Region Of RAG1 Research Articles

Nuclease-independent functions of RAG1 direct distinct DNA damage responses in B cells.

Developing B cells generate DNA double-stranded breaks (DSBs) to assemble immunoglobulin receptor (Ig) genes necessary for the expression of a mature B cell receptor. These physiologic DSBs are made by the RAG endonuclease, which is comprised of the RAG1 and RAG2 proteins. In pre-B cells, RAG-mediated DSBs activate the ATM kinase to coordinate canonical and non-canonical DNA damage responses (DDR) that trigger DSB repair and B cell developmental signals, respectively. Whether this broad cellular response is distinctive to RAG DSBs is poorly understood. To delineate the factors that direct DDR signaling in B cells, we express a tetracycline-inducible Cas9 nuclease in Rag1-deficient pre-B cells. Both RAG- and Cas9-mediated DSBs at Ig genes activate canonical DDR. In contrast, RAG DSBs, but not Cas9 DSBs, induce the non-canonical DDR-dependent developmental program. This unique response to RAG DSBs is, in part, regulated by non-core regions of RAG1. Thus, B cells trigger distinct cellular responses to RAG DSBs through unique properties of the RAG endonuclease that promotes activation of B cell developmental programs.

Risky business blockade: RAG2 basic region blocks V(D)J recombinase function upon genotoxic stress in DNA damage response

Abstract The V(D)J recombinase, composed of the recombination activating gene (RAG) proteins, RAG1 and RAG2, generate functional antigen receptor genes through the repeated generation of DNA double strand breaks and DNA repair. However, this process can lead to increased genomic instability particularly in the presence of excess DNA breaks. Previously, we showed in pre-B cells that full length RAG2, a nuclear protein, will export to the cytoplasm with DNA damage in an ATM-dependent manner with eventual recovery back into the nucleus following DNA repair. We propose that re-localization of RAG2 prevents RAG-mediated DNA cleavage in the presence of excess DNA ends. Here, by using fluorescently tagged RAG proteins, we show that DNA damage-induced export of the V(D)J recombinase can be recapitulated in non-lymphoid cells. Further, we determined that mutations in the intrinsically-disordered region of RAG2 have distinct effects on this process. Specifically, a basic region mutant of RAG2 (K/R 518–524 A) alters the localization pattern of the V(D)J recombinase, and yet its pre-DNA damage localization pattern is re-established upon DNA repair. Conversely, a T490A mutant blocks relocalization of RAG2 following genotoxic stress for both the single T490A mutant and the combined T490A/basic region mutant. Together, these results demonstrate the role of the intrinsically-disordered region in RAG2 in regulating the mobility and subcellular localization of the V(D)J recombinase upon cellular conditions that may be deemed too risky for continued DNA rearrangements. Supported by NIA T32AG052363, NIH AI128137, OCAST

Missense mutation in non-recombinase domain of RAG1 results in B-cell developmental defect

Abstract Objective To examine how a single amino acid change to a region of RAG1 that is not related to VDJ recombination affects B-cell development. Methods We studied B-cells within a Rag1 P326G missense mutant mouse (Rag1PG) utilizing flow cytometric analysis to phenotype B-cell development and peripheral populations, as well as biochemical studies examining the B-cell Receptor (BCR) molecular complex and signaling function. We also created a mixed bone marrow chimera to determine which alterations were cell-intrinsic. Results We find that overall there is an ~50% decrease in total B-cells in the Rag1PG mouse compared to wildtype (WT) due to a Pro-B to Pre-B-cell developmental block. Peripheral B cell cytopenias are in the Follicular Zone B-cell compartment, with sparing of the Marginal Zone B-cells. There is decreased Syk expression in Rag1PG—however no differences in BCR signaling strength were appreciated in Rag1PG. In the mixed bone marrow chimera, we find a competitive advantage in the WT developing B-cells, but peripheral B cell populations survive equally well. Conclusion Despite the Rag1PG protein being a single amino acid change to a region of RAG1 not involved in recombination, we nevertheless find a developmental defect consistent with leaky SCID. Given that there is no evidence of altered BCR signaling, and the developmental block occurs at the stage in B-cell development in which VDJ recombination is first required, these data suggest a recombination defect. The mutation is in a ubiquitin ligase domain, and it is possible this alters currently unappreciated roles of RAG1, such as histone degradation to allow RAG1 access to DNA. Our findings illustrate how mutations in the non-recombinase domain of RAG1 may result in leaky SCID.

