his tag

[brownk12]

I put an HA and His6 tag on the N-terminus of my protein and infected 4 different cell lines with the retrovirus that encodes for the protein and the cells died after 4 days (survived and looked "normal" after 2 days selection). The cells infected with the tag, alone, did not exhibit any cell death. I checked the sequence and it looks fine. I have over expressed this protein with an N-terminal FLAG tag before without any toxicity and I have used this dual tagged system with 4 other proteins in the past without any trouble. In addition, overexpression of the protein is not known to be toxic.
Can anyone give me an explanation for why I see cell death with this tag? The only explanation I can think of is that maybe with the two tags, the protein is getting caught up in the ER or something and is causing cell death. Has anyone made a similar observation? I appreciate your help. Thanks.