May be involved in response to oxidative stress (Probable). Toxic component of a type I toxin-antitoxin (TA) system. When overexpression is induced in situ in the absence of its cognate antisense RNA antitoxin IsoA1 it leads to cell growth arrest and cell death without lysis. Neutralized by IsoA1 RNA which forms an extensive duplex with the mRNA (PubMed:28077560, PubMed:33229580, PubMed:31476357). Binds artificial prokaryotic and eukaryotic lipid membranes, with 30-fold higher affinity for prokaryotic membranes. Molecular dynamics suggests the peptide penetrates the membrane leading to lipid reorganization and thinning of the bilayer (PubMed:31476357). Induction of toxin in the absence of antitoxin RNA causes a fast conversion of cells from spiral-shaped to coccoid forms; cells have no visible membrane defects and resemble wild-type 'aging coccoids'. Toxin causes a moderate decrease in membrane potential and ATP content and alterations in peptidoglycan muropeptide abundance; GlcNAc-MurNAc dipeptides increase while GlcNAc-MurNAc tripeptides decrease (i.e. a faster phenocopy of cell aging). Deletion of all 6 AapA/IsoA TA loci in strain B128 leads to slower than wild-type conversion of H2O2-treated cells to the coccoid form. This suggests oxidative stress triggers coccoid transformation via these type I TA systems, although other factors eventually drive the morphology change (PubMed:33229580). Transcription of the aapA1 gene generates a full-length transcript whose folding impedes translation. Processing of the 3' end of the aapA1 message generates a shorter transcript that becomes translatable after a structural rearrangement. The processing also makes it more susceptible to forming dsRNA with IsoA1 which leads to duplex RNA degradation by RNase 3 (rnc). Constitutively expressed, increases slightly as cells get older. Does not respond to acid, rifampicin, tetracycline or Ni(2+) stress. Antisense RNA expression decreases strongly under oxidative stress (H2O2 or paraquat), while aapA1 expression decreases only marginally in H2O2 but decreases strongly in response to paraquat. A decrease in antisense RNA leads to an increase in toxin production. Has 3 regions; the N-terminal 8 residues are disordered and not required for toxicity, the central helical segment (residues 9-28) probably crosses the membrane, and the charged C-terminal 2 residues which are required for toxicity. Addition of an SPA-tag or GFP to the C-terminus of the protein renders it non-toxic, but detectable by antibodies. As Helicobacter ages cells change from a helicoidal shape with multiple flagella at one pole to coccoid forms by 70 hours in culture. As they age peptidoglycan muropeptide abundance alters; GlcNAc-MurNAc dipeptides increase while GlcNAc-MurNAc tripeptides decrease. Belongs to the AapA toxin family.