Antitoxin component of a type VII toxin-antitoxin (TA) system. Upon cloning in E.coli neutralizes the effect of cognate toxin HepT. Neutralization is mostly due to di-AMPylation of toxin by this enzyme. Successively di-AMPylates HepT on 'Tyr-109'. In vitro will use ATP, dATP, GTP, dGTP, TTP or UTP to generate a mono-modified protein, but requires a purine nucleotide for the second modification reaction (ATP, dATP or GTP). 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] Mn(2+) works as well as Mg(2+), Co(2+), Ni(2+) and Zn(2+) work less well (PubMed:33290744). Bind 2 Mg(2+) per subunit (By similarity). Forms a complex with HepT, probably MntA(1):HepT(2) in vivo; can only be purified when both 'Arg-102' and 'Tyr-109' (or 'His-107' and 'Tyr-109') of HepThave been mutated. The fully di-AMPylated HepT homodimer is not found in a complex with MntA. Part of a locus that includes subtype I-D CRISPR-Cas genes. Belongs to the MntA antitoxin family.