From Proteopediaproteopedia link
Crystal structure of human XLF/Cernunnos, a non-homologous end-joining factor
[NHEJ1_HUMAN] Defects in NHEJ1 are the cause of severe combined immunodeficiency due to NHEJ1 deficiency (NHEJ1-SCID) [MIM:611291]; also known as autosomal recessive T-cell-negative, B-cell-negative, NK cell-positive, severe combined immunodeficiency with microcephaly, growth retardation and sensitivity to ionizing radiation or NHEJ1 syndrome. SCID refers to a genetically and clinically heterogeneous group of rare congenital disorders characterized by impairment of both humoral and cell-mediated immunity, leukopenia and low or absent antibody levels. Patients with SCID present in infancy with recurrent, persistent infections by opportunistic organisms. The common characteristic of all types of SCID is absence of T-cell-mediated cellular immunity due to a defect in T-cell development. NHEJ1-SCID is characterized by a profound T- and B-lymphocytopenia associated with increased cellular sensitivity to ionizing radiation, microcephaly and growth retardation. Some patients may manifest SCID with sensitivity to ionizing radiation without microcephaly and mild growth retardation, probably due to hypomorphic NHEJ1 mutations.    Note=A chromosomal aberration involving NHEJ1 is found in a patient with polymicrogyria. Translocation t(2;7)(q35;p22).
[NHEJ1_HUMAN] DNA repair protein involved in DNA nonhomologous end joining (NHEJ) required for double-strand break (DSB) repair and V(D)J recombination. May serve as a bridge between XRCC4 and the other NHEJ factors located at DNA ends, or may participate in reconfiguration of the end bound NHEJ factors to allow XRCC4 access to the DNA termini. It may act in concert with XRCC6/XRCC5 (Ku) to stimulate XRCC4-mediated joining of blunt ends and several types of mismatched ends that are noncomplementary or partially complementary.  
Publication Abstract from PubMed
The recently characterised 299-residue human XLF/Cernunnos protein plays a crucial role in DNA repair by non-homologous end joining (NHEJ) and interacts with the XRCC4-DNA Ligase IV complex. Here, we report the crystal structure of the XLF (1-233) homodimer at 2.3 A resolution, confirming the predicted structural similarity to XRCC4. The XLF coiled-coil, however, is shorter than that of XRCC4 and undergoes an unexpected reverse in direction giving rise to a short distorted four helical bundle and a C-terminal helical structure wedged between the coiled-coil and head domain. The existence of a dimer as the major species is confirmed by size-exclusion chromatography, analytical ultracentrifugation, small-angle X-ray scattering and other biophysical methods. We show that the XLF structure is not easily compatible with a proposed XRCC4:XLF heterodimer. However, we demonstrate interactions between dimers of XLF and XRCC4 by surface plasmon resonance and analyse these in terms of surface properties, amino-acid conservation and mutations in immunodeficient patients. Our data are most consistent with head-to-head interactions in a 2:2:1 XRCC4:XLF:Ligase IV complex.
Crystal structure of human XLF/Cernunnos reveals unexpected differences from XRCC4 with implications for NHEJ.,Li Y, Chirgadze DY, Bolanos-Garcia VM, Sibanda BL, Davies OR, Ahnesorg P, Jackson SP, Blundell TL EMBO J. 2008 Jan 9;27(1):290-300. Epub 2007 Nov 29. PMID:18046455
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.