Function
Recombination activating gene (RAG) 2 is a key protein in the rearrangement and recombination of genes that encode immunoglobulins and T-cell receptors. Both RAG1 and RAG2 form a V(D)J recombinase complex that is essential in maturing B- and T-cells, both of which are critical for adaptive immunity (Sadofsky). RAG cleaves dsDNA between the antigen receptor and the recombination signal sequence. These segments are then rejoined in a variable fashion that allows for the diversity within an organism's immune system. The recombination allows for the T- and B-cells to mature and ultimately act in their specified manner. More specifically RAG2 has a plant homeodomain (PHD) that recognizes a methylated lysine on histone H3. This domain is crucial to the function of RAG.
Disease
Five specific residues were implicated in patients with severe combined immunodeficiency (SCID). These residues are involved in histone recognition. (Ramón-Maiques) One mechanism is in the disruption of the overall protein structure results in T(-)B(-) SCID. This autosomal recessive disease is caused by an inability to form fully mature T- and B-cells, thus rendering them ineffective within the patient. Natural Killer cells are still present within the patient and the thalamus is typically hypoplastic. The condition can usually be treated with a bone marrow transplant. (Fischer)
Failure to bind to H3K4me-3 Omenn Syndrome is a special type of SCID that is usually classified by a failure to thrive and elevated serum IgE levels. The autosomal recessive disease renders patients with low immunoglobulin levels and no B-cells. There are levels of T-cells in the blood but they are functionally impaired. (Aleman) Hypomorphic mutations of RAG contribute to Omenn Syndrome/
Interestingly, infants born with the same RAG mutations can develop either SCID or Omenn Syndrome indicating that there may be an environmental component that contributes to disease developmemt.
Relevance
Structural highlights
RAG2 alone has 82 residues and is comprised of three chains(RCSB). It is bound to multiple zinc ions which help to stabilize the DNA during recombination and also act in conjunction with the PHD helping to recognize H3. The structure also demonstrates multiple N-Trimethyllysines. RAG2 contains a PHD near its C-terminus that specifically recognizes a methylated lysine 4 on histone H3. One unique component of the RAG2-PHD complex is that an peptide N-terminal to the RAG2-PHD can bind to the substrate-binding site. The ability of protein segments down the line to bind to an earlier section demonstrates the potential autoregulatory nature of this peptide(Ramón-Maiques). The peptide is also modified to yield enhanced binding to the H3K4me3. Normal interaction involves a carboxylate with an arginine 2 (R2). The RAG2-PHD replaces this carboxylate with a tyrosine (T445) which gives an interaction that is enhanced by the dimethylation of the R2 rather than inhibited.
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References
Sadofsky MJ. The RAG proteins in V(D)J recombination: more than just a nuclease. Nucleic Acids Res. 2001;29(7):1399–1409. doi:10.1093/nar/29.7.1399
1. Ramon-Maiques, S., et al., The plant homeodomain finger of RAG2 recognizes histone H3 methylated at both lysine-4 and arginine-2. Proc Natl Acad Sci U S A, 2007. 104(48): p. 18993-8.
Fischer A. Severe combined immunodeficiencies (SCID). Clin Exp Immunol. 2000;122(2):143–149. doi:10.1046/j.1365-2249.2000.01359.x
