Abstract
Enzymes of the AID/APOBEC family, characterized by the targeted deamination of cytosine to generate uracil within DNA, mediate numerous critical immune responses. One family member, activation-induced cytidine deaminase (AID), selectively introduces uracil into antibody variable and switch regions, promoting antibody diversity through somatic hypermutation or class switching. Other family members, including APOBEC3F and APOBEC3G, play an important role in retroviral defense by acting on viral reverse transcripts. These enzymes are distinguished from one another by targeting cytosine within different DNA sequence contexts; however, the reason for these differences is not known. Here, we report the identification of a recognition loop of 9-11 amino acids that contributes significantly to the distinct sequence motifs of individual family members. When this recognition loop is grafted from the donor APOBEC3F or 3G proteins into the acceptor scaffold of AID, the mutational signature of AID changes toward that of the donor proteins. These loop-graft mutants of AID provide useful tools for dissecting the biological impact of DNA sequence preferences upon generation of antibody diversity, and the results have implications for the evolution and specialization of the AID/APOBEC family.
Highlights
The polynucleotide cytosine deaminases have been identified as key contributors to both the adaptive and innate immune responses to pathogens
Identification of a Potential DNA Sequence Recognition Loop— recent structures of the C-terminal catalytic domain of APOBEC3G have increased our understanding of the overall enzymatic-fold, there are currently no structures of a polynucleotide cytosine deaminase bound to nucleic acid to elucidate how hot spot sequences are recognized [26, 27]
This work identifies a recognition loop that significantly accounts for the different substrate sequence preferences in the activation-induced cytidine deaminase (AID)/APOBEC family
Summary
The polynucleotide cytosine deaminases have been identified as key contributors to both the adaptive and innate immune responses to pathogens This enzyme family includes activation-induced cytidine deaminase (AID), which initiates antibody diversification, and the APOBEC3 enzymes, which inhibit retroviral infection [1, 2]. AID is a B-cell-specific enzyme whose catalytic action on mammalian antibody genes initiates somatic hypermutation and class switch recombination [4, 5]. The activity of AID is restricted to relatively small 3-kb regions within the immunoglobulin locus around rearranged variable genes and heavy chain switch regions. Within these regions, the targeting of AID is partially dictated by its DNA sequence preferences. Biochemical assays with heterologously expressed AID have confirmed that this mutational footprint reflects the inherent sequence preferences of AID for deamination within WRC motifs [12,13,14]
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