Secretory expression, immunoaffinity purification and metal-binding ability of recombinant metallothionein (ShMT) from freshwater crab Sinopotamon henanense

Abstract

Metallothioneins (MTs) are a super-family of ubiquitous, low-molecular-weight, cysteine-rich and metal-binding proteins. They are thought to play a predominant role in mediating metal metabolism and antioxidation. However, the accurate functions of MTs remain unclear in the physiological processes due to native proteins deficiency and little information of their metal-binding character. Freshwater crab Sinopotamon henanense is a decapod crustacean widely distributed in northern China, in which only one MT isoform (ShMT) has been reported so far. In order to shed light on the accurate role of ShMT, a novel recombinant ShMT in native form was over-expressed by phoA secreted expression system in Escherichia coli (E. coli). Then the ShMT proteins were purified using a one-step gentle immunoaffinity chromatography with a polyol-responsive mAb (PR-mAb) to ShMT, which was generated by conventional hybridoma technology followed by ELISA-elution. The Zn-, Cu-, and Cd-ShMT complexes were prepared by recombinant synthesis in metal-enriched media and reconstitution with metal ions, respectively. Further analysis about metal-binding capacity showed recombinant ShMT has high ability to bind Zn, Cu and Cd metals, although the recombinantly expressed and reconstituted metal-ShMT complexes have different metal-to-protein stoichiometry. Moreover, the affinity of recombinant protein for metal ions has been analyzed using competitive reaction with 5, 5-dithiobis (2-nitrobenzoic acid) (DTNB). The results demonstrated the affinity of recombinant ShMT for metals was as follows: Cu＞Cd＞Zn. In summary, the experimental procedure we have developed facilitates production of recombinant ShMT with native characteristics for further research and the study of metal-binding ability could help further clarify the accurate functions of ShMT.