3-M primordial dwarfism is an inherited disease characterized by severe pre- and postnatal growth retardation and by mutually exclusive mutations in 3 genes, CUL7, OBSL1, and CCDC8. The mechanism underlying 3-M dwarfism is not clear. We showed here that CCDC8, derived from a retrotransposon Gag protein in placental mammals, exclusively localized on the plasma membrane and was phosphorylated by CK2 and GSK3. Phosphorylation of CCDC8 resulted in its binding first with OBSL1, and then CUL7, leading to the membrane assembly of the 3-M E3 ubiquitin ligase complex. We identified LL5β, a plasma membrane protein that regulates cell migration, as a substrate of 3-M ligase. Wnt inhibition of CCDC8 phosphorylation or patient-derived mutations in 3-M genes disrupted membrane localization of the 3-M complex and accumulated LL5β. Deletion of Ccdc8 in mice impaired trophoblast migration and placental development, resulting in intrauterine growth restriction and perinatal lethality. These results identified a mechanism regulating cell migration and placental development that underlies the development of 3-M dwarfism.
Pu Wang, Feng Yan, Zhijun Li, Yanbao Yu, Scott E. Parnell, Yue Xiong
3-M E3 ligase ubiquitinates and degrades plasma membrane protein LL5β.