Internalization of biotinylated compounds into cancer cells is promoted by a molecular Trojan horse based upon core streptavidin and clostridial C2 toxin

The C2 toxin produced by Clostridium botulinum is a binary AB-type exotoxin composed of the enzyme subunit C21 and the binding/translocation moiety C2II. After proteolytic activation, C2IIa mediates the subsequent internalization of C2I into the cytosol of mammalian target cells. The N-terminal domain of C21 (C2IN) is necessary for C2IIa-dependent uptake, but lacks the enzyme domain that is responsible for cytotoxicity. In the present study, we generated a delivery system building on C21N and a truncated core streptavidin (Stv13) with enhanced solubility for the C2IIa-dependent internalization of biotinylated cargo molecules into mammalian cells. C2IN Stv13 fusion protein expressed in Escherichia coli was obtained in high yields and purity. The affinity-purified protein formed tetramers and a defined higher order species in solution as shown by gel filtration and retained its biotin-binding properties, however with an obvious reduction in affinity. Uptake of C2IN Stv13 into the cytosol of HeLa and other cancer cell lines was observed by immunoblot analysis, which was corroborated by confocal microscopy. In addition, the fusion protein was not cytotoxic and did not inhibit cell proliferation as determined by MTS assay. Finally, we demonstrated the C2IN-Stv13/C211a-mediated uptake of biocytin Alexa 488 as cargo into HeLa cells, underscoring the functionality of the generated transport system.