The multidrug/oligosaccharidyl-lipid/polysaccharide (MOP) flippase superfamily
TC# 2.A.66
is a group of integral membrane protein families. The MOP flippase superfamily includes twelve distantly related families, six for which functional data are available: # One ubiquitous family (MATE) specific for drugs
(TC# 2.A.66.1)
The Multi Antimicrobial Extrusion (MATE) Family # One (PST) specific for polysaccharides and/or their lipid-linked precursors in prokaryotes
(TC# 2.A.66.2)
The Polysaccharide Transport (PST) Family # One (OLF) specific for lipid-linked oligosaccharide precursors of glycoproteins in eukaryotes
(TC# 2.A.66.3)
The Oligosaccharidyl-lipid Flippase (OLF) Family # One (MVF) lipid-peptidoglycan precursor flippase involved in cell wall biosynthesis
(TC# 2.A.66.4)
The Mouse Virulence Factor (MVF) Family # One (AgnG) which includes a single functionally characterized member that extrudes the antibiotic, Agrocin 84
(TC# 2.A.66.5)
The Agrocin 84 Antibiotic Exporter (AgnG) Family # And finally, one (Ank) that shuttles inorganic pyrophosphate (PP_i)
(TC# 2.A.66.9)
The Progressive Ankylosis (Ank) Family All functionally characterized members of the MOP superfamily catalyze

efflux Efflux may refer to: * Efflux (microbiology), a mechanism responsible for moving compounds out of cells * e-flux, a publishing platform and archive See also * Efflux time, part of a measure of paint viscosity * Flux (biology) In general, flux ...

of their substrates, presumably by

cation An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

antiport An antiporter (also called exchanger or counter-transporter) is a cotransporter and integral membrane protein involved in secondary active transport of two or more different molecules or ions across a phospholipid membrane such as the plasma mem ...

Functionally characterized families

2.A.66.1 The Multi Antimicrobial Extrusion (MATE) Family

The MATE family is made up of several members and includes a functionally characterized multidrug efflux system from '' Vibrio parahaemolyticus'' NorM
TC# 2.A.66.1.1
, and several homologues from other closely related bacteria that function by a drug:Na⁺

mechanism, a putative

ethionine Ethionine is a non-proteinogenic amino acid structurally related to methionine, with an ethyl group in place of the methyl group. Ethionine is an antimetabolite and methionine antagonist. It prevents amino acid incorporation into proteins and ...

resistance protein of '' Saccharomyces cerevisiae'' (ERC1 (YHR032w)
TC# 2.A.66.1.5
, a cationic drug efflux pump in ''

A. thaliana ''Arabidopsis thaliana'', the thale cress, mouse-ear cress or arabidopsis, is a small flowering plant native to Eurasia and Africa. ''A. thaliana'' is considered a weed; it is found along the shoulders of roads and in disturbed land. A winter a ...

'' (i.e., AtDTX1 aka AT2G04040
TC# 2.A.66.1.8
and the functionally uncharacterized DNA damage-inducible protein F (DinF
TC# 2.A.66.1.4
of '' E. coli.'' The family includes hundreds of functionally uncharacterized but sequenced homologues from bacteria, archaea, and all eukaryotic kingdoms. A representative list of proteins belonging to the MATE family can be found in th
Transporter Classification Database

Structure

The bacterial proteins are of about 450 amino acyl residues in length and exhibit 12 putative transmembrane segments (TMSs). They arose by an internal gene duplication event from a primordial 6 TMS encoding genetic element. The yeast proteins are larger (up to about 700 residues) and exhibit about 12 TMSs.

hMATE1

Human MATE1 (hMATE1) is an electroneutral H⁺/organic cation (OC) exchanger responsible for the final excretion step of structurally unrelated toxic organic cations in kidney and liver. Glu273, Glu278, Glu300 and Glu389 are conserved in the transmembrane regions. Substitution with alanine or

aspartate Aspartic acid (symbol Asp or D; the ionic form is known as aspartate), is an α-amino acid that is used in the biosynthesis of proteins. Like all other amino acids, it contains an amino group and a carboxylic acid. Its α-amino group is in the pro ...

reduced export of tetraethylammonium (TEA) and

cimetidine Cimetidine, sold under the brand name Tagamet among others, is a histamine H2 receptor antagonist that inhibits stomach acid production. It is mainly used in the treatment of heartburn and peptic ulcers. The development of longer-acting H2 rec ...

, and several had altered substrate affinities. Thus, all of these

glutamate Glutamic acid (symbol Glu or E; the ionic form is known as glutamate) is an α-amino acid that is used by almost all living beings in the biosynthesis of proteins. It is a non-essential nutrient for humans, meaning that the human body can syn ...

residues are involved in binding and/or transport of TEA and cimetidine, but their roles are different.

