Effects of the Fc-III tag on activity and stability of green fluorescent protein and human muscle creatine kinase.

The Fc-III tag is a newly developed fusion tag that can be applied to protein purification and detection. In the present work, we use the Fc-III-tagged green fluorescent protein (GFP) and human muscle creatine kinase (CK) as model systems to investigate effects of the Fc-III tag on activities and stabilities of the expressed multicysteine-containing proteins. Our results show the Fc-III tag has no adverse effects on the fluorescence of GFP and reduces the occurrence of GFP misfolding due to incorrect Cys oxidation compared with the His-tagged protein. The activity and stability of the Fc-III-tagged CK is slightly lower than that of the tag-free CK, but is higher than that of the His-tagged CK as determined by the ratio of the oxidized versus reduced CK. A major portion of His-tagged CK is in its oxidized form, while that of the Fc-III-tagged CK is in its reduced form. A folding model of CK with different tags was proposed, which may provide insights into the effect of the Fc-III tag on the conformations of disulfide-bridged proteins.