Overproduction of fungal ribotoxin alpha-sarcin in Escherichia coli: generation of an active immunotoxin.

alpha-Sarcin is a ribonucleolytic protein secreted by the mold Aspergillus giganteus. DNA encoding alpha-sarcin was isolated from the host and cloned into T7 promoter based E. coli expression vectors. Using bacterial outer membrane protein A (OmpA) signal sequence, properly processed recombinant (re-) protein was secreted into the culture medium while in the absence of a signal sequence protein remained insoluble in the bacterial inclusion bodies. The re-alpha-sarcin was purified to homogeneity by simple chromatographic techniques both from the insoluble and soluble sources with respective yields of 40-50 microg/ml and 2-3 microg/ml. The re-ribotoxin was functionally as active as the native toxin and preserved its specificity. The re-alpha-sarcin was used in the construction of an active immunotoxin targeted at the human cancer cells overexpressing transferrin receptor (TFR).