The placental protein NRK promotes cell death through its plasma membrane-localizing CNH domain

Placental development is regulated by the balance between cell proliferation and death. The placental protein NRK (NIK-related kinase) plays a role in preventing excessive placenta growth. We previously demonstrated that NRK underwent rapid molecular evolution in the ancestor of placental mammals and acquired the functional regions, including the phospholipid-binding citron homology (CNH) domain, by which NRK inhibits cell proliferation. NRK is also potentially responsible for cell death; caspases cleave NRK during apoptosis, releasing the C-terminal fragment that promotes cell death. Here, we explored the molecular mechanisms underlying the cell death-promoting effects of NRK. Our experimental data using HeLa, placenta trophoblast BeWo (human), and Rcho-1 (rat) cells indicated that the CNH domain of NRK was required and sufficient to promote cell death. In vitro and in silico studies showed the NRK CNH domain bound to phospholipids via its polybasic clusters and remains at the plasma membrane (PM) during apoptosis. Evolutional analyses indicated that these clusters formed in the ancestor of placental mammals. Mutations in these clusters (CNH-18A) hindered the cell death-promoting activity of the CNH domain. We concluded that NRK promotes cell death through its plasma membrane-localizing CNH domain and suggested its active role in PM-associated events during cell death.