The <scp>E3</scp> ubiquitin ligase, <scp>RNF219</scp> , suppresses <scp>CNOT6L</scp> expression to exhibit antiproliferative activity

Despite the increasing evidence of the role of CCR4-NOT complex in posttranscriptional gene regulation, relatively little is known about its mode of action. In a search for novel CCR4-NOT interacting partners, we carried out mass spectrometry analysis of immunoprecipitates with antibodies against four different CCR4-NOT subunits and identified RNF219, ring finger protein 219. A pull-down assay revealed that the C-terminal part of RNF219 directly binds to the CNOT1 DUF3819 domain and is associated with ubiquitin ligase activity. RNF219 knock-down in HEK293T cells resulted in elevated expression of CNOT6L, accompanied by increased cell proliferation. The apparent antiproliferative activity of RNF219 was inversely correlated with the level of CNOT6L. Furthermore, RNF219 ubiquitinated CNOT6L in vitro. Our data suggest that RNF219 suppress CNOT6L expression through proteasome-mediated protein degradation. Intriguingly, low expression of RNF219 was associated with poor prognosis of triple-negative breast cancer patients. However, further studies would be required to confirm whether the impact of RNF219 activity on cancer progression is mediated by the CCR4-NOT complex.