Mutations in the PKD1 gene are responsible for 85% of cases of autosomal dominant polycystic kidney disease (ADPKD). This gene encodes a large membrane associated glycoprotein, polycystin-1, which is predicted to contain a number of extracellular protein motifs, including a C-type lectin domain between amino acids 403--532. We have cloned and expressed the PKD1 C-type lectin domain, and have demonstrated that it binds carbohydrate matrices in vitro, and that Ca(2+) is required for this interaction. This domain also binds to collagens type I, II and IV in vitro. This binding is greatly enhanced in the presence of Ca(2+) and can be inhibited by soluble carbohydrates such as 2-deoxyglucose and dextran. These results suggest that polycystin-1 may be involved in protein-carbohydrate interactions in vivo. The data presented indicate that there may a direct interaction between the PKD1 gene product and an ubiquitous extracellular matrix (ECM) protein.