand their role in signal transduction and tubular morphogenesis. In addition, we will screen for extracellular PC1 ligands using high-resolution proteomics. In order to genetically dissect the function of PC1 in vivo, we will study the function of the PKD1 homologs in Drosophila. We have generated a Drosophila mutant for the fly Pkd1 homolog CG42685 using homologous recombination. Taking advantage of the extensive genetic toolbox in Drosophila, we will study the phenotypes of loss and gain of function Pkd1 transgenic flies in order to unravel evolutionarily conserved signaling functions of PC1. Candidates from the biochemical screens will be investigated in Drosophila and in vertebrate models, in collaboration with members of the CRC. The synergy between these complementary approaches will provide insights into the signaling mechanisms of the enigmatic polycystin signaling pathway.