Structural and functional analysis of AniA, the major anaerobically induced outer membrane protein of Neisseria gonorrhoeae

AniA, the major anaerobically induced outer membrane protein of Neisseria gonorrhoeae, is a key molecule in gonococcal adaptation to anaerobiosis and an important antigen in acute gonococcal infection in women. AniA was recognized as having a low level of homology to the copper-containing nitrite reductases which are highly conserved among themselves. AniA also conserved seven amino acids which have been shown to act as copper ion ligands in this family of proteins. Preliminary evidence showed a correlation between AniA presence and nitrite reductase activity, both in gonococcal outer membrane preparations and in a biochemically complemented strain of Escherichia coli. AniA was successfully purified using the glutathione S-transferase gene fusion system, and shown to reduce nitrite. Similar to other copper-containing nitrite reductases, AniA nitrite reduction was dependent on copper reconstitution, had a pH optimum at or below 5.8, and was shown to form trimers in its native state. The AniA UV/visible absorption spectrum had a different profile from that of other copper-containing nitrite reductases, which may be a reflection of AniA divergence from these proteins. AniA has proline-rich repeat regions which were shown to be involved in protein stability and may represent protein interaction domains. Data revealed AniA to be located on the external surface of N. gonorrhoeae. AniA may not be the only gonococcal enzyme to reduce nitrite as a putative sulfite reductase was identified, however attempts to insertionally mutate the gene were unsuccessful. Finally, AniA was shown to have a role in gonococcal pathogenesis when aerobically grown N. gonorrhoeae which constitutively expressed AniA were protected from killing by normal human sera, while an isogenic, non-AniA expressing strain was rapidly killed. The confirmation that AniA can function as a nitrite reductase, as well as its similarities to copper-containing nitrite reductases suggests that the product of nitrite reduction will be nitric oxide. Given that nitric oxide is now recognized as a participant in a number of human signal transduction pathways, the suggestion that N. gonorrhoeae is capable of generating this molecule on its outer surface has important and exciting implications about the pathogenic mechanisms of this organism.