Arginine methylation of USP9X promotes its interaction with TDRD3 and its anti-apoptotic activities in breast cancer cells.

The Tudor domain-containing proteins are characterized by their specific interactions with methylated protein motifs, including methyl-arginines and methyl-lysines. The Tudor domain-containing protein 3 (TDRD3) is one of the major methyl-arginine effector molecules that recognizes methylated arginine residues on histones and the C-terminal domain of RNA polymerase II, and activates transcription. However, majority of the cellular TDRD3 localizes to the cytoplasm and its functions there are still elusive. Here, we have identified ubiquitin-specific protease 9 X-linked (USP9X) as a TDRD3-interacting protein by GST (glutathione -transferase) pull-down and co-immunoprecipitation. Detailed characterization suggests that the interaction between TDRD3 and USP9X is mediated through the Tudor domain of TDRD3 and the arginine methylation of USP9X. This interaction plays a critical role in TDRD3 protein stability, as knockdown of USP9X expression leads to increased TDRD3 ubiquitination. We also found that USP9X co-localizes with TDRD3 in cytoplasmic stress granules and this localization is diminished in Tdrd3-null mouse embryonic fibroblast cells, suggesting that TDRD3 is essential for USP9X stress granule localization. Furthermore, we found that one of the USP9X de-ubiquitination targets, myeloid cell leukemia protein 1, is regulated by TDRD3, indicating that TDRD3 potentially regulates USP9X de-ubiquitinase activity. Finally, we show that knockdown of TDRD3 expression sensitizes breast cancer cells to chemotherapeutic drug-induced apoptosis, likely due to its regulation of USP9X. This study provides a novel candidate strategy for targeting apoptosis pathways in cancer therapy.