RAG2 interactions with H3K4me3 are regulated by Thr490 proximal to the RAG2 PHD region.

Abstract Association of RAG2 with the epigenetic marker H3K4me3 through a plant homeodomain (PHD) in the RAG2 non core domain is important for relieving auto inhibition of the RAG recombinase activity for V(D)J recombination. However, little is known regarding the properties of RAG2 that regulate its PHD-dependent interactions with H3K4me3. Accordingly, we measured the localization and dynamics of an N-terminal GFP fusion protein of full length RAG2 (GFP-FL); a T490A mutant of full length RAG2 (GFP-T490A) that does not undergo a regulatory phosphorylation proximal to the PHD region of RAG2; and GFP-labeled core domain of RAG2 (GFP-Core). The proteins were expressed in RAG2−/− pro-B cells, and the H3K4me3 was labeled using monoclonal antibody. Cross-correlation analysis showed that GFP-FL and GFP-Core exhibited similar colocalization with H3K4me3, which was significantly less than colocalization of GFP-T490A with H3K4me3. Blocking H3K4 demethylation using 2,4-pyridinedicarboxylic acid (PDA) caused GFP-FL/H3K4me3 colocalization to approach that of GFP-T490A and H3K4me3, whereas GFP-Core/H3K4me3 colocalization was unchanged. Super resolution imaging showed that RAG2 interactions with H3K4me3 were restricted to interfaces with H3K4me3-enriched puncta, and that GFP-T490A exhibited greater overlap with the H3K4me3-enriched puncta than GFP-FL. Finally, measurement of fluorescence recovery after photobleaching demonstrated that GFP-T490A had a lower mobile fraction and rate of diffusion than GFP-FL in conditions where RAG2 interactions with H3K4me3 were enhanced. These data suggest that RAG2 interactions with H3K4me3 are regulated by Thr490 proximal to the PHD region, such as by a phosphoryation/dephosphorylation cycle.

Prevalence and clinical challenges among adults with primary immunodeficiency and recombination-activating gene deficiency

Prevalence and clinical challenges among adults with primary immunodeficiency and recombination-activating gene deficiency

RAG1 targeting in the genome is dominated by chromatin interactions mediated by the non-core regions of RAG1 and RAG2.

The RAG1/RAG2 endonuclease initiates V(D)J recombination at antigen receptor loci but also binds to thousands of places outside of these loci. RAG2 localizes directly to lysine 4 trimethylated histone 3 (H3K4me3) through a plant homeodomain (PHD) finger. The relative contribution of RAG2-dependent and RAG1-intrinsic mechanisms in determining RAG1 binding patterns is not known. Through analysis of deep RAG1 ChIP-seq data, we provide a quantitative description of the forces underlying genome-wide targeting of RAG1. Surprisingly, sequence-specific DNA binding contributes minimally to RAG1 targeting outside of antigen receptor loci. Instead, RAG1 binding is driven by two distinct modes of interaction with chromatin: the first is driven by H3K4me3, promoter-focused and dependent on the RAG2 PHD, and the second is defined by H3K27Ac, enhancer-focused and dependent on ‘non-core’ portions of RAG1. Based on this and additional chromatin and genomic features, we formulated a predictive model of RAG1 targeting to the genome. RAG1 binding sites predicted by our model correlate well with observed patterns of RAG1-mediated breaks in human pro-B acute lymphoblastic leukemia. Overall, this study provides an integrative model for RAG1 genome-wide binding and off-target activity and reveals a novel role for the RAG1 non-core region in RAG1 targeting.

Elucidating regulatory mechanisms of RAG2 in response to DNA damage.

Abstract V(D)J recombination of lymphocyte antigen receptor genes occurs via the formation of DNA double strand breaks (DSBs) through the activity of RAG1 and RAG2. The co-existence of RAG-independent DNA DSBs generated by genotoxic stressors potentially increases the risk of incorrect repair and chromosomal abnormalities. However, it is not known whether cellular responses to DSBs by genotoxic stressors affect the RAG complex. Using cellular imaging and subcellular fractionation approaches, we show that formation of DSBs by treating cells with DNA damaging agents causes export of nuclear RAG2. Within the cytoplasm, RAG2 exhibited substantial enrichment at the centrosome. Further, RAG2 export was sensitive to inhibition of ATM, and was reversed following DNA repair. Live cell imaging reveals export of RAG2 within minutes of DNA damage, demonstrating a rapid response to genotoxic stress. Intriguingly, dose response assessment exhibited RAG2 export following the generation of very few DNA DSBs, indicating that DNA damage-triggered export of RAG2 can occur under normal physiological conditions. The core region of RAG2 was sufficient for export, but not centrosome targeting, and RAG2 export was blocked by mutation of Thr490. In summary, DNA damage triggers relocalization of RAG2 from the nucleus to centrosomes, suggesting a novel mechanism for modulation of cellular responses to DSBs in developing lymphocytes.

