Figure 3

The cellular activities of SLT-1A^IYSNKLM are linked to its catalytic activity. (A) Dose-response curves indicating a comparable cytotoxicity for the purified, furin-cleaved A subunit of SLT-1A^IYSNKLM (A₁ and A₂ subunits remain linked by a disulfide bridge between C242 and C261; white squares) and the A₁ subunit alone of SLT-1A^IYSNKLM (white circles) towards 518-A2 cells. The catalytically-inactive forms (Y77S) of these same molecules (black squares, black circles) were not cytotoxic towards 518-A2 human melanoma cells at concentrations of up to 10 μM. (B) Western blot analysis demonstrating the cleavage of PARP following treatment of 518-A2 cells with SLT-1A^IYSNKLM. (C) Effect of exposing 518-A2 (black bars) and PC-3 cells (grey bars) to SLT-1A^IYSNKLM on the activation of caspase-3 as measured using a fluorescent peptide substrate for caspase-3. PC-3 cells are insensitive to SLT-1A^IYSNKLM. Error bars represent s.e.m. for experiments performed in quadruplicate.