Pichia pastoris as a host for production and isolation of mutagenic AID/APOBEC enzymes involved in cancer and immunity

Abstract

AID/APOBEC3 enzymes are cytidine deaminases that mutate antibody and retroviral genes and also mediate extensive tumor genome mutagenesis. The study of purified AID/APOBEC3 proteins is challenged by difficulties with their expression and purification arising from genotoxicity in expression hosts, extensive non-specific protein-protein/DNA/RNA interactions and haphazard oligomerization. To date, expression hosts for purification of AID/APOBEC3 enzymes include bacteria, insect and mammalian cells. Here the establishment and optimization of a yeast expression/secretion system for AID/APOBEC3s are reported, followed by comparison with the same enzymes expressed in bacterial and mammalian hosts. AID and APOBEC3G were expressed successfully in Pichia pastoris, each either with an N-terminal GST tag, C-terminal V5-His tag or as untagged native form. It was verified that the yeast-expressed enzymes exhibit identical biochemical properties to those reported using bacterial and mammalian expression, indicating high fidelity of protein folding. It was demonstrated that the system can be adapted for secretion of the enzymes into the media which was used directly in various enzyme assays. The system is also amenable to elimination of bulky fusion tags, providing native untagged enzymes. Thus, P. pastoris is an advantageous expression factory for AID/APOBEC3 enzymes, considering the cost, time, efficiency and quality of the obtained enzymes. The first report is also provided here of a functionally active, untagged, secreted AID, which may become a useful research reagent. A comprehensive comparison is made of the effect of fusion tags and expression hosts on the biochemical actions of AID and APOBEC3G.