Disruption of the RAG2 zinc finger motif impairs protein stability and causes immunodeficiency

Although the RAG2 core domain is the minimal region required for V(D)J recombination, the noncore region also plays important roles in the regulation of recombination, and mutations in this region are often related to severe combined immunodeficiency. A complete understanding of the functions of the RAG2 noncore region and the potential contributions of its individual residues has not yet been achieved. Here, we show that the zinc finger motif within the noncore region of RAG2 is indispensable for maintaining the stability of the RAG2 protein. The zinc finger motif in the noncore region of RAG2 is highly conserved from zebrafish to humans. Knock-in mice carrying a zinc finger mutation (C478Y) exhibit decreased V(D)J recombination efficiency and serious impairment in T/B-cell development due to RAG2 instability. Further studies also reveal the importance of the zinc finger motif for RAG2 stability. Moreover, mice harboring a RAG2 noncore region mutation (N474S), which is located near C478 but is not zinc-binding, exhibit no impairment in either RAG2 stability or T/B-cell development. Taken together, our findings contribute to defining critical functions of the RAG2 zinc finger motif and provide insights into the relationships between the mutations within this motif and immunodeficiency diseases.

Mapping and Quantitation of the Interaction between the Recombination Activating Gene Proteins RAG1 and RAG2

The RAG endonuclease consists of RAG1, which contains the active site for DNA cleavage, and RAG2, an accessory factor whose interaction with RAG1 is critical for catalytic function. How RAG2 activates RAG1 is not understood. Here, we used biolayer interferometry and pulldown assays to identify regions of RAG1 necessary for interaction with RAG2 and to measure the RAG1-RAG2 binding affinity (KD ∼0.4 μM) (where RAG1 and RAG2 are recombination activating genes 1 or 2). Using the Hermes transposase as a guide, we constructed a 36-kDa "mini" RAG1 capable of interacting robustly with RAG2. Mini-RAG1 consists primarily of the catalytic center and the residues N-terminal to it, but it lacks a zinc finger region in RAG1 previously implicated in binding RAG2. The ability of Mini-RAG1 to interact with RAG2 depends on a predicted α-helix (amino acids 997-1008) near the RAG1 C terminus and a region of RAG1 from amino acids 479 to 559. Two adjacent acidic amino acids in this region (Asp-546 and Glu-547) are important for both the RAG1-RAG2 interaction and recombination activity, with Asp-546 of particular importance. Structural modeling of Mini-RAG1 suggests that Asp-546/Glu-547 lie near the predicted 997-1008 α-helix and components of the active site, raising the possibility that RAG2 binding alters the structure of the RAG1 active site. Quantitative Western blotting allowed us to estimate that mouse thymocytes contain on average ∼1,800 monomers of RAG1 and ∼15,000 molecules of RAG2, implying that nuclear concentrations of RAG1 and RAG2 are below the KD value for their interaction, which could help limit off-target RAG activity.

An interdomain boundary in RAG1 facilitates cooperative binding to RAG2 in formation of the V(D)J recombinase complex.

V(D)J recombination assembles functional antigen receptor genes during lymphocyte development. Formation of the recombination complex containing the recombination activating proteins, RAG1 and RAG2, is essential for the site-specific DNA cleavage steps in V(D)J recombination. However, little is known concerning how complex formation leads to a catalytically-active complex. Here, we combined limited proteolysis and mass spectrometry methods to identify regions of RAG1 that are sequestered upon association with RAG2. These results show that RAG2 bridges an interdomain boundary in the catalytic region of RAG1. In a second approach, mutation of RAG1 residues within the interdomain boundary were tested for disruption of RAG1:RAG2 complex formation using fluorescence-based pull down assays. The core RAG1 mutants demonstrated varying effects on complex formation with RAG2. Interestingly, two mutants showed opposing results for the ability to interact with core versus full length RAG2, indicating that the non-core region of RAG2 participates in binding to core RAG1. Significantly, all of the RAG1 interdomain mutants demonstrated altered stoichiometries of the RAG complexes, with an increased number of RAG2 per RAG1 subunit compared to the wild type complex. Based on our results, we propose that interaction of RAG2 with RAG1 induces cooperative interactions of multiple binding sites, induced through conformational changes at the RAG1 interdomain boundary, and resulting in formation of the DNA cleavage active site.

