Antitoxin component of a type VII toxin-antitoxin (TA) system. Upon overexpression in situ or in E.coli neutralizes the effect of cognate toxin HepT (PubMed:26112399, PubMed:29555683, PubMed:33045733). Neutralization is mostly due to tri-AMPylation of toxin by this enzyme. Successively tri-AMPylates HepT on 'Tyr-104' (PubMed:33045733). Binds its own promoter, probably repressing its expression. The TA system confers plasmid stability in E.coli (PubMed:26112399). 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] ATP + O-[5'-(adenylyl-(5'->3')-adenylyl)]-L-tyrosyl-[protein] = diphosphate + O-[5'-(adenylyl-(5'->3')-adenylyl-(5'->3')-adenylyl)]-L-tyrosyl-[protein] Binds 2 Mg(2+) per subunit. Forms a complex with cognate toxin HepT, may form a homodimer by itself (PubMed:26112399). Forms a heterooctamer with cognate toxin HEPN with stoichiometry HepT(6):MntA(2). The 2 MNT subunits do not contact each other in the heterooctamer, each contacts 3 HEPN subunits, blocking access to the toxin active site (PubMed:29555683). Expressed during exponential growth, part of the mntA-hepT operon. Under autocontrol by MntA. Has 2 alpha-helices that are important for interaction with cognate toxin HEPN; deletion of helix 2 (residues 52-65) or the C-terminus of helix 4 (residues 114-125) reduces antitoxin function about 100-fold, while deletion of the N-terminus of helix 4 (residues 98-113) reduces antitoxin function over 1000-fold. When mntA-hepT is deleted, has a sight reduction in swimming motility. This gene cannot be deleted when the hepT gene is intact. Belongs to the MntA antitoxin family.