Evaluation of employing poly-lysine tags versus poly-histidine tags for purification and characterization of...

Quantitative characterization of metalloproteins at molecular and atomic levels generally requires tens of milligrams of highly purified samples, a situation frequently challenged by problems in generating unmodified native forms. A variety of affinity tags, such as the popular poly-histidine tag, have been developed to facilitate purification but they generally rely on expensive affinity resins and their presence may interfere with protein characterization. This paper documents that addition of a poly-lysine tag to the C-terminus enables, for the copper-binding proteins examined, ready purification in large scale via cost-effective cation-exchange chromatography. The tag may be removed readily by the enzyme carboxypeptidase B to generate the native protein with no extra residues. However, this cleavage step is normally not necessary since the poly-lysine tag is shown to have no detectable affinity for either Cu(I) or Cu(II) and imposes no interference to the copper binding properties of the target proteins. In contrast, the poly-histidine tag possesses a sub-picomolar affinity for Cu(I) and -nanomolar affinity for Cu(II) and may need to be removed for reliable characterization of the target proteins. These conclusions may be extended to the study of other metallo-proteins and metallo-enzymes.