RAG1-mediated ubiquitylation of histone H3 is required for chromosomal V(D)J recombination.

RAG1 and RAG2 proteins are key components in V(D)J recombination. The core region of RAG1 is capable of catalyzing the recombination reaction; however, the biological function of non-core RAG1 remains largely unknown. Here, we show that in a murine-model carrying the RAG1 ring-finger conserved cysteine residue mutation (C325Y), V(D)J recombination was abrogated at the cleavage step, and this effect was accompanied by decreased mono-ubiquitylation of histone H3. Further analyses suggest that un-ubiquitylated histone H3 restrains RAG1 to the chromatin by interacting with the N-terminal 218 amino acids of RAG1. Our data provide evidence for a model in which ubiquitylation of histone H3 mediated by the ring-finger domain of RAG1 triggers the release of RAG1, thus allowing its transition into the cleavage phase. Collectively, our findings reveal that the non-core region of RAG1 facilitates chromosomal V(D)J recombination in a ubiquitylation-dependent pathway.

Spatio-temporal regulation of RAG2 following genotoxic stress.

V(D)J recombination of lymphocyte antigen receptor genes occurs via the formation of DNA double strand breaks (DSBs) through the activity of RAG1 and RAG2. The co-existence of RAG-independent DNA DSBs generated by genotoxic stressors potentially increases the risk of incorrect repair and chromosomal abnormalities. However, it is not known whether cellular responses to DSBs by genotoxic stressors affect the RAG complex. Using cellular imaging and subcellular fractionation approaches, we show that formation of DSBs by treating cells with DNA damaging agents causes export of nuclear RAG2. Within the cytoplasm, RAG2 exhibited substantial enrichment at the centrosome. Further, RAG2 export was sensitive to inhibition of ATM, and was reversed following DNA repair. The core region of RAG2 was sufficient for export, but not centrosome targeting, and RAG2 export was blocked by mutation of Thr(490). In summary, DNA damage triggers relocalization of RAG2 from the nucleus to centrosomes, suggesting a novel mechanism for modulating cellular responses to DSBs in developing lymphocytes.

The RAG2 acidic hinge regulates pathway choice, maintains genomic stability, and prevents lymphomagenesis (P4398)

Abstract VDJ recombination-associated DNA double-stranded breaks are normally repaired by the high fidelity, classical NHEJ machinery. Previous studies have implicated the RAG/DNA post-cleavage complex (PCC) in regulating pathway choice by preventing access to inappropriate alternative repair mechanisms such as HR and alternative NHEJ (aNHEJ). We report that RAG2’s acidic hinge is critical for several steps. Mutations reducing the negative charge in this region destabilize the PCC, disrupt pathway choice, permit repair of RAG-mediated DSBs by the low-fidelity, translocation-prone aNHEJ machinery, and reduce genomic stability in developing lymphocytes. Structural predictions and experimental data support our hypothesis that flexibility of the acidic hinge is necessary for these functions. Similar human RAG2 sequence variants alter the predicted flexibility and permit repair by aNHEJ, indicating the potential for hazardous physiological consequences. Mice bearing similar neutralizing mutations (RAG2Neut), in the absence of p53, have an increased incidence of lymphomagenesis with hallmarks of VDJ recombination errors. Thus, the acidic hinge, along with stabilizing the PCC and ensuring proper pathway choice, aids in the prevention of lymphomagenesis by maintaining genomic integrity. This novel regulatory role that helps to ensure the safe development of a diverse immune repertoire, is to the best of our knowledge, the first function ascribed to this conserved region of RAG2.

Genetic mutation of recombination activating gene 1 in Dahl salt-sensitive rats attenuates hypertension and renal damage.

