Polycystin 1, the product of the PKD1 gene, is mutated in autosomal dominant polycystic kidney disease, a disease characterized by renal cyst formation and progressive renal failure. We show that expression of the C-terminal domain of human polycystin-1 (PKD1-CT) triggers spreading of isolated inner medullary collecting duct cells, a process mediated by Erk. As inner medullary collecting duct cells spread, PKD1-CT localizes to cell-extracellular matrix contacts, interacts with focal adhesion proteins Fak and paxillin, and stimulates Fak phosphorylation, paxillin phosphorylation, Fak-paxillin association, and formation of small focal complexes. PKD1-CT-mediated spreading requires membrane localization and the integrity of the C-terminal protein binding sites. We additionally show that Pkd1 null proximal tubule cells generated from Pkd1(flox/-):TSLargeT mice by in vitro Cre recombinase transfection demonstrate diminished spreading when compared with Pkd(flox/-) heterozygous parental cells. These findings suggest that membrane-bound PC1 has a central role in regulating morphogenic protein signaling at cell-matrix interfaces in non-confluent cells.