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 the 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).

A FeoB homologue is present in Helicobacter pylori. This system takes up Fe²⁺ with high affinity (0.5 uM) in a process that is inhibited by FCCP, DCCD and vanadate, indicating that uptake is energized by ATP 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 it is essential for colonization of the murine gastric mucosa. A similar FeoB homologue in the spirochete Leptospira biflexa has been implicated in Fe²⁺ uptake (Louvel et al., 2005).

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).

The Feoic transport system consists of three proteins: FeoA, FeoB, and FeoC. The N-terminal domain of NFeoB 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).

The generalized transport reaction catalyzed by FeoB is presumably:

Fe² (out) energy %u2192 (out) Fe² (in).