( 67) noted that a mammalian phosphate:Na + symporter is a distant member of this family Paulsen et al. The family was then expanded to include organophosphate:phosphate exchangers and oligosaccharide:H +symport permeases ( 51). Subsequent studies revealed that drug efflux systems and Krebs cycle metabolites belong to this family ( 30, 62). The MFS was originally believed to function primarily in the uptake of sugars ( 36, 46). The importance of these two families of transport systems to living organisms can therefore not be overestimated. They are also prevalent in higher organisms. Although well over 100 families of transporters have now been recognized and classified ( 73), the ABC superfamily and MFS account for nearly half of the solute transporters encoded within the genomes of microorganisms ( 63). While ABC family permeases are in general multicomponent primary active transporters, capable of transporting both small molecules and macromolecules in response to ATP hydrolysis ( 59), the MFS transporters are single-polypeptide secondary carriers capable only of transporting small solutes in response to chemiosmotic ion gradients. These are the ATP-binding cassette (ABC) superfamily ( 15, 21, 37, 44) and the major facilitator superfamily (MFS), also called the uniporter-symporter-antiporter family ( 7, 28, 30, 35, 51). Two such families have been found to occur ubiquitously in all classifications of living organisms. Recent genome-sequencing data and a wealth of biochemical and molecular genetic investigations have revealed the occurrence of dozens of families of primary and secondary transporters ( 63). The analyses reported serve to characterize one of the largest and most diverse families of transport proteins found in living organisms. All 17 families are shown to exhibit the common feature of a well-conserved motif present between transmembrane spanners 2 and 3. All permeases of the MFS possess either 12 or 14 putative or established transmembrane α-helical spanners, and evidence is presented substantiating the proposal that an internal tandem gene duplication event gave rise to a primordial MFS protein prior to divergence of the family members. Protein members of some MFS families are found exclusively in bacteria or in eukaryotes, but others are found in bacteria, archaea, and eukaryotes. Compounds transported by MFS permeases include simple sugars, oligosaccharides, inositols, drugs, amino acids, nucleosides, organophosphate esters, Krebs cycle metabolites, and a large variety of organic and inorganic anions and cations. Phylogenetic analyses revealed the occurrence of 17 distinct families within the MFS, each of which generally transports a single class of compounds. All homologous MFS protein sequences in the public databases as of January 1997 were identified on the basis of sequence similarity and shown to be homologous. It is present ubiquitously in bacteria, archaea, and eukarya and includes members that can function by solute uniport, solute/cation symport, solute/cation antiport and/or solute/solute antiport with inwardly and/or outwardly directed polarity. The major facilitator superfamily (MFS) is one of the two largest families of membrane transporters found on Earth. EVIDENCE FOR REPEAT SEQUENCES IN MFS PROTEINS.MFS FAMILY-SPECIFIC SIGNATURE SEQUENCES.POSSIBLE INCLUSION OF THE PROTON-DEPENDENT OLIGOPEPTIDE TRANSPORTER (POT) FAMILY IN THE MFS.FAMILY 17: CYANATE PERMEASE (CP) FAMILY.FAMILY 16: UNKNOWN MAJOR FACILITATOR (UMF) FAMILY.FAMILY 15: AROMATIC ACID:H +SYMPORTER (AAHS) FAMILY.FAMILY 14: ANION:CATION SYMPORTER (ACS) FAMILY.FAMILY 13: MONOCARBOXYLATE PORTER (MCP) FAMILY.FAMILY 12: SIALATE:H + SYMPORTER (SHS) FAMILY.FAMILY 11: OXALATE:FORMATE ANTIPORTER (OFA) FAMILY.FAMILY 10: NUCLEOSIDE:H + SYMPORTER (NHS) FAMILY.FAMILY 9: PHOSPHATE:H + SYMPORTER (PHS) FAMILY.FAMILY 8: NITRATE-NITRITE PORTER (NNP) FAMILY.FAMILY 7: FUCOSE-GALACTOSE-GLUCOSE:H + SYMPORTER (FGHS) FAMILY.FAMILY 6: METABOLITE:H + SYMPORTER (MHS) FAMILY.FAMILY 5: OLIGOSACCHARIDE:H + SYMPORTER (OHS) FAMILY.FAMILY 4: ORGANOPHOSPHATE:INORGANIC PHOSPHATE ANTIPORTER (OPA) FAMILY.FAMILY 3: DRUG:H + ANTIPORTER (12-SPANNER) (DHA12) DRUG EFFLUX FAMILY.FAMILY 2: DRUG:H + ANTIPORTER (14-SPANNER) (DHA14) DRUG EFFLUX FAMILY.ESTABLISHMENT OF HOMOLOGY FOR MFS PROTEINS.
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