Hypertension and renal damage in Dahl SS rats are associated with increased infiltrating immune cells in the kidney. To examine the role of infiltrating immune cells in this disease process, a zinc finger nuclease targeting bases 672-706 of recombination-activating gene 1 (Rag1) was injected into the pronucleus of Dahl SS (SS/JrHsdMcwi) strain embryos and implanted in pseudopregnant females. This strategy yielded a rat strain with a 13-base frame-shift mutation in the target region of Rag1 and a deletion of immunoreactive Rag1 protein in the thymus. Flow cytometry demonstrated that the Rag1-null mutant rats have a significant reduction in T and B lymphocytes in the circulation and spleen. Studies were performed on SS and Rag1-null rats fed a 4.0% NaCl diet for 3 wk. The infiltration of T cells into the kidney following high-salt intake was significantly blunted in the Rag1-null rats (1.7 ± 0.6 × 10(5) cells/kidney) compared with the Dahl SS (5.6 ± 0.9 × 10(5) cells/kidney). Accompanying the reduction in infiltration of immune cells in the kidney, mean arterial blood pressure and urinary albumin excretion rate were significantly lower in Rag1-null mutants (158 ± 3 mmHg and 60 ± 16 mg/day, respectively) than in SS rats (180 ± 11 mmHg and 251 ± 37 mg/day). Finally, a histological analysis revealed that the glomerular and tubular damage in the kidneys of the SS rats fed a high-salt diet was also attenuated in the Rag1 mutants. These studies demonstrate the importance of renal infiltration of immune cells in the pathogenesis of hypertension and renal damage in Dahl SS rats.

A Dual Interaction between the DNA Damage Response Protein MDC1 and the RAG1 Subunit of the V(D)J Recombinase

The first step in V(D)J recombination is the formation of specific DNA double-strand breaks (DSBs) by the RAG1 and RAG2 proteins, which form the RAG recombinase. DSBs activate a complex network of proteins termed the DNA damage response (DDR). A key early event in the DDR is the phosphorylation of histone H2AX around DSBs, which forms a binding site for the tandem BRCA1 C-terminal (tBRCT) domain of MDC1. This event is required for subsequent signal amplification and recruitment of additional DDR proteins to the break site. RAG1 bears a histone H2AX-like motif at its C terminus (R1Ct), making it a putative MDC1-binding protein. In this work we show that the tBRCT domain of MDC1 binds the R1Ct motif of RAG1. Surprisingly, we also observed a second binding interface between the two proteins that involves the Proline-Serine-Threonine rich (PST) repeats of MDC1 and the N-terminal non-core region of RAG1 (R1Nt). The repeats-R1Nt interaction is constitutive, whereas the tBRCT-R1Ct interaction likely requires phosphorylation of the R1Ct motif of RAG1. As the C terminus of RAG1 has been implicated in inhibition of RAG activity, we propose a model in which phosphorylation of the R1Ct motif of RAG1 functions as a self-initiated regulatory signal.

Elucidating the domain architecture and functions of non-core RAG1: The capacity of a non-core zinc-binding domain to function in nuclear import and nucleic acid binding

BackgroundThe repertoire of the antigen-binding receptors originates from the rearrangement of immunoglobulin and T-cell receptor genetic loci in a process known as V(D)J recombination. The initial site-specific DNA cleavage steps of this process are catalyzed by the lymphoid specific proteins RAG1 and RAG2. The majority of studies on RAG1 and RAG2 have focused on the minimal, core regions required for catalytic activity. Though not absolutely required, non-core regions of RAG1 and RAG2 have been shown to influence the efficiency and fidelity of the recombination reaction.ResultsUsing a partial proteolysis approach in combination with bioinformatics analyses, we identified the domain boundaries of a structural domain that is present in the 380-residue N-terminal non-core region of RAG1. We term this domain the Central Non-core Domain (CND; residues 87-217).ConclusionsWe show how the CND alone, and in combination with other regions of non-core RAG1, functions in nuclear localization, zinc coordination, and interactions with nucleic acid. Together, these results demonstrate the multiple roles that the non-core region can play in the function of the full length protein.

Autoinhibition of DNA cleavage mediated by RAG1 and RAG2 is overcome by an epigenetic signal in V(D)J recombination

Gene assembly of the variable domain of antigen receptors is initiated by DNA cleavage by the RAG1-RAG2 protein complex at sites flanking V, D, and J gene segments. Double-strand breaks are produced via a single-strand nick that is converted to a hairpin end on coding DNA and a blunt end on the neighboring recombination signal sequence. We demonstrate that the C-terminal regions of purified murine RAG1 (aa 1009-1040) and RAG2 (aa 388-520, including a plant homeodomain [PHD domain]) collaborate to inhibit the hairpinning stage of DNA cleavage. The C-terminal region of RAG2 stabilizes the RAG1/2 heterotetramer but destabilizes the RAG-DNA precleavage complex. This destabilization is reversed by binding of the PHD domain to a histone H3 peptide trimethylated on lysine 4 (H3K4me3). The addition of H3K4me3 likewise alleviates the RAG1/RAG2 C-terminus-mediated inhibition of hairpinning and the PHD-mediated inhibition of transposition activity. Thus a negative regulatory function of the noncore regions of RAG1/2 limits the RAG endonuclease activity in the absence of an activating methylated histone tail bound to the complex.

