The neisserial 37 kDa ferric binding protein (FbpA).

The ferric binding protein (FbpA) is one of the major proteins regulated by the level of environmental iron in the genus Neisseria. Its conservation in all species of pathogenic Neisseria has been demonstrated, and the possible role that it plays in the iron uptake mechanisms in these bacteria has been postulated. Similar proteins in Haemophilus influenzae (HitA) and in Serratia marcescens (SfuA) have been described, but relationships with the meningococcal FbpA could not be proven. Although supposedly periplasmic, the exact location of FbpA remains controversial because some molecules, or parts of them, have been found exposed to the bacterial outer surface. The DNA sequence downstream of the fbpA gene has been recently analysed, finding an operon composed of three open reading frames: fbpA, encoding for FbpA; fbpB, that codifies a cytoplasmic permease, and fbpC, that contains the information for a nucleotide binding protein. These proteins would form an iron transport system through the periplasmic space. FbpA is highly antigenic in mice when injected in purified form, shows intraspecies and interspecies antigenic homogenicity, and specific anti-FbpA antibodies are fully cross-reactive; nevertheless, the in vivo induction of anti-FbpA antibodies in man is still polemical. Recent studies reveal that the purified FbpA induces a fair response of bactericidal antibodies in mice.