In line with the association between AID and ROD1, AID exhibits similar RNA binding profiles to ROD1, and the interaction is ROD1-dependent. (SHM) modifies antibody variable regions to improve antigen-binding affinity. Open in a separate window Fig. 1 Illustration of three pathways of antibody diversification. The diagram is for illustration purpose only and is not drawn to scale. The germline IgH locus is shown at the top. In B cell progenitors, V(D)J recombination joins various combinations of VH, D, and JH segments together to form the complete V(D)J exon, which encodes the Ig variable region. In na?ve B cells, C and C are expressed in association with the assembled V(D)J exon to form IgM and IgD, respectively. In response to antigenic challenge and B cell activation, class switch recombination occurs between switch region (S) in front of C and one of the downstream switch regions, for instance, S in the diagram. As a result, the downstream constant region (e.g., C in the diagram) replaces C and C, leading to the expression of a new class of antibody with distinct effector functions. Additionally, somatic hypermutation introduces high levels of mutations in the V(D)J exon. The microenvironment of germinal center selects for mutations that improve antigen-binding affinity CSR takes place Haloxon between switch regions that precede constant regions at the Haloxon IgH locus (Fig.?1). SHM introduces primarily point mutations into the variable region exons of both Ig heavy and light chains. Despite their apparent differences, CSR and SHM share similar mechanisms.2,3 At the core of both processes is Activation Haloxon Induced Cytidine Deaminase (AID). AID initiates CSR by deaminating cytidines in switch regions. The deamination leaves uracil in DNA. Uracil DNA glycosylase excises the uracil to form abasic sites. Apurinic endonuclease cleaves the abasic sites to break switch regions. Non-Homologous End Joining machinery ligates two broken switch regions to form the CSR product. During SHM, AID deaminates cytidines in Ig variable region exons. Subsequent processing of uracil by DNA replication and various repair pathways mutate the variable region. The mutagenic activity of AID is largely confined to switch regions and Ig variable regions. The restriction is not absolute, and AID acts on certain non-Ig loci at low levels. Despite the leakiness, the targeting mechanism minimizes deleterious mutations in B cells during an immune response. Oncogenic transformation of B lymphocyte is relatively rare in relation to innumerable B cell activation events throughout the lifespan of an individual. Understanding the AID targeting mechanism has been a major goal of the field. AID preferentially deaminates cytidines in the context of WRCY or the complementary RGYW motif. Such a Haloxon prevalent motif cannot be sufficient to unequivocally mark AID targets. Although AID is usually relatively nondiscriminatory with respect to substrates at the primary sequence level, the enzyme functions Haloxon purely on single-stranded DNA. As transcription transiently unwinds DNA template, the process is usually a major source of single-stranded DNA in genome. Consistent with this link, both CSR and SHM require transcription. Since AID does not take action on all transcribed regions, transcription at AID targets may be special in some way. Indeed, AID off-targets in non-Ig loci tend to become convergently transcribed.4,5 How convergent transcription Rabbit Polyclonal to CtBP1 attracts AID to off-targets, and whether or not it is involved in on-target activity at Ig loci has not been fully elucidated. The study by Chen and colleagues, which is definitely reported recently in em Cell Study /em ,6 shed some light on this query. Chen and colleagues investigated the AID focusing on mechanism by searching for factors that interact with AID and guide AID to genomic focuses on. The approach is definitely a well-trodden path and, over the years, offers yielded a bewilderingly large collection of AID-interacting proteins. However, the part of many of these putative co-factors remains to be rigorously validated by practical assays in vivo. With this important respect, the AID-interacting protein recognized by Chen and colleagues, Pole1/PTBP3,7 stands out from the masses. To test the function of Pole1, the authors knocked out its gene in mice. Amazingly, Pole1 deficiency led to severe reductions in both CSR and SHM. The phenotype is not an indirect result of problems in related processes. In this regard, Pole1-knockout mice contain normal B cell populations in different developmental phases, including germinal center B cells, where CSR and SHM take place. In response to B cell activation signals in vitro, the Pole1-deficient B cells proliferate, upregulate AID manifestation, and transcribe numerous switch regions at similar levels as wild-type B cells. In light of the direct connection between AID.
In line with the association between AID and ROD1, AID exhibits similar RNA binding profiles to ROD1, and the interaction is ROD1-dependent
- by globalhealth