Expression, one-step purification, and immobilization of HaloTag ^TM fusion proteins on chloroalkane-functionalized magnetic beads

The presented work introduces a novel method to immobilize enzymes either purified or directly out of a crude extract onto magnetic particles in the micrometer range. This method is based on the creation of a fusion protein consisting of the enzyme of choice and a mutant dehalogenase. The dehalogenase gene is commercially available from the company Promega under the name HaloTag^TM. When the fusion protein is contacted with magnetic beads having chemically synthesized, chloroalkane ligands on their surface, the dehalogenase and the ligand undergo a covalent coupling leading to stable and spatially defined immobilization. The principle was proved with a lipase fused to the HaloTag^TM gene and magnetic poly(methyl)methacrylate beads as carriers. The solubility of the tagged lipase was strongly increased by fusion of the malE gene at the N-terminal end of the HaloTag^TM lipase gene. This tripartite protein was purified on amylose resin and used for immobilization. About 13 μg protein could be immobilized per 1 mg of beads within a few minutes. Due to the defined binding site, no activity loss was observed in the course of the immobilization. The resulting enzyme carrier was tested with the same beads up to six times for lipase activity over a storage period of 36 days at 8 °C. No loss of activity was found during this time.