Identification of new tag sequences with differential and selective recognition properties for the anti-FLAG monoclonal antibodies M1, M2 and M5

The FLAG peptides DYKDDDDK and MDYKDDDDK are widely used affinity tags. Here we describe new variants of the FLAG peptides which, in direct ELISA, showed selective and differential binding to the commercially available anti-FLAG monoclonal antibodies M1, M2 and M5. Variants of the FLAG peptides were synthesized on polymer-grafted plastic pins, and in an ELISA incubated with mAbs M1, M2 and M5. Among the newly identified tag sequences are those that bind only one of the anti-FLAG mAbs and those that bind only two or all three of the anti-FLAG mAbs. Examples of new tag sequences are MDFKDDDDK (which binds mAb M5 and does not bind mAbs M1 and M2) and MDYKAFDNL (which binds mAb M2 and does not bind mAbs M1 and M5). The sensitivity in direct ELISA of some variants was increased, e.g. using mAb M2 it was found that replacing DDDDK in MDYKDDDDK by AFDNL increased the sensitivity in ELISA at least 10-fold. The activity of this peptide was studied in more detail. In different direct ELISAs, in which MDYKAFDNL was synthesized on polyethylene pins, coated onto polystyrene microtiter plates or onto nitrocellulose paper, the activity of this peptide was similar, i.e. increased at least 10-fold over that of MDYKDDDDK. Remarkably, in competitive ELISA the binding activity of soluble MDYKAFDNL was decreased 10-fold over those of soluble MDYKDDDDK or DYKDDDDK. The results seem to suggest that, in solution, the conformation of MDYKAFDNL is more 'unstructured' compared to its conformation when coated or linked to a carrier. We postulate that the newly described tag sequences may be used as affinity tags to separately detect, quantify and purify multiple co-expressed proteins and/or subunits.