Our lab works on a variety of different biological systems including DNA replication/repair, nuclear receptors, and heparan/chondroitin sulfate biosynthesis. Listed below are some notable results from our research in this past year: 1) SMCHD1 (structural maintenance of chromosome flexible hinge domain containing 1) is a 2005 amino acid protein involved in gene silencing, with reported roles in X inactivation, genomic imprinting, and non-homologous end joining (NHEJ). Missense mutations in SMCHD1 are associated with arhinia, and Facioscapulohumeral Muscular Dystrophy Type 2 (FSHD2). Last year, we solved the initial X-ray crystal structure of the SMCHD1 ATPase module (aa 24-580) ,and this year focused on structural refinement and analysis of the protein. The ATPase module is composed of three domains: ubiquitin-like domain (UBL), ATPase domain, and transducer domain. We determined that while both the ATPase and transducer domains are required for ATPase activity, the UBL domain is not. However, the UBL is required for dimerization, consistent with the domain-swap observed in the crystal structure dimer. We also characterized 11 variants associated with arhinia and FSHD2. Most of the mutants maintained near wild-type activity and all of the arhinia mutants dimerized. Interestingly, dimerization was greatly diminished in the FSHD2 variants tested. Most notably, the variant Y283C showed a slight increase in ATPase activity even in the absence of dimerization. 2) Classical non-homologous end-joining (NHEJ) is the pathway through which DNA double strand breaks, resulting from immunoglobulin gene maturation or genotoxic DNA damage, are repaired. Human X family polymerase Mu is involved in faithful repair of these breaks by incorporating correctly paired nucleotides to fill gaps in the broken DNA, which are subsequently sealed by DNA ligase IV (LigIV). LigIV utilizes ATP to catalyze the ligation step between the 3-hydroxyl and the 5-phosphate of the break to generate a new phosphodiester bond. In support of research carried out in Dr. Thomas Kunkels Replication Fidelity Group, our lab published novel structures of DNA polymerase Mu engaged with DNA substrates containing an 8-oxoG in the template strand to better understand how Pol Mu bypasses this type of damage using both deoxyribo- and ribonucleotides. Our work demonstrates that Pol Mu prefers to misinsert adenine rather than correctly paired cytidine nucleotides opposite the template 8-oxoG, which suggests that, given cellular nucleotide concentrations, the most likely nucleotide inserted would be a ribo- rather than deoxyriboadenosine. This is consistent with our crystal structures, which demonstrate that adenosine is easily accommodated opposite a syn 8-oxoG with no conformational distortion within the active site.
