Antitoxin component of a type VII toxin-antitoxin (TA) system. Overexpression in E.coli neutralizes the toxic effect of cognate toxin HepT. Neutralization is mostly due to AMPylation of the toxin by this enzyme. ATP + L-tyrosyl-[protein] = diphosphate + O-(5'-adenylyl)-L-tyrosyl-[protein] ATP + O-(5'-adenylyl)-L-tyrosyl-[protein] = diphosphate + O-[5'-(adenylyl-(5'->3')-adenylyl)]-L-tyrosyl-[protein] Binds 2 Mg(2+) ions. Belongs to the MntA antitoxin family.