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 nuclease activity of CdiA, its cognate toxin protein, but not against CdiA from E.coli strain EC93 or D.dadantii strain 3937. Binding to CdiA-CT in the CdiA-CysK complex probably blocks the tRNA-binding site of CdiA-CT (PubMed:27531961). Interacts with cognate CdiA-CT but not CdiA-CT from D.dadantii strain 3937 (PubMed:21085179). CysK forms a complex with CdiA-CT/CdiI (PubMed:22333533). One CdiA toxin subunit binds to each subunit of the CysK homodimer, and one CdiI immunity protein binds to each toxin subunit; the immune complex is thus a dimer of trimers (PubMed:27531961).