Calcium-assisted sortase A cleavage of SUMOylated metallothionein constructs leads to high-yield production of human MT3

Mammalian metallothioneins (MTs) are small (6-7 kDa), intracellular, cysteine-rich, metal-binding proteins involved, inter alia, in the homeostasis of zinc and copper, detoxification of heavy metals, antioxidation against reactive oxygen species, and protection against DNA damage. The high cysteine content (~ 30%) in MTs makes them toxic to bacterial cells during protein production, resulting in low yield. To address this issue, we present for the first time a combinatorial approach using the small ubiquitin-like modifier (SUMO) and/or sortase as fusion tags for high-level expression of human MT3 in E. coli and its purification by three different strategies.

The SUMO/sortase-based strategy used as the production system for MT3 resulted in a very high expression level and protein production yield. The apo-MT3 purified by this strategy contained an additional glycine residue and had similar metal binding properties as WT-MT3. This SUMO-sortase fusion system is a simple, robust, and inexpensive one-step purification approach for various MTs as well as other toxic proteins with very high yield via immobilized metal affinity chromatography (IMAC).

Three different plasmids were generated using SUMO, sortase A pentamutant (eSrtA), and sortase recognition motif (LPETG) as removable fusion tags for high-level expression and purification of human MT3 from the bacterial system. In the first strategy, SUMOylated MT3 was expressed and purified using Ulp1-mediated cleavage. In the second strategy, SUMOylated MT3 with a sortase recognition motif at the N-terminus of MT3 was expressed and purified using sortase-mediated cleavage. In the final strategy, the fusion protein His6-SUMO-eSrtA-LPETG-MT3 was expressed and purified by one-step sortase-mediated inducible on-bead autocleavage. Using these three strategies the apo-MT3 was purified in a yield of 11.5, 11, and 10.8 mg/L, respectively, which is the highest yield achieved for MT expression and purification to date. No effect of MT3 on Ni2+-containing resin was observed.