The core regions acted as focal points of subsequent research, mainly

because they were more soluble than their full-length counterparts. Using surface plasmon resonance and in vivo one-hybrid experiments, it was shown that the SCH 900776 in vivo N-terminus of cRAG1 (amino acids 384–460) harbours the nonamer binding region.[28] The heptamer recognition region of RAGs still remains obscure. The DDE motif (a triad of three acidic amino acids: D600, D708 and E962) of RAG1 forms the catalytic centre of the RAG1/RAG2 complex,[64-66] which plays a role in chelating the two divalent metal ions essential for catalysis.[67] The N-terminal non-core region (amino acids 1–383) contains a RING domain fold, which exhibits ubiquitin ligase activity.[68] Studies by Rodgers’s group[63] using limited proteolysis showed that murine cRAG1 is composed of topologically independent domains that can function individually. These include the N-terminal, the central and the C-terminal domains. The central domain has the heptamer binding site, RAG2 binding site and zinc

finger motif. The C-terminal domain has the dimerization region and binds DNA co-operatively. Murine cRAG1 was successfully expressed in Escherichia coli as a fusion protein with Maltose binding protein (MBP) tag with high yield and solubility and was active when combined with cRAG2 expressed in human embryonic kidney cell line.[69] However, there is no report of successful bacterial expression of RAG2. Murine ‘core RAG2’ consists of amino GSK126 acids 1–383 out of the total 527. The molecular function of core RAG2 remains elusive. RAG2 consists of an N-terminal 6-bladed beta-propeller domain and a C-terminal plant homeo domain (PHD).[70, 71] The PHD is a motif characteristic of chromatin remodelling proteins.[72] It has been predicted to facilitate the ordered Dimethyl sulfoxide rearrangement of IgH chains and the binding of core histone proteins.[72-74] The C-terminus of RAG2 contains a threonine residue (T490)

that acts as a target of Chk2 kinase.[75] Phosphorylation of this amino acid regulates the proteosomal degradation of RAG2 at the G1/S transition of the cell cycle.[76] This regulatory mechanism ensures that RAG2 is degraded in a cell-cycle-dependent manner preventing RAG-induced DNA breaks during replication. Biochemical analysis of recombinant RAG2 has identified several basic residue mutants defective in catalysis. Accordingly, Schatz’s group[77] has proposed a model for the interaction of RAG2 with DNA in which the amino acids K119 and K283 directly contact DNA. It was shown that the PHD finger specifically recognizes histone 3 trimethylated at lysine 4 (H3K4me3).[78] The H3K4me3 increases the catalytic turnover number (Kcat) of RAGs as well as tethering it to DNA.