I have a human olfactory receptors transfected into 293 cells which is a 40 kDa protein runs as a 200KDa 'smear'. Masks antibody epitope (presumably) - tried 2 antibodies and positive control is fine. It is HA tagged and GFP tagged, and I can see it under a microscope and see it on a gel only when probing for the tag....it is much larger than it should be. It almost looks like it is forming new disulphide bonds. I am using a strong reducing agent and boiling a lot before loading. I've used many tagged proteins in 293 cells and I have never seen this before. HA or GFP tag effect? Unusual covalent modification in Hek 293 cells? Appreciate any input.