The surface charge distribution on the toxin monomers is critical for CDC activity. Modification of surface charges affects ring formation, as we demonstrated by mutagenesis both for LLO (Köster et al., 2014) and PLY (van Pee et al., 2016). The alternating positive and negative charges of the HTH motif (Figure 5D) and the positive charges at the tips of the β-hairpins (Figure 5E,F) demonstrate the importance of charge complementarity for PLY ring formation. A similar HTH motif was found in the 8 Å cryoEM map of poly-C9 component of the human membrane attack complex (Dudkina et al., 2016). This HTH motif was already present in the soluble monomer of the C6 component (Aleshin et al., 2012) As in PLY, the HTH in the poly-C9 pore forms an α-barrel that determines the effective pore diameter.