The A subunit is responsible for inhibiting protein synthesis through the catalytic inactivation of 60S ribosomal subunits. After endocytosis, the A subunit is cleaved by furin in two fragments, A1 and A2: A1 is the catalytically active fragment, and A2 is essential for holotoxin assembly with the B subunits. Endohydrolysis of the N-glycosidic bond at one specific adenosine on the 28S rRNA. Shiga toxin contains a single subunit A and five copies of subunit B. Belongs to the ribosome-inactivating protein family.