Mercury transporter

The mercury transporter superfamily (TC# 1.A.72) is a family of transmembrane bacterial transporters of mercury ions. The common origin of all Mer superfamily members has been established.^[2] The common elements between family members are included in TMSs 1-2. A representative list of the subfamilies and proteins that belong to those subfamilies is available in the Transporter Classification Database.

Subfamilies

• 1.A.72.1: The MerF Mercuric Ion (Hg²⁺) Uptake (MerF) Family
• 1.A.72.2: The MerH Mercuric Ion (Hg²⁺) Permease (MerH) Family
• 1.A.72.3: The MerTP Mercuric Ion (Hg²⁺) Permease (MerTP) Family
• 1.A.72.4: The MerC Mercuric Ion (Hg²⁺) Permease (MerC) Family
• 1.A.72.5: The MerE Mercuric Ion (Hg²⁺) Permease (MerE) Family

Transport Reaction

The transport reaction catalyzed by Mer Superfamily members is:

Hg²⁺ or methyl-Hg²⁺ (out) → Hg²⁺ or methyl-Hg²⁺ (in)

MerF

The MerF protein encoded on plasmid pMER327/419 is an 81 residue polypeptide with two putative TMSs.^[3] It catalyzes uptake of Hg²⁺ in preparation for reduction by mercuric reductase. The MerF gene is found on mercury resistant plasmids from many gram-negative bacteria, but the sequence of the protein from these plasmids is the same. Limited sequence similarity is shown with the first two TMSs of MerT (TC# 1.A.72.3) and MerC (TC# 1.A.72.4). MerF has two vicinal pairs of cysteine residues which are involved in the transport of Hg(II) across the membrane and are exposed to the cytoplasm.^[4] Some members of the MerF family have been designated MerH.^[5]

Crystal structures

PDB: 1WAZ​, 2H3O​, 2LJ2​, 2M67​, 2MOZ​

MerTP

The MerTP permeases catalyze uptake into bacterial cells of Hg²⁺ in preparation for its reduction by the MerA mercuric reductase. The Hg^o produced by MerA is volatile and passively diffuses out of the cell. The merT and merP genes are found on mercury resistance plasmids and transposons of gram-negative and gram-positive bacteria but are also chromosomally encoded in some bacteria. MerT consists of about 130 amino acids and has 3 transmembrane helical segments. Evidence for direct interactions between the cytoplasmic face of MerT and the N-terminus of MerA have been presented.^[6] Operon analyses have been reported.^[3][7][8]

MerP

MerP is a periplasmic Hg²⁺-binding receptor of about 70-80 amino acyl residues, synthesized with a cleavable N-terminal leader. It is homologous to the N-terminal heavy metal binding domains of the copper-and cadmium-transporting P-type ATPases. The 3-D structure of MerP from Ralstonia metallidurans has been solved to 2 Å resolution (PDB: 1OSD​).^[9][10] It is 91 amino acyl residues (aas) long with its leader sequence, is monomeric, and binds a single Hg²⁺ ion. Hg²⁺ is bound to a sequence GMTCXXC found in metallochaperones as well as metal-transporting ATPases. The fold is βαββαβ, called the ''ferridoxin-like fold''.

MerT

MerT homologues have been identified in which the 3 TMS MerT is fused to a MerP ''heavy metal associated'' (HMA) domain, possibly via a linker region that includes a fourth TMS (see 1.A.72.3.3). HMA domains of ~30 aas are found in MerP, copper chaperone proteins, mercuric reductase, and at the N-termini of both copper and heavy metal P-type ATPases, sometimes in multiple copies.^[11]

MerC

The MerC protein encoded on the IncJ plasmid pMERPH of the Shewanella putrefaciens mercuric resistance operon is 137 amino acids in length and possesses four putative transmembrane α-helical spanners (TMSs). It has been shown to bind and take up Hg²⁺ ions. merC genes are encoded on several plasmids of gram-negative bacteria and may also be chromosomally encoded. MerC proteins are homologous to other bacterial Hg²⁺ bacterial transporters.^{[2][12][13][14]}

MerE

The merE gene of transposon Tn21, a pE4 plasmid that contains the merR gene of plasmid pMR26 within Pseudomonas strain K-62, and the merE gene of Tn21 from the Shigella flexneri plasmid NR1 (R100) conferred hypersensitivity to CH₃Hg²⁺ and Hg²⁺, taking up significantly more CH₃Hg²⁺ and Hg²⁺ than the isogenic strain.^[15] The MerE protein encoded by pE4 was localized in the membrane cell fraction, but not in the soluble fraction. Kiyono et al. (2009) suggested that the merE gene is a broad mercury transporter mediating the transport of both CH₃Hg²⁺ and Hg²⁺ across the bacterial membrane.^[15]

References

This article incorporates text from the public domain Pfam and InterPro IPR021091