A histidine-rich fusion tag enables real-time monitoring of recombinant protein expression by Pauly reaction-based colorimetric assay

Commercially available recombinant expression systems always use fusion tags to facilitate target protein purification and SDS-PAGE analysis followed by Coomassie Brilliant Blue (CBB) staining is the classical method to validate the expression level of target protein, which is time-consuming, although not very laborious. Previously, we found that a histidine-rich elastin-like polypeptide (HRELP) tag could make its fusion proteins being quickly and specifically stained with Pauly's reagent. In this study, we designed a Pauly reaction-based colorimetric assay to real-time monitoring of the expression level of recombinant protein tagged HRELP and found that the absorption value of post-induction E. coli cells stained with Pauly's reagent correlated well with both the band intensity of the target protein from Pauly's reagent-stained and CBB-stained gels. Moreover, we found the colorimetric assay could also be helpful to roughly estimate the expression efficiency by using a poly-histidine-tagged protein, which has only 1.17% histidine residue. In our opinion, Pauly reaction-based colorimetric assay could significantly shorten the time to validate the over-expression of recombinant protein tagged with either HRELP or poly-histidine. And HRELP seemed to be an ideal fusion tag for it can not only facilitate protein purification but also simplify protein detection.