TonB-Dependent Transporters Expressed by Neisseria gonorrhoeae

Abstract

Neisseria gonorrhoeae causes the common sexually transmitted infection, gonorrhea. This microorganism is an obligate human pathogen, existing nowhere in nature except in association with humans. For growth and proliferation, N. gonorrhoeae requires iron and must acquire this nutrient from within its host. The gonococcus is well-adapted for growth in diverse niches within the human body because it expresses efficient transport systems enabling use of a diverse array of iron sources. Iron transport systems facilitating the use of transferrin, lactoferrin, and hemoglobin have two components: one TonB-dependent transporter and one lipoprotein. A single component TonB-dependent transporter also allows N. gonorrhoeae to avail itself of iron bound to heterologous siderophores produced by bacteria within the same ecological niche. Other TonB-dependent transporters are encoded by the gonococcus but have not been ascribed specific functions. The best characterized iron transport system expressed by N. gonorrhoeae enables the use of human transferrin as a sole iron source. This review summarizes the molecular mechanisms involved in gonococcal iron acquisition from human transferrin and also reviews what is currently known about the other TonB-dependent transport systems. No vaccine is available to prevent gonococcal infections and our options for treating this disease are compromised by the emergence of antibiotic resistance. Because iron transport systems are critical for the survival of the gonococcus in vivo, the surface-exposed components of these systems are attractive candidates for vaccine development or therapeutic intervention.

Keywords: Neisseria gonorrhoeae; TonB; iron; transferrin; xenosiderophores.

Figure 1

Two component gonococcal systems for acquisition of iron from host proteins. The TonB dependent outer membrane transporters are shown as barrels traversing the outer membrane (OM). The lipid-modified companion proteins are shown tethered to the outer membrane surface. TonB, ExbB, and ExbD (gold) are depicted as attached to or imbedded within the cytoplasmic membrane (CM). The periplasmic binding protein, FbpA, is responsible for transporting iron from the outer membrane transporters, TbpA and LbpA, to the cytoplasmic membrane permease, comprised of FbpB. FbpC is the ATPase that energizes transport through FbpB.

Figure 2

Hypothetical two-dimensional topology model of gonococcal TbpA. The sequence shown is of the TbpA protein from gonococcal strain FA19. The model includes an amino-terminal plug domain of 161 residues, 11 surface-exposed loops (numbered L1–L11), and 22 transmembrane beta-strands. Residues highlighted in red are those that are divergent among a panel of five gonococcal TbpA sequences. Cysteine residues are highlighted in yellow. The EYE residues that were mutated within the plug domain are highlighted in green. The position of HA epitopes inserted into putative loops and the plug domain are shown as gray-shaded triangles. The TonB-box region within the plug domain is also indicated.

Figure 3

Model of the mechanism of iron transport from transferrin through the gonococcal outer membrane. TbpA is shown as a bisected barrel with an amino-terminal plug domain exposed in the periplasm. TbpB is shown as a bi-lobed protein protruding from the gonococcal cell surface, tethered to the outer membrane by an amino-terminal lipid moiety. Human transferrin is shown as a bi-lobed structure, each lobe of which is capable of binding an atom of iron. Seven putative steps in iron acquisition from transferrin are listed on the right. The arrows depict the direction of iron import and the hypothetical, transient interaction between iron and the plug domain.

Figure 4

Single component iron transport systems. FetA and the uncharacterized TonB-dependent transporters (TdfF, TdfG, TdfH, and TdfJ) are shown as barrels traversing the outer membrane. TonB, ExbB, and ExbD (gold) are depicted as attached to or imbedded within the cytoplasmic membrane (CM). The periplasmic binding protein FetB is encoded downstream of FetA and is expected to transport ferric-siderophores across the periplasm to a cytoplasmic membrane permease. Genes located immediately downstream of that encoding FetB are hypothesized to fulfill this function. Encoded near TdfF is a putative periplasmic binding protein annotated as FetB2, reflecting its sequence similarity with FetB.

Figure 5

Model comparing TonB-dependent and TonB-independent iron acquisition systems. Left: Ferric-catecholate siderophores are imported through the outer membrane in a TonB- and FetA-dependent mechanism. Use of these ferric-siderophores is expected to require proteins encoded by genes located downstream of fetA. Right: In gonococcal strain FA19, use of ferric-catecholate siderophores is limited to a TonB-independent mechanism that involves the participation of the FbpABC system. Gonococcal strain FA1090 appears to be able to employ both TonB-dependent and TonB-independent mechanisms for the use of ferric siderophores, including enterobactin, DHBS, and salmochelin.