Receptor-binding subunit of subtilase cytotoxin SubAB5. Required for receptor-binding and thus correct trafficking in the host cell. Has specificity for host glycans terminating in the sialic acid N-glycolyl-alpha-neuraminic acid (Neu5Gc); each subunit in the SubB pentamer binds one Neu5Gc. The protease subunit (SubA) cleaves host BiP/HSPA5, inducing the host endoplasmic reticulum stress response and eventual cell death. Culture supernatant of E.coli expressing both subA and subB are toxic for Vero cells (African green monkey kidney cell line), Chinese hamster ovary cells and Hct-8 cells (human colonic epithelial cell line); the subunits are not toxic individually. Purified SubAB5 is highly toxic, <0.1 pg is able to kill at least 50% of 30'000 Vero cells in a microtiter plate assay after 3 days; no cytotoxicity is seen at 24 hours. Preabsorption with cells expressing a ganglioside GM2 mimic reduced cytotoxicity of SubAB5 by 93% in the Vero cytotoxicity assay. Intraperitoneal injection of 200 ng of purified SubAB5 kills mice; the higher the dose the faster the mice die. Animals injected with purified SubAB5 have microvascular thrombi in the brain and other organs, including the renal tubules and glomeruli. Mice fed E.coli cells expressing cloned SubAB5 experience drastic weight loss and appear ill and lethargic. SubB alone at 2.5 ug/ml causes vacuolation of Vero cells, which requires the V-type ATPase proton pump; treated cells die. Protein synthesis in Vero cells is transiently inhibited by SubAB5; both subunits are required for this effect. Inhibition of protein synthesis is prevented by brefeldin A; cells are arrested in the G1 phase. SubAB5 at 100 ng/ml induced caspase-dependent apoptosis in Vero cells through mitochondrial membrane damage