Toxic component of a type II toxin-antitoxin (TA) system. Its toxic activity is neutralized by cognate antitoxin SpoIISB. Expression in the absence of SpoIISB permits sporulation to stage II, when plasmolysis zones and holes in the peptidoglycan layer are observed, resulting in cell death. Lethal when synthesized during vegetative growth in the absence of SpoIISB. In E.coli both the membrane bound and soluble domain are required in cis for toxin activity. Probably forms an oligomer; X-ray data suggests the inactive complex forms a heterotetramer of SpoIISA(2)-SpoIISB(2), which inactivates the toxic activity of SpoIISA. Expressed at low levels during exponential growth, it increases dramatically at the onset of sporulation (at protein level). A member of the spoIISA-spoIISB operon. No change in sporulation efficiency. Belongs to the SpoIISA toxin family.