Question

Explain the discovery, mechanism of action and function of AID (activation-induced deaminase
SOMATIC HYPERMUTATION (Figures 7 and 8). Indeed, the discovery of AID provided the missing MSH2-deficient mice had previously been taken to indicate that somatic mutation is initiated by diversification at C:G pairs with MSH2-mediated recognition then triggering a subsequent stage of diversification at A:T pairs. Such a two-stage model was supportedations by the observation that CG-biased mutation was also a characteristic of somatic mutation in B-c nes9 and had also been noted in the frog* as well as in a mutating mouse pre-B-cell line*7 (although the Figure 7 Mechanism of somatic mutation. AID-mediated cytosine relevance of the mutation in the pre-B line to physiological antibody deamination in the IgV creates U:G mismatches. Replication across the hypermutation had been controversial; Figure 4b). While a process resultant UNG-generated abasic site leads to nucleotide substitutions at C.o that initially targets mutations to C:G pairs and only subsequently pairs. Recognition of the U:G mismatch by MSH2/MSH6 yields to an error- mutates A:T pairs is not readily compatible with a simple error-prone generating patch repair, which generates mutations predominantly at polymerase mechanism in the mould of the Brenner-Milstein mo pairs. In chicken, gene conversion results I am not aware that any significant thought had been given prior to the discovery of AID of how such C:G-targeted mutation might be achieved. However, once AID was in the picture, direct deamination of a cytidine in DNA should have provided a ready explanation. (templated on the Igv pseudogenes) of the UNG-generated abasic site. Although the phenotype of AID-deficient mice and humans provided a dramatic pointer towards switch recombination and somatic mutation exhibiting some commonality in their carly steps, there were also earlier hints in that direction coming from other sources. Thus, not only were the intrinsic hotspots of somatic essentially identical to one of the constituent motifs of S regions; it switch breakpoints in a manner analogous to the refocusing of somatic constitute alternative ways of processing a common initiating DNA chicken DT40 cells could be shifted from one of gene conversion to Figure 8 AD-med ated DNA deamination triggers multiple pathways of antibody diversification. a) AID acts by deaminating cytosine in DNA to generating a U:G mismatch that is recognized by components of transitions that was evident in in vio antibody in man and mouse was both base-excision (UNG, uracil-DNA glycosylase) and mismatch (MSH, not retained in the CG-biased IgV hypermutation that occurred in hamologue of bacterial mutS) pathways. (b) Targeted deamination in the Igv leads to lesions which are resolved by forms of lesion bypass (yielding Although there was no precedent for any physiological programme omatic hypermutation) or recombinational repair (yielded lgv gene national repair. Interestingly,however, the preference for nucleotideuracil in vertebrates that depended on targeted deamination of DNA, invoking such a mechanism of AID action immediately suggested conversion). see Figure 7 for more detail.) Targeted deamination in the vicinity of the switch regions appears to lead (following UNG action) to double-strand DNA breaks, which result in class-switch recombination explanations for these disparate observations that had previously lowing non-homologous (ar, in some cases, microhomology-mediated TESTING THE DNA DEAMINATION MECHANISM The DNA deamination schemes made many predictions, several of significant part of the story.5s8 AID does not target all cytidines equally which were readily testable. It did, of course, predict that AID could but rather shows preferences that accord with a preference for a act to deaminate cytidine in DNA. This was confirmed using both 5,-WRC substrate where R-purine and W N. The intrinsic hot- bacterial genetic and biochemical assays.45,327 The enzyme has, spots of antibody hypermutation (WRCY, where Y-pyrimidine with however, been relatively refractory to detailed biochemical assay and AGCI being the preferred exemplification) might then be explained structural determination since intact active recombinant protein is not by different subunits of an AID oligomer acting on a substrate that readily produced in high yield. Nevertheless, it is evident that contains overlapping WRC motifs on opposite DNA strands.5 recombinant AID as well as AID rescued from B cells will deaminateWhile biochemistry revealed that AID could act on cytidine in cytidine in the context of a single-stranded DNA substrate. ThisDNA, genetics provided strong support for such targeted DNA obviously raises questions about how the Ig target is rendered deamination constituting the trigger for antibody diversification. A single-stranded in vivo in order for AID to gain access; an association clear prediction of the DNA deamination scheme is that deficiency in with the single-stranded protein RPA appears to be at least a
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Answer 1

AID is the Activation-induced cytidine deaminase, also known as AICDA. It is expressed in germinal center centroblasts.  It is encoded by the AICDA gene. It is 24 kDa enzyme. It causes mutation in DNA which can change the cytosine base,in to uracil by deamination of cytosine base. Foe ex.  C:G base pair change in to U:G mismatch.This mismatch can be recognized by the DNA mismatch repair (MMR) machinery, that is particular for MutSα(alpha) complex.  MSH2 and MSH6 are present in MutSα(alpha) complex.  The induction of AID takes place through the transcription factors E47, HoxC4, Irf8 and Pax5, and inhibited by Blimp1 and Id2.

Function of AID:

The main function of AID is  regulating secondary antibody diversification by the  initiation of three separate immunoglobulin (Ig) diversification processes known as Somatic hypermutation (SHM), Class switch recombination (CSR) and Gene conversion (GC). AID also  require for active transcription .AID is also used for cut or cleave specific target DNA. AID is a mutator of G:C base pair.