9.A.8 The Ferrous Iron Uptake (FeoB) Family

The FeoB protein of E. coli is an integral membrane protein of 773 amino acyl residues which is predicted to span the membrane 8-13 times as α-helices (Kammler et al., 1993). Homologous proteins are encoded within the genomes of many bacteria and archaea. The E. coli protein possesses an N-terminal 300 amino acyl residue hydrophilic domain that bears at its N-terminus a regulatory ATP/GTP binding motif as well as an S domain. The N-terminal hydrophilic domain is homologous to prokaryotic and eukaryotic GTP binding proteins including the E. coli Era protein (P06616).  GTP binding is required for efficient Fe²⁺ uptake, but GTP is hydrolyzed very slowly (Marlovits et al., 2002).  The C-terminal transmembrane domain of FeoB catalyzes transport  (Hantke, 2003; Hung et al. 2012).  Transport is probably regulated by or energized by the N-terminal intramolecular G-protein-like domain. Based on x-ray crystallographic data, the G-doman transmits information to the transmembrane domain in a fashion possibly similar to energy transfer in ABC transporters (Köster et al., 2009).   Biochemical analyses demonstrated that the GTPase activity of FeoB is activated by K⁺, which leads to a 20-fold acceleration in its hydrolysis rate (Ash et al. 2010). Analysis of the structure identified a conserved asparagine residue likely to be involved in K⁺ coordination, and mutation of this residue abolished K⁺-dependent activation.  Ash et al. (2010) suggested that this, together with a second asparagine residue that is critical for the structure of the Switch I loop, allows K⁺-dependent activation in G proteins. The accelerated hydrolysis rate opens up the possibility that FeoB might indeed function as an active transporter. Iron uptake in eukaryotes has been reviewed (Sherman et al. 2018) as has that in prokaryotes (Sestok et al. 2018).  Thus the mechanism of energy coupling to FeoB transport is an open question.

A FeoB homologue is present in Helicobacter pylori. This system takes up Fe²⁺ with high affinity (0.5 μM) in a process that is inhibited by FCCP, DCCD and vanadate, indicating that uptake is energized by ATP or GTP hydrolysis (Velayudhan et al., 2000). Fe³⁺ is first converted to Fe²⁺ by an extracytoplasmic Fe³⁺ reductase, and the resultant Fe²⁺ is taken up by FeoB. FeoB appears to provide the major pathway for Fe²⁺ uptake in H. pylori and C. perfringens, and it is essential for colonization of the murine gastric mucosa in H. pylori. A similar FeoB homologue in the spirochete Leptospira biflexa has been implicated in Fe²⁺ uptake (Louvel et al., 2005). A general protocol for the expression and purification of the intact transmembrane  FeoBs from several bacteria has been described (Sestok et al. 2022). Additionally, seasonal patterns in microbial carbon and iron transporter expression in the Southern Ocean has been examined (Debeljak et al. 2023).

Prokaryotic FeoB proteins are involved in G protein coupled Fe²⁺ transport. They are unique in that the G protein is directly tethered to the membrane domain. Guilfoyle et al., 2009 reported the structure of the soluble domain of FeoB, including the G protein domain, and its assembly into a trimer. Comparisons between nucleotide free and liganded structures reveal the closed and open state of a central cytoplasmic pore, respectively. In addition, these data provide the first observation of a conformational switch in the nucleotide-binding G5 motif, defining the structural basis for GDP release. From these results, structural parallels are drawn to eukaryotic G protein coupled membrane processes (Guilfoyle et al., 2009).  Diverse gut bacteria communities protect against harmful pathogens by nutrient blocking.

