# Bacterial Type I Toxins: Folding and Membrane Interactions
## 3 Membrane-Associated Type I Toxins Inducing Membrane Perturbations as a Primary Detected Effect  
### Membrane-Associated Type I Toxins Inducing Pore Formation  
The two well-characterized membrane-associated type I toxins HokB and TisB disrupt membrane integrity though pore formation in _E. coli_. The ectopic overexpression of these two pore-forming toxins causes membrane depolarization and generates dead cells so-called \"ghost cells\" with an unusual morphology characterized by cell material at the poles and a translucent cell center [9; 30]. The Hok type I toxin is expressed from the _hok/Sok_ locus that is the first type I TA system discovered on the R1 plasmid in _E. coli_ where it confers plasmid maintenance through PSK [9]. The _hok/Sok_ locus codes for three genes: _hok_ (for host killing), _sok_ (for suppression of killing), and _mok_ (for mediation of killing). The _hok_ gene encodes a 52 amino acid membrane-associated peptide whose ectopic overexpression provokes loss of membrane potential and arrest of respiration, and kills bacteria within 30 min upon induction (Figure 2) [9]. The activation of _hok_ translation in R1 plasmid-free cells is linked not only to the absence of the unstable Sok RNA antitoxin but also to the 3'-end processing and refolding of the _hok_ mRNA, which increase its activity and stability [31]. In plasmid-carrying cells,  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{ \begin{tabular}{c} **Bacteria** \\ **Species** \\ \end{tabular}  } & \multirow{2}{*}{**Toxins**} & \multirow{2}{*}{**Size (asas)**} & \multirow{2}{*}{**Amino Acids Sequence with the \(\alpha\)-Helical Transmembrane Domain**} & \multicolumn{2}{c}{**PDB ID and**} & \multirow{2}{*}{**Charge**} & \multicolumn{1}{c}{**GRAVY**} \\  & & & & & **Insights** & & **Index** \\ \hline \multirow{8}{*}{_E. coli_} & **HokB** & 49 & \begin{tabular}{c} **HokB** \\ **TisB** \\ **LdrA** \\ \end{tabular}  & 35 & \begin{tabular}{c} **HokB** \\ **TisB** \\ **LdrD** \\ \end{tabular}  & 29 & \begin{tabular}{c} **HokB** \\ **TisB** \\ **LdrD** \\ \end{tabular}  & 2.13 & 0.59 \\  & **TisC** & 29 & \begin{tabular}{c} **HokB** \\ **TisB** \\ **LdrD** \\ \end{tabular}  & 29 & \begin{tabular}{c} **HokB** \\ **TisB** \\ **LdrD** \\ \end{tabular}  & 29 & \begin{tabular}{c} **HokB** \\ **TisB** \\ **LdrD** \\ \end{tabular}  & 29 & \ \begin{tabular}{c} **HokB** \\ **TisB** \\ **LdrD** \\ \end{tabular}  & 2.13 & 0.59 \\  & **TisB** & 29 &
\begin{tabular}{c} **HokB** \\ **Tthe Sok RNA antitoxin binds to the _mock-hok_ mRNA at the level of the Shine-Dalgarno (SD) sequence of _mok_ encoding the Mok peptide, upstream of _hok_. The degradation of this RNA duplex by the RNAse III prevents translation of the _hok_ mRNA, leading to cell growth [31]. Five _hok_/_Sok_ homologues were discovered later in the _E. coli_ chromosome, but many of them are non-functional due to mutations, insertions, or large rearrangements [10]. By single-cell approach, the [PERSON] group demonstrated that the GTPase ObgE promotes _E. coli_ persistence through transcriptional activation of _hok_B expression, requiring the stringent response alarmone (p)ppGpp [32]. The GDP or ppGpp binding seems to be required for the GTPase ObgE to induce _hokB_ transcription [33]. The deletion of _hokB_ does not impact _E. coli_ persistence, but the ectopic overexpression of _hokB_ increases the number of persister cells in response to ofloxacin or tobramycin exposure [32]. The relationship between the mode of action of the 49 amino acid membrane-associated peptide HokB and _E. coli_ persistence was elucidated in 2018 [34]. Using in vitro conductance measurements with synthetic or natural planar lipid bilayers, the authors showed that HokB targets the lipid bilayer and forms pores with an estimated diameter of 0.59-0.64 nm [34].