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Autosomal dominant polycystic kidney disease (ADPKD) is a common inherited nephropathy responsible for 4%–10% of end-stage renal disease cases. Mutations in the genes encoding polycystin-1 (PC1, PKD1) or polycystin-2 (PC2, PKD2) cause ADPKD, and PKD1 mutations are associated with more severe renal disease. PC1 has been shown to form a complex with PC2, and the severity of PKD1-mediated disease is associated with the level of the mature PC1 glycoform. Here, we demonstrated that PC1 and PC2 first interact in the ER before PC1 cleavage at the GPS/GAIN site and determined that PC2 acts as an essential chaperone for PC1 maturation and surface localization. The chaperone function of PC2 was dependent on the presence of the distal coiled-coil domain and was disrupted by pathogenic missense mutations. In Pkd2–/– mice, complete loss of PC2 prevented PC1 maturation. In Pkd2 heterozygotes, the 50% PC2 reduction resulted in a nonequimolar reduction (20%–25%) of the mature PC1 glycoform. Interbreeding between various Pkd1 and Pkd2 models revealed that animals with reduced levels of functional PC1 and PC2 in the kidney exhibited severe, rapidly progressive disease, illustrating the importance of complexing of these proteins for function. Our results indicate that PC2 regulates PC1 maturation; therefore, mature PC1 levels are a determinant of disease severity in PKD2 as well as PKD1.