MATE (NorM) Transport Reaction

The probable transport reaction catalyzed by NorM, and possibly by other proteins of the MATE family is: Antimicrobial (in) + nNa⁺ (out) → Antimicrobial (out) + nNa⁺ (in).

2.A.66.2 The Polysaccharide Transport (PST) Family

Analyses conducted in 1997 showed that members of the PST family formed two major clusters. One is concerned with lipopolysaccharide O-antigen ( undecaprenol pyrophosphate-linked O-antigen repeat unit) export (flipping from the cytoplasmic side to the periplasmic side of the inner membranes) in Gram-negative bacteria. On the periplasmic side, polymerization occurs catalyzed by Wzy. The other is concerned with exopolysaccharide or capsular polysaccharide export in both Gram-negative and Gram-positive bacteria. However, archaeal and eukaryotic homologues are now recognized. The mechanism of energy coupling is not established, but homology with the MATE family suggests that they are secondary carriers. These transporters may function together with auxiliary proteins that allow passage across just the cytoplasmic membrane or both membranes of the Gram-negative bacterial envelope. They may also regulate transport. Thus, each Gram-negative bacterial PST system specific for an exo- or capsular polysaccharide functions in conjunction with a cytoplasmic membrane-periplasmic auxiliary (MPA) protein with a cytoplasmic ATP-binding domain (MPA1-C; TC# 3.C.3) as well as an outer membrane auxiliary protein (OMA; TC #3.C.5). Each Gram-positive bacterial PST system functions in conjunction with a homologous MPA1 + C pair of proteins equivalent to an MPA1-C proteins of Gram-negative bacteria. The C-domain has been shown to possess tyrosine protein kinase activity, so it may function in a regulatory capacity. The lipopolysaccharide exporters may function specifically in the translocation of the lipid-linked O-antigen side chain precursor from the inner leaflet of the cytoplasmic membrane to the outer leaflet. In this respect, they correlate in function with the flippase activities of members of the oligosaccharidyl-lipid flippase (OLF) family of the MVF families.

Structure

The protein members of the PST family are generally of 400-500 amino acyl residues in length and traverse the membrane as putative α-helical spanners twelve times.

PST Transport Reaction

The generalized transport reaction catalyzed by PST family proteins is: Lipid-linked polysaccharide precursor (in) + energy → Lipid-linked polysaccharide precursor (out).

2.A.66.3 The Oligosaccharidyl-lipid Flippase (OLF) Family

The OLF family is found in the endoplasmic reticular membranes of eukaryotes. N-linked glycosylation in eukaryotic cells follows a conserved pathway in which a tetradecasaccharide substrate (Glc3Man9GlcNAc2) is initially assembled in the ER membrane as a dolichylpyrophosphate (Dol-PP)-linked intermediate before being transferred to an asparaginyl residue in a lumenal protein. An intermediate, Man5GlcNAc2-PP-Dol is made on the cytoplasmic side of the membrane and translocated across the membrane so that the oligosaccharide chain faces the ER lumen where biosynthesis continues to completion. The flippase that catalyzes the translocation step is dependent on the Rft1 protein
TC# 2.A.66.3.1
of ''

S. cerevisiae ''Saccharomyces cerevisiae'' () (brewer's yeast or baker's yeast) is a species of yeast (single-celled fungus microorganisms). The species has been instrumental in winemaking, baking, and brewing since ancient times. It is believed to have bee ...

.''

Homologues

Homologues are found in plants, animals and fungi including ''

C. elegans ''Caenorhabditis elegans'' () is a free-living transparent nematode about 1 mm in length that lives in temperate soil environments. It is the type species of its genus. The name is a blend of the Greek ''caeno-'' (recent), ''rhabditis'' (r ...

'', '' D. melanogaste'' r, ''

H. sapiens Humans (''Homo sapiens'') are the most abundant and widespread species of primate, characterized by bipedalism and exceptional cognitive skills due to a large and complex brain. This has enabled the development of advanced tools, culture, ...

'', ''

'' and '' S. pombe''. These proteins are distantly related to MATE and PST family members and therefore are believed to be secondary carriers.

Structure

The yeast protein, called the nuclear division Rft1 protein
TC# 2.A.66.3.1
, is 574 aas with 12 putative TMSs. The homologue in ''A. thaliana'' is 401 aas in length with 8 or 9 putative TMSs while that in ''C. elegans'' is 522 aas long with 11 putative TMSs.

