C-terminal GTPase-like domain of F-Box Only Protein 4
F-box proteins are involved in substrate recognition as part of SCF (Skp1-Cul1-Rbx1-F-box protein) ubiquitin ligase complexes. Fbx4 (or Fbxo4) binds to the telomere repeat binding factor 1 (TRF1), whose activity at telomeres is regulated in part by selective ubiquitination and degradation. This ubiquitination of TRF1 is mediated by Fbx4, which binds to the TRFH domain of TRF1, via the C-terminal domain characterized by this model, a module resembling a small GTPase domain that lacks the GTP-binding site. When bound to telomeres, TIN2 acts to protect TRF1 from SCF-Fbx4 mediated ubiquitination. Tankyrase-mediated ADP-ribosylation releases TRF1 from telomeres, rendering them susceptible to ubiquitination and degradation, which in turn promotes telomere elongation. Fbx4 has also been reported to target cyclin D1 for degradation by the proteasome, a mechanism ensuring the fidelity of DNA replication. More recently, these findings have been disputed.
Jiyao W et al.(2023). "The conserved domain database in 2023.",