Antitoxin component of a type II toxin-antitoxin (TA) system. Antitoxin that counteracts the effect of its cognate VapC4 toxin. Upon expression in situ, in M.smegmatis or E.coli neutralizes the effect of cognate toxin VapC4. Interacts with cognate toxin VapC4. The C-terminal domain (residues 55-85) is sufficient to interact with and neutralize VapC4; exchanging the N-terminus (residues 1-54) with VapB5 allows neutralization, while exchanging the C-terminal domain (residues 55-85) does not. Many single residue mutations in the C-terminal domain construct but not in full-length protein prevent toxin neutralization, they are not all annotated here. Belongs to the phD/YefM antitoxin family.