The Feo transport system consists of three proteins: FeoA, FeoB, and FeoC. The N-terminal domain (N-FeoB) has been shown to form a trimeric pore that may function as a Fe²⁺ gate. FeoC is a small winged-helix protein possessing four conserved cysteine residues with a consensus sequence that may provide binding sites for an [Fe-S]-cluster. Therefore, FeoC may be an [Fe-S]-cluster-dependent regulator that directly controls transcription of the feo operon. Hung et al. (2012) showed that Klebsiella pneumoniae FeoC (KpFeoC) forms a tight complex with the intracellular N-terminal domain of FeoB (KpNFeoB). The crystal structure of the complex revealed that KpFeoC binds to KpNFeoB between the switch II region of the G-protein domain and the effector S domain, and that the long KpFeoC W1 loop lies above the KpNFeoB nucleotide-binding site. These interactions suggest that KpFeoC modulates guanine nucleotide-mediated signal transduction. Binding of KpFeoC disrupts pore formation by interfering with KpNFeoB trimerization. Thus, KpFeoC may play a crucial role in regulating Fe²⁺ transport as well as  gene regulation. FeoA is a 75aa protein homologous to the N-terminus of FeoB2 of Porphyromonas gingivalis (TC#9.A.8.1.6) and some similarity  to an internal hydrophilic segment of the RND heavy metal porter, CzcA of Myxococcus xanthus (TC#2.A.6.1.7).

In Vibrio cholerae the feo operon consists of three genes, feoABC. feoB encodes an 83 kDa protein with an amino terminal GTPase domain and a carboxy terminal domain predicted to be embedded in the inner membrane.  In V. cholerae, FeoA and FeoC, as well as the more highly conserved FeoB, are all required for iron acquisition (Weaver et al. 2013). FeoC interacts with the cytoplasmic domain of FeoB, and two conserved amino acids in FeoC were found to be necessary for the interaction with FeoB. 

FeoB normally consists of a cytoplasmic soluble domain termed NFeoB and a C-terminal polytopic transmembrane domain. NFeoB has two structural subdomains: a canonical GTPase domain and a five-helix helical domain. The GTPase domain hydrolyses GTP to GDP through a well characterized mechanism, a process which is required for Fe²⁺ transport. The structure of the cytoplasmic domain of FeoB from Gallionella capsiferriformans has been determined (Deshpande et al. 2013). The G. capsiferriformans NFeoB structure does not contain a helical domain, and the crystal structures of both the apo and GDP-bound protein reveals a domain-swapped dimer. I

Insertional inactivation of feoB in C. perfringens yielded altered growth properties and a markedly reduced total iron and manganese content compared to the wild type. Thus, under anaerobic conditions, FeoB is the major protein required for iron uptake into the cell and it may play an important role in the pathogenesis of C. perfringens infections (Awad et al. 2016). 

Bacterial FeoBs possess either GTPase or an NTPase with substrate promiscuity. This difference in nucleotide preference alters cellular requirements for monovalent and divalent cations. While the hydrolytic activity of the GTP-dependent FeoBs was stimulated by potassium, the action of the NTP-dependent FeoBs was not. Mutation of Asn11, having a role in potassium-dependent GTP hydrolysis, changed nucleotide specificity of the NTP-dependent FeoB, resulting in loss of ATPase activity (Shin et al. 2020).

Feo transporters are the most widespread systems for ferrous iron uptake in bacteria and are critical for virulence in some species. The canonical architecture of Feo consists of a large transmembrane nucleoside triphosphatase (NTPase) protein, FeoB, and two small accessory cytoplasmic proteins, FeoA and FeoC. Gómez-Garzón et al. 2022 conducted comparative analyses of Feo protein sequences. They found that FeoC, while absent in most lineages, is largely present in the γ-proteobacteria although its sequence is poorly conserved. FeoC may couple FeoB NTPase activity with pore opening and was an ancestral element that has been dispensed with through mutations in FeoA and FeoB. FeoC-independent mutants of the Vibrio cholerae Feo system were isolated, and the Shewanella oneidensis FeoAB does not require FeoC; thus, the Vibrio FeoC sequences may resemble transitional forms on an evolutionary pathway toward FeoC-independent transporters. FeoC may have different functions in different species that retain this protein, and the loss of FeoC may be promoted by mutations in FeoA or by the fusion of FeoA and FeoB (Gómez-Garzón et al. 2022).

The generalized transport reaction catalyzed by FeoB is presumably:

Fe²⁺  (out) +  energy (GTP hydrolysis) →  Fe²⁺  (in)