Toxic component of a type I toxin-antitoxin (TA) system. When overexpressed kills cells within minutes; causes collapse of the transmembrane potential and arrest of respiration (PubMed:10361310). Its toxic effect is probably neutralized by antisense antitoxin RNA SokC (PubMed:10361310). Homodimer; disulfide-linked. The periplasmic portion confers toxicity; dimerization is not required for toxicity. Interrupted by an IS186 element, which silences the locus in strain K12 MG1655. Belongs to the Hok/Gef family.