The V(D)J recombination process that assembles functional immunoglobulin and T cell receptor genes in lymphoid cells is essential for generating the diversity of the immune response. The reaction is now known to occur in two stages. In the first stage, specific double- strand breaks are made at the target sites by the RAG1 and RAG2 proteins acting together. The later steps, which join the broken ends in new combinations, are known to be relatively non-specific and to share many protein factors with DNA double-strand break repair. We have found novel activities of some of these proteins.(1) In addition to its known function as a cofactor of the DNA-dependent protein kinase, Ku protein can greatly stimulate joining of blunt or near-blunt DNA ends by mammalian DNA ligases. This observation correlates with biological results showing that Ku-deficient cells have more severe defects than those lacking DNA PKCS. (2) The Xrcc4 protein is known to bind to DNA ligase IV and to stimulate its activity. The radiation-sensitivity of Xrcc4-deficient cells, and their failure to join broken ends in V(D)J recombination, have been attributed to this defect. We find that Xrcc4 protein also binds to DNA, and that its biological activity correlates better with DNA binding than with its effect on ligase IV. (3) In yeast, the Mre11/Rad50/Xrs2 complex is known to be important for non homologous DNA end-joining as well as meiotic recombination. Human homologs of Mre11 and Rad50 have been cloned, and the human Nbs1 protein is thought to be the homolog of yeast Xrs2. We found that Mre11 is a 3 to 5 exonuclease, more active when associated with Rad50. Mre11 also has an endonuclease activity on single-stranded DNA. In the triple complex, new activities are revealed. There is now a limited ATP- dependent DNA unwinding, an ATP-dependent incision to remove 3- overhanging tails, and an efficient cutting of perfect hairpins. These activities suggest explanations for several biological functions of the complex, and imply that Mre11/Rad50/Nbs1 may be the biologically relevant hairpin-cutting enzyme in V(D)J recombination. - Immunoglobulin, T cell receptor, V(D)J recombination, transposition, evolution, translocation, leukemia
| Jones, Jessica M; Gellert, Martin (2004) The taming of a transposon: V(D)J recombination and the immune system. Immunol Rev 200:233-48 |
| Modesti, Mauro; Junop, Murray S; Ghirlando, Rodolfo et al. (2003) Tetramerization and DNA ligase IV interaction of the DNA double-strand break repair protein XRCC4 are mutually exclusive. J Mol Biol 334:215-28 |
| Jones, Jessica M; Gellert, Martin (2003) Autoubiquitylation of the V(D)J recombinase protein RAG1. Proc Natl Acad Sci U S A 100:15446-51 |
| Paull, T T; Cortez, D; Bowers, B et al. (2001) Direct DNA binding by Brca1. Proc Natl Acad Sci U S A 98:6086-91 |
| Jones, J M; Gellert, M (2001) Intermediates in V(D)J recombination: a stable RAG1/2 complex sequesters cleaved RSS ends. Proc Natl Acad Sci U S A 98:12926-31 |
| Jones, J M; Gellert, M; Yang, W (2001) A Ku bridge over broken DNA. Structure 9:881-4 |
| Paull, T T; Rogakou, E P; Yamazaki, V et al. (2000) A critical role for histone H2AX in recruitment of repair factors to nuclear foci after DNA damage. Curr Biol 10:886-95 |
| Paull, T T; Gellert, M (2000) A mechanistic basis for Mre11-directed DNA joining at microhomologies. Proc Natl Acad Sci U S A 97:6409-14 |
| Roth, D B; Gellert, M (2000) New guardians of the genome. Nature 404:823-5 |
| Junop, M S; Modesti, M; Guarne, A et al. (2000) Crystal structure of the Xrcc4 DNA repair protein and implications for end joining. EMBO J 19:5962-70 |
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