Engineering a switchable toxin: the Potential Use of PDZ Domains in the Expression, Targeting and Activation of Modified Saporin Variants

In this paper we describe an engineered form of the ribosome inactivating protein saporin (sapVSAV), bearing a C-terminal extra sequence that is recognized and bound by PDZ2 domain from the mouse PTP-BL protein. The co-expression of sapVSAV and PDZ2 genes in E. coli BL21 cells, greatly enhances the production of the toxin in a soluble form. This increase was not due to protection from bacterial autointoxication due to the saporin enzymatic activity, but may arise from a stabilization effect of PDZ2 on the toxin molecule during biosynthesis of sapVSAV. We found that once purified, sapVSAV is stable but shows no toxicity to free ribosomes, while it is fully active against human cancer cells. Thus, our strategy of co-expression of a toxin moiety with a soluble PDZ domain may represent a new system to increase the yields in recombinant toxic proteins, allowing for the selection of new C-terminal sequences to target specific PDZ domains of mammalian intracellular domains.