Immunity protein component of a toxin-immunity protein module, which functions as a cellular contact-dependent growth inhibition (CDI) system. CDI modules allow bacteria to communicate with and inhibit the growth of closely related neighboring bacteria in a contact-dependent fashion. Protects cells against the tRNA endonuclease activity of cognate toxin CdiA-STECO31 but not other non-cognate toxins, inhibits the tRNase activity of the isolated CT fragment of CdiA-STECO31. Binds the CT fragment of CdiA-STECO31, between two subdomains. Forms a 1:1 complex with cognate toxin CdiA-STECO31, binding between 2 toxin subdomains. Truncated N-terminus.