C-terminal tagging impairs AGO2 function

Abstract

A full understanding of RNA silencing requires appropriate molecular biology tools to explore the roles of Argonaute 2 (AGO2) and the RNA-induced silencing complex (RISC). Approaches relying on affinity tagging and antibodies have important limitations that can lead to artificial results. Both the N- and C-terminal domains of AGO2 have been shown to be important for correct activity and yet the consequences of appending tags to either terminus have not been fully investigated. N-terminal tags are frequently used to study AGO2 biology. Recently, an N-terminal HaloTag-Ago2 fusion was reported and examined in mice. While the versatile HaloTag provided new opportunities to study RISC biology, the tagged construct showed certain activity changes compared to unmodified AGO2. CRISPaint, a new CRISPR-Cas9 technique, permits the creation of endogenous C-terminal tag fusions. We used CRISPaint to generate the first reported recombinant AGO2 construct with a C-terminal tag: an endogenous C-terminal HaloTag fusion to AGO2 (AGO2HALO) in human (A549) cells. We found that the AGO2HALO fusion protein has a reduced capacity to interact with the key protein binding partner TNRC6A and that the C-terminal HaloTag does not affect cell viability. However, the AGO2HALO fusion significantly impairs RNA cleavage and RNA silencing activity compared to control cells and reduces nuclear localisation of the fusion protein. Using plasmid constructs and transient transfection, we compared AGO2 tagged with EGFP at the N- or C- terminus in siRNA and miRNA reporter gene assays, and cellular localisation. N-terminally tagged AGO2 functioned and localised similarly to WT untagged AGO2, whereas, C-terminally tagged AGO2 was impaired in siRNA and miRNA silencing and exhibited poor nuclear and P-body localisation. We conclude that the fusion of a C-terminal HaloTag to AGO2 is not appropriate for studying AGO2 and RISC. Our results assert the importance of comprehensively validating recombinant tagging strategies to ensure that any experimental results generated do not arise from, or are not obscured by critical functional defects.