Structural Insights into the C1q Domain of Caprin-2 in Canonical Wnt Signaling

Background: Caprin-2 is a newly identified regulator in canonical Wnt signaling. Results: Mutants targeting trimer contacts of Caprin-2 CRD rather than calcium-binding sites affect the function of Caprin-2 in canonical Wnt signaling. Conclusion: Caprin-2 CRD forms a flexible homotrimer mediated by calcium, and this trimeric assembly is required for the function of Caprin-2. Significance: This work facilitates our understanding of how Caprin-2 functions in canonical Wnt signaling. Previously, we have identified Caprin-2 as a new regulator in canonical Wnt signaling through a mechanism of facilitating LRP5/6 phosphorylation; moreover, we found that its C-terminal C1q-related domain (Cap2_CRD) is required for this process. Here, we determined the crystal structures of Cap2_CRD from human and zebrafish, which both associate as a homotrimer with calcium located at the symmetric center. Surprisingly, the calcium binding-deficient mutant exists as a more stable trimer than its wild-type counterpart. Further studies showed that this Caprin-2 mutant disabled in binding calcium maintains the activity of promoting LRP5/6 phosphorylation, whereas the mutations disrupting Cap2_CRD homotrimer did impair such activity. Together, our findings suggested that the C-terminal CRD domain of Caprin-2 forms a flexible homotrimer mediated by calcium and that such trimeric assembly is required for Caprin-2 to regulate canonical Wnt signaling.