Highly-Efficient Purification of Native Polyhistidine-Tagged Proteins by Multivalent NTA-Modified Magnetic Nanoparticles

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Abstract
A new bis-nitrilotriacetic acid (NTA) chelate with catechol anchor was synthesized and immobilized on superparamagnetic iron oxide nanoparticles. When loaded with Ni(II), these bis-NTA-immobilized nanoparticles were shown to bind polyhistidine (His × 6-tagged) fusion proteins in their native, folded conformations that commercial microbeads failed to bind under identical conditions. Control experiments with a mono-NTA chelate immobilized on iron oxide nanoparticles indicate a similarly high affinity for His × 6-tagged native proteins, suggesting that the high density of the mono-NTA chelate presented by the nanoparticles allows the binding of the His × 6-tag to more than one Ni-NTA moiety on the surface. This study shows that the multivalency strategy can be utilized to enhance the binding of His × 6-tagged proteins in their native, folded conformations. We further demonstrated the selective purification of His × 6-tagged proteins from crude cell lysates by using the Ni(II)-loaded iron oxide nanoparticles. The present platform is capable of efficient purification of His × 6-tagged proteins that are expressed at low levels in mammalian cells. This work thus presents a novel nanoparticle-based high-capacity protein purification system with shorter incubation times, proportionally large washes, and significantly smaller elution volumes compared to commercially available microbeads.