Mercury transporter

Transporter MerF
structure of MerF. It has two trans-membrane helices. PDB 2h3o.[1]
Identifiers
Symbol	MerF
Pfam	PF11431
InterPro	IPR021091
TCDB	1.A.72
OPM superfamily	218
OPM protein	2lj2
Available protein structures: Pfam   structures / ECOD   PDB RCSB PDB; PDBe; PDBj PDBsum structure summary

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.

• 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

The transport reaction catalyzed by Mer Superfamily members is:

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

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]

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

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.^[6] Operon analyses have been reported.^[3][7][8]

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 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]

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]}

See Kiyono, Masako; Sone, Yuka; Nakamura, Ryosuke; Pan-Hou, Hidemitsu; Sakabe, Kou (2009-04-02). "The MerE protein encoded by transposon Tn21 is a broad mercury transporter in Escherichia coli". FEBS Letters. 583 (7): 1127–1131. doi:10.1016/j.febslet.2009.02.039. ISSN 1873-3468. PMID 19265693. S2CID 27100434.