Secretory nucleoside diphosphate kinases from both intra- and extracellular pathogenic bacteria are functionally indistinguishable.

Nucleoside diphosphate kinase (NDK), responsible for the maintenance of NTP pools, is an ATP-utilizing enzyme secreted by different pathogens. We found that NDK from Salmonella enterica serovar Typhimurium (S. Typhimurium) is also secretory in nature. Secretory NDK is known to play a crucial role in the survival of pathogenic microbes within host cells through their interaction with extracellular ATP. To elucidate this aspect, we assessed the contribution of secretory products containing NDK from intracellular (Mycobacterium tuberculosis and S. Typhimurium) and extracellular (Vibrio cholerae) pathogens to the process of ATP-induced J774 mouse macrophage cell lysis by monitoring lactate dehydrogenase (LDH) release in the culture medium. Compared with an untreated control, our results demonstrate that S. Typhimurium secretory products caused a greater than twofold decrease in LDH release from J774 macrophage cells treated with ATP. Furthermore, the secretory products from an ndk-deleted strain of S. Typhimurium did not display such behaviour. Contrary to this observation, the secretory products containing NDK of V. cholerae were found to be cytotoxic to J774 cells. At the amino acid level, the sequences of both the NDKs (S. Typhimurium and V. cholerae) exhibited 65 % identity, and their biochemical characteristics (autophosphorylation and phosphotransfer activities) were indistinguishable. However, to our surprise, the secretory product of an ndk-deleted strain of S. Typhimurium, when complemented with V. cholerae ndk, was able to prevent ATP-induced cytolysis. Taken together, our results unambiguously imply that the intrinsic properties of secretory NDKs are identical in intra- and extracellular pathogens, irrespective of their mode of manifestation.