SpoIISA is a toxin which causes lysis of vegetatively growing cells. It forms part of a type II toxin-antitoxin system, where the SpoIISB protein, Pfam:PF14185, acts as an antitoxin. It is a transmembrane protein, with a cytoplasmic domain accounting for approximately two-thirds of the protein. The structure of the cytoplasmic domain resembles that of the GAF domains, Pfam: PF01590. SpoIISB binds to the cytoplasmic domain of SpoIISA with high affinity [1].

1: The structure and interactions of SpoIISA and SpoIISB, a toxin-antitoxin system in Bacillus subtilis. Florek P, Levdikov VM, Blagova E, Lebedev AA, Skrabana R, Resetarova S, Pavelcikova P, Barak I, Wilkinson AJ; J Biol Chem. 2011;286:6808-6819. PMID:21147767