2.A.66.4 The Mouse Virulence Factor (MVF) Family

One member of the MVF family, MviN
TC# 2.A.66.4.1
of '' Salmonella typhimurium,'' has been shown to be an important virulence factor for this organism when infecting the mouse. In several bacteria, ''mviN'' genes occur in operons including ''glnD'' genes that encode the uridyl transferase that participates in the regulation of nitrogen metabolism. It is thought that MviN may flip the Lipid II peptidoglycan (PG) precursor from the cytoplasmic side of the inner membrane to the periplasmic surface acting as a putative lipid flippase in '' Salmonella typhimurium''. In ''E. coli'', MviN is an essential protein which when defective results in the accumulation of polyprenyl diphosphate-N-acetylmuramic acid-(pentapeptide)-N-acetyl-glucosamine, thought to be the peptidoglycan intermediated exported via MviN. In ''Mycobacterium tuberculosis,'' MviN is thought to play an essential role in peptidoglycan biosynthesis. Another MVF protein, MurJ, functions as a peptidoglycan biosynthesis protein. A 3-d structural model shows that MurJ contains a solvent-exposed cavity within the plane of the membrane. MurJ has 14 TMSs, and specific charged residues localized in the central cavity are essential for function. This structural homology model suggests that MurJ functions as an essential transporter in PG biosynthesis. Based on an in vivo assay, MurJ acts as a flippase for the lipid-linked cell wall precursor, polyisoprenoid-linked disaccharide-peptapeptide. There is controversy about the role of this porter and FtsW/RodA which on the basis of an in vitro assay, were thought to be flippases for the same intermediate.

2.A.66.5 The Agrocin 84 Antibiotic Exporter (AgnG) Family

Agrocin 84 is a disubstituted adenine nucleotide antibiotic made by and specific for ''Agrobacteria''. It is encoded by the pAgK84 plasmid of ''A. tumefaciens'' and targets a tRNA synthetase. The ''agnG'' gene encodes a protein of 496 aas with 12-13 putative TMSs and a short hydrophilic N-terminal domain of 80 residues.
TCDB
Blast search with 2 iterations shows that members of the AgnG family are related to the U-MOP12 family (TC# 2.A.66.12) and the PST family (TC# 2.A.66.2) and more distantly related to the OLF (TC# 2.A.66.3), MVF (TC# 2.A.66.4), and LPS-F (TC# 2.A.66.10) families.

Transport Reaction

The reaction catalyzed by AgnG is: agrocin (in) → agrocin (out) AgnG homologue 2 of ''Lyngbya sp.'' (TC# 2.A.66.5.3) is thought to be a polysaccharide exporter.

2.A.66.9 The Progressive Ankylosis (Ank) Family

Craniometaphyseal dysplasia Craniometaphyseal dysplasia is a rare skeletal disorder that results from a mutation in the ANKH or GJA1 genes. The condition is characterized abnormal facial features, impairment of cranial nerves, and malformation of the long bones in the limbs ...

(CMD) is a bone dysplasia characterized by overgrowth and sclerosis of the craniofacial bones and abnormal modeling of the

metaphyses The metaphysis is the neck portion of a long bone between the epiphysis and the diaphysis. It contains the growth plate, the part of the bone that grows during childhood, and as it grows it ossifies near the diaphysis and the epiphyses. The metaph ...

of the tubular bones. Hyperostosis and sclerosis of the skull may lead to cranial nerve compressions resulting in hearing loss and facial palsy. An autosomal dominant form of the disorder has been linked to chromosome 5p15.2-p14.1 within a region harboring the human homolog (ANKH
TC# 2.A.66.9.1
of the mouse progressive ankylosis (''ank'') gene. The ANK protein spans the cell membrane and shuttles inorganic pyrophosphate (PP_i), a major inhibitor of physiologic and pathologic calcification, bone mineralization and bone resorption.

Structure

The ANK protein has 12 membrane-spanning helices with a central channel permitting the passage of PP_i. Mutations occur at highly conserved amino acid residues presumed to be located in the cytosolic portion of the protein. The PP_i carrier ANK is concerned with bone formation and remodeling.

Other Families

* 2.A.66.6 - The Putative Exopolysaccharide Exporter (EPS-E) Family * 2.A.66.7 - Putative O-Unit Flippase (OUF) Family * 2.A.66.8 - Unknown MOP-1 (U-MOP1) Family * 2.A.66.10 - LPS Precursor Flippase (LPS-F) Family * 2.A.66.11 - Uncharacterized MOP-11 (U-MOP11) Family * 2.A.66.12 - Uncharacterized MOP-12 (U-MOP12) Family

References

{{Dual, source=Transporter Classification Database, sourcepath=http://www.tcdb.org/search/result.php?tc=2.A.66, sourcearticle=2.A.66 The Multidrug/Oligosaccharidyl-lipid/Polysaccharide (MOP) Flippase Superfamily, date=19:37, 24 February 2016 (UTC), author=Saier Lab Protein families Solute carrier family