Functional analysis of the Notch ligand Jagged1 missense mutant proteins underlying Alagille syndrome

Heterozygous mutations in the JAG1 gene, encoding Notch ligand Jagged1, cause Alagille syndrome (ALGS). As most of the mutations are nonsense or frameshift mutations producing inactive truncated proteins, haplo-insufficiency is considered the major pathogenic mechanism of ALGS. However, the molecular mechanisms by which the missense mutations cause ALGS remain unclear. Here we analyzed the functional properties of four ALGS missense mutant proteins, P163L, R184H, G386R and C714Y, using transfected mammalian cells. P163L and R184H showed Notch-binding activities similar to that of the wild-type when assessed by immunoprecipitation. However, their trans-activation and cis-inhibition activities were almost completely impaired. These mutant proteins localized mainly to the endoplasmic reticulum (ER), suggesting that the mutations induced improper protein folding. Furthermore, the mutant proteins bound more strongly to the ER chaperone proteins calnexin and calreticulin than the wild-type did. C714Y also localized to the ER, but possessed significant trans-activation activity and lacked enhanced binding to the chaperones, indicating a less severe phenotype. The properties of G386R were the same as those of the wild-type. Dominant-negative effects were not detected for any mutant protein. These results indicate that accumulation in the ER and binding to the chaperones correlate with the impaired signal-transduction activities of the missense mutant proteins, which may contribute to the pathogenic mechanism of ALGS. Our findings, which suggest the requirement for cell-surface localization of Jagged1 for cis-inhibition activities, also provide important information for understanding the molecular basis of Notch-signaling pathways.