To produce natively-folded scorpion toxin directly from a bacterial expression system, we sub-cloned the Bj-xtrIT-(His6) gene into the pASK-IBA expression vector, thereby introducing an OmpA secretion signal at the toxin N-terminus. This sequence directs polypeptide secretion via the Sec pathway in the oxidative compartment of the E. coli periplasm. The signal is subsequently cleaved by an endopeptidase, producing the target gene product without an N-terminal adduct. The level of Bj-xtrIT-(His6) expression was extremely low ( Fig. 2A; lane 1). We attempted to use immobilized-metal affinity chromatography to isolate soluble toxin from the periplasmic extract in order to obtain a more concentrated solution of protein for visualization using SDS-PAGE. The toxin in its reduced form should migrate with a molecular mass of ~ 9.4 kDa but no band was observed on a 4–12% SDS-PAGE gel ( Fig. 2B; lane 1). These results are representative of Bj-xtrIT-(His6) expression trials using three individual E. coli transformants and suggest that the secreted protein may be inherently labil