Lee, Tae HoPerrem, KilianHarper, J. WadeLu, Kun PingZhou, Xiao Zhen2019-10-142006Lee, Tae Ho, Kilian Perrem, J. Wade Harper, Kun Ping Lu, and Xiao Zhen Zhou. 2006. “The F-Box Protein FBX4 Targets PIN2/TRF1 for Ubiquitin-Mediated Degradation and Regulates Telomere Maintenance.” Journal of Biological Chemistry 281 (2): 759–68. doi:10.1074/jbc.M509855200.0021-92581083-351Xhttp://nrs.harvard.edu/urn-3:HUL.InstRepos:41543035Pin2/TRF1 was identified previously as both a protein (TRF1) that binds to telomeric DNA repeats and as a protein (Pin2) that associates with the kinase NIMA and suppresses its mitosis inducing activity. Pin2/TRF1 negatively regulates telomere length and also plays a critical role in cell cycle checkpoint control. Pin2/TRF1 is down-regulated in many human cancers and may be degraded by the ubiquitin-proteasome pathway, but components of the pathway involved in Pin2/TRF1 turnover have not been elucidated. By using the two-hybrid system, we recently identified Pin2/TRF1-interacting proteins, PinX1-4, and we demonstrated that PinX1 is a conserved telomerase inhibitor and a putative tumor suppressor. Here we report the characterization of PinX3. PinX3 was later found to be identical to Fbx4, a member of the F-box family of proteins, which function as substrate-specific adaptors of Cul 1-based ubiquitin ligases. Fbx4 interacts with both Pin2 and TRF1 isoforms and promotes their ubiquitination in vitro and in vivo. Moreover, overexpression of Fbx4 reduces endogenous Pin2/TRF1 protein levels and causes progressive telomere elongation in human cells. In contrast, inhibition of Fbx4 by RNA interference stabilizes Pin2/TRF1 and promotes telomere shortening, thereby impairing cell growth. These results demonstrate that Fbx4 is a central regulator of Pin2/TRF1 protein abundance and that alterations in the stability of Pin2/TRF1 can have a dramatic impact on telomere length. Thus, Fbx4 may play a critical role in telomere maintenance.en-USJournal Article2019-10-1410.1074/jbc.M509855200