The mechanisms of thermosensitivity of human and bony fish activation-induced cytidine deaminase (AID)

Abstract

Activation-induced cytidine deaminase (AID) is an enzyme that initiates the secondary antibody diversification events of somatic hypermutation (SHM) and class switch recombination (CSR). AID is expressed in activated B lymphocytes and deaminates cytidines to uridines, thus introducing mutations and/or double-strand breaks throughout the immunoglobulin (Ig) locus. Although AID activity is mostly targeted to the Ig locus, it mutates many other genes at lower rates and this off-target activity induces genomic lesions leading to transformation of B cells. The structure of AID has not been solved, which is partially the reason that several important questions remain unanswered about its mechanism of action. Our lab has recently shown that AID from different species has disparate rates of deamination, and that the deamination rate of AID from each species is thermosensitive, such that fish AID are cold-adapted relative to human AID. Here, we have measured the activity of purified AID from human, zebrafish, channel catfish, pufferfish (takifugu), and hybrid AID containing various parts of the aforementioned, over a wide range and fine increments of temperatures. Our aim was to elucidate the relationship between thermosensitivity of AID and its primary structure, by identifying region(s) and/or residue(s) within the enzyme that affect thermosensitivity. We found that this characteristic is mostly determined by the C-terminus in the sense that swapping this region resulted in transferring thermosensitivity patterns from the donor enzyme. We also found that AID activity is not always optimal at native physiological temperature, such that human AID is optimal at 32°C instead of 37°C. Based on our data, we postulated that this finding may have implications for evolution of AID. Overall, our work has shed light on mechanistic features of AID as well as aspects on its evolution.