This helical domain is part of the Xrn1 catalytic core []. Xrn1 is a cytoplasmic 5'-3' exonuclease that degrades decapped mRNAs. This domain can also be found in Xrn2 [].
XRN2 is an essential eukaryotic exoribonuclease that processes and degrades various substrates. The ~80-residue XRN2-binding domain (XTBD) constitutes an XRN2-binding module that is employed by different metazoan proteins to link to XRN2 [, ], such as:Caenorhabditis elegans Partner of Xrn-Two protein 1, or PAXT-1 for short. Plays a role in maintenance of steady-state concentration and turnover ofmicroRNAs (miRNA) by degradation of mature miRNA in complex with theexoribonuclease XRN-2 [].Mammalian CDKN2A-interacting protein (CDKN2AIP) or Collaborator of ARF(CARF) that regulates DNA damage response in a dose-dependent manner through a number of signaling pathways involved in cell proliferation, apoptosis [, ]. Mammalian CDKN2AIP N-terminal-like protein (CDKN2AIPNL).The XTBD domain folds into a globular four-helix bundle (H1-H4) connected by three loops (L1-L3). H1-H3 form an antiparallel helical array and H4 folds back on top of H2 and H3 at a 90degree angle. The four-helical bundle is mainly stabilized by hydrophobic helix-helix interactions together with additional polar interactions between side chains located on neighbouring helices. The four-helix bundle of XTBD represents a structurally unique arrangement for XRN2 binding [].
