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. 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 th
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. 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 Gram-negative bacteria are bacteria that do not retain the crystal violet stain used in the Gram staining method of bacterial differentiation. They are characterized by their cell envelopes, which are composed of a thin peptidoglycan cell wall ...

, but the sequence of the protein from these

plasmid A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; how ...

s 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 Concentration, also known as Memory, Shinkei-suijaku (Japanese meaning "nervous breakdown"), Matching Pairs, Match Match, Match Up, Pelmanism, Pexeso or simply Pairs, is a card game in which all of the cards are laid face down on a surface and tw ...

cysteine Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile. When present as a deprotonated catalytic residue, sometime ...

residue Residue may refer to: Chemistry and biology * An amino acid, within a peptide chain * Crop residue, materials left after agricultural processes * Pesticide residue, refers to the pesticides that may remain on or in food after they are applied ...

s which are involved in the

transport Transport (in British English), or transportation (in American English), is the intentional movement of humans, animals, and goods from one location to another. Modes of transport include air, land (rail and road), water, cable, pipeline, an ...

of Hg(II) across the

membrane A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. B ...

and are exposed to the

cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. The ...

. Some members of the MerF family have been designated MerH.

Crystal structures

, , , ,

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. Operon analyses have been reported.

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 (). 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 (se
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.

MerC

The MerC protein encoded on the IncJ plasmid pMERPH of the ''

Shewanella putrefaciens ''Shewanella putrefaciens'' is a Gram-negative pleomorphic bacterium. It has been isolated from marine environments, as well as from anaerobic sandstone in the Morrison Formation in New Mexico. ''S. putrefaciens'' is also a facultative anaerobe ...

'' 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.

MerE

See

References

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