Analysis of mutations and recombination activity in RAG-deficient patients

Mutations in the recombination activating genes (RAG1 or RAG2) can lead to a variety of immunodeficiencies. Herein, we report 5 cases of RAG deficiency from 5 families: 3 of Omenn syndrome, 1 of severe combined immunodeficiency, and 1 of combined immunodeficiency with oligoclonal TCRγδ(+) T cells, autoimmunity and cytomegalovirus infection. The genetic defects were heterogeneous and included 6 novel RAG mutations. All missense mutations except for Met443Ile in RAG2 were located in active core regions of RAG1 or RAG2. V(D)J recombination activity of each mutant was variable, ranging from half of the wild type activity to none, however, a significant decrease in average recombination activity was demonstrated in each patient. The reduced recombination activity of Met443Ile in RAG2 may suggest a crucial role of the non-core region of RAG2 in V(D)J recombination. These findings suggest that functional evaluation together with molecular analysis contributes to our broader understanding of RAG deficiency.

A Zinc Site in the C-Terminal Domain of RAG1 Is Essential for DNA Cleavage Activity

The recombination-activating protein, RAG1, a key component of the V(D)J recombinase, binds multiple Zn2+ ions in its catalytically required core region. However, the role of zinc in the DNA cleavage activity of RAG1 is not well resolved. To address this issue, we determined the stoichiometry of Zn2+ ions bound to the catalytically active core region of RAG1 under various conditions. Using metal quantitation methods, we determined that core RAG1 can bind up to four Zn2+ ions. Stripping the full complement of bound Zn2+ ions to produce apoprotein abrogated DNA cleavage activity. Moreover, even partial removal of zinc-binding equivalents resulted in a significant diminishment of DNA cleavage activity, as compared to holo-Zn2+ core RAG1. Mutants of the intact core RAG1 and the isolated core RAG1 domains were studied to identify the location of zinc-binding sites. Significantly, the C-terminal domain in core RAG1 binds at least two Zn2+ ions, with one zinc-binding site containing C902 and C907 as ligands (termed the CC zinc site) and H937 and H942 coordinating a Zn2+ ion in a separate site (HH zinc site). The latter zinc-binding site is essential for DNA cleavage activity, given that the H937A and H942A mutants were defective in both in vitro DNA cleavage assays and cellular recombination assays. Furthermore, as mutation of the active-site residue E962 reduces Zn2+ coordination, we propose that the HH zinc site is located in close proximity to the DDE active site. Overall, these results demonstrate that Zn2+ serves an important auxiliary role for RAG1 DNA cleavage activity. Furthermore, we propose that one of the zinc-binding sites is linked to the active site of core RAG1 directly or indirectly by E962.

The roles of the RAG1 and RAG2 “non-core” regions in V(D)J recombination and lymphocyte development

The enormous repertoire of the vertebrate specific immune system relies on the rearrangement of discrete gene segments into intact antigen receptor genes during the early stages of B-and T-cell development. This V(D)J recombination is initiated by a lymphoid-specific recombinase comprising the RAG1 and RAG2 proteins, which introduces double-strand breaks in the DNA adjacent to the coding segments. Much of the biochemical research into V(D)J recombination has focused on truncated or "core" fragments of RAG1 and RAG2, which lack approximately one third of the amino acids from each. However, genetic analyses of SCID and Omenn syndrome patients indicate that residues outside the cores are essential to normal immune development. This is in agreement with the striking degree of conservation across all vertebrate classes in certain non-core domains. Work from multiple laboratories has shed light on activities resident within these domains, including ubiquitin ligase activity and KPNA1 binding by the RING finger domain of RAG1 and the recognition of specific chromatin modifications as well as phosphoinositide binding by the PHD module of RAG2. In addition, elements outside of the cores are necessary for regulated protein expression and turnover. Here the current state of knowledge is reviewed regarding the non-core regions of RAG1 and RAG2 and how these findings contribute to our broader understanding of recombination.