ADDITION OF A POLY-(GX) HIS TAG TO MILK BUNDLE-1 AND PURIFICATION USING IMMOBILIZED METAL-AFFINITY CHROMATOGRAPHY

Milk Bundle 1 (MB-1) is a de novo designed protein enriched in M, T, K , and L, Its future application is as a high-quality dietary protein s ource for ruminants, The protein is currently expressed in Escherichia coli and is being characterized to solve its folded conformation. MB- 1 has marginal stability at room temperature, which has hindered our a ttempts at characterization. To increase the stability of the protein at room temperature, the purification procedure was examined and chang ed to hopefully increase its effectiveness. We describe here the produ ction and purification of a new MB-1 with six His residues at the C-te rminal end. This allows the new mutant (MB-1-His) to bind metal ions a nd to be purified with immobilized metal-affinity chromatography (IMAC ). MB-1-His obtained using IMAC was purer on SDS-PAGE than both MB-1 o r MB-1-His isolated using the current protocol. The IMAC protocol is m ore economical and more efficient; preliminary results show that the p rotein purified by this method is also quite stable at room temperatur e. (C) 1998 Academic Press.