Shiga toxin subunit A; The A subunit is responsible for inhibiting protein synthesis through the catalytic inactivation of 60S ribosomal subunits. After endocytosis, the A subnit 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 (315 aa)