Local Sequence Targeting in the AID/APOBEC Family Differentially Impacts Retroviral Restriction and Antibody

Nucleic acid cytidine deaminases of the activation-induceddeaminase (AID)/APOBEC family are critical players in activeand innate immune responses, playing roles as target-directed,purposeful mutators. AID specifically deaminates the hostimmunoglobulin (Ig) locus to evolve antibody specificity,whereas its close relative, APOBEC3G (A3G), lethally mu-tates the genomes of retroviral pathogens such as HIV. Un-derstanding the basis for the target-specific action of theseenzymes is essential, as mistargeting poses significant risks,potentially promoting oncogenesis (AID) or fostering drugresistance (A3G). AID prefers to deaminate cytosine inWRC (W role in the anti-HIV activity of A3G (3–6). The importance ofA/T, R A/G) motifs, whereas A3G favorsdeamination of CCC motifs. This specificity is largely dic-tated by a single, divergent protein loop in the enzyme fam-ily that recognizes the DNA sequence. Through grafting ofthis substrate-recognition loop, we have created enzymevariants of A3G and AID with altered local targeting to di-rectly evaluate the role of sequence specificity on immunefunction. We find that grafted loops placed in the A3G scaf-fold all produced efficient restriction of HIV but that for-eign loops in the AID scaffold compromised hypermutationand class switch recombination. Local targeting, therefore,appears alterable for innate defense against retroviruses byA3G but important for adaptive antibody maturation cata-lyzed by AID. Notably, AID targeting within the Ig locus isproportionally correlated to its