Classification of Membrane Transport Proteins

Continued from properties of families 3

Properties of Families of Transport Proteins Included in the TC System (continued)

Families 4 to 9

4. Group Translocators

4.A Phosphotransferase systems

Components of the bacterial phosphotransferase system PTS are included in this subclass. The product of the reaction, derived from extracellular sugar, is a cytoplasmic sugar phosphate. No transporters of the PTS group have been identified either in archebacteria or in eukaryotes.

TC no. a Family Substrates b Size range c Number of transmembrane segmentsd Distributione nf Examples
4.A.1Glcglucose; N-acetylglucosamine; a- and b-glucosides (maltose; trehalose; sucrose; arbutin; arbutin, cellobiose, salicin)c. 2000 (3 domains; dimeric)(8)2B2glucose IICB-IIA of Escherichia coli
4.A.2Frufructose; mannitolc. 2000 (3 domains; dimeric)(6)2B2fructose IIB'BC-IIAMH of Escherichia coli
4.A.3Laclactose; cellobiose, N,N'-diacetylchitobiose; lichenan oligosaccharidesc. 2000 (3 domains; dimeric)c. (8)2B2lactose IICB-IIA of Staphylococcus aureus
4.A.4Gutglucitolc. 2000 (3 domains; dimeric)(8)2B1glucitol IICBC'-IIA of Escherichia coli
4.A.5Gatgalactitol, D-arabinitolc. 2000 (3 domains; dimeric)c. (8)2B1galactitol IIC-IIB-IIA of Escherichia coli
4.A.6Manglucose, mannose, fructose; glucosamine, N-acetylglucosamine; sorbose; galactosamine; N-acetylgalactosaminec. 2000 (4 domains; probably dimeric)(6(IIC) + 1(IID))B!mannose IIAB-IIC-IID of Escherichia coli

5. Transmembrane Electron Carriers

This is a newly emerged class of a few proteins which act as translocators of electrons across the membrane, e.g., from the cytoplasm to the periplasm of Escherichia coli, to keep important cysteine residues in the reduced form. Members of subclass 5.B couple electron and proton transport in connection with the activity of superoxide-generating "burst" oxidases.

5.A Two-electron carriers

TC no. a Family Substrates b Size range c Number of transmembrane segmentsd Distributione nf Examples
5.A.1DsbD2e-150-8006-9A, B2DsbD of E.coli
5.A.2DsbB2e-150-2004B1DsbB of Escherichia coli

5.B One-electron carriers

TC no. a Family Substrates b Size range c Number of transmembrane segmentsd Distributione nf Examples
5.B.1Gp91 phoxe-450-7506B, E2Gp91phox of Homo sapiens

8. Accessory Factors Involved in Transport

8.A Auxiliary transport proteins

Proteins that in some way facilitate transport across one or more biological membranes but do not themselves participate directly in the transmembrane translocation of a substrate are included in this subclass. They may provide a function connected with energy coupling to transport, play a structural role in complex formation, serve a biogenic or stability function or function in regulation.

TC no. a Family Substrates b Size range c Number of transmembrane segmentsd Distributione nf Examples
8.A.1MFPproteins; peptides; lipooligosaccharides; drugs, dyes, signalling molecules; heavy metal ions; etc.350-5001B2EmrA of Escherichia coli
8.A.2SALproteins, peptides100-1500-1B(G-)1PulS of Klebsiella pneumonia
8.A.3MPA1complex polysaccharides600-8002B2ExoP of Rhizobium meliloti
8.A.4MPA2complex polysaccharides300-4002B2KpsE of Escherichia coli
8.A.5VICbK+200-5502B, E2b1a of Homo sapiens
8.A.6ANTnutrientsc. 30002E0CSF1 of Saccharomyces cerevisiae
8.A.7EIsugars500-6000B1phosphotransferase enzyme 1 of Escherichia coli
8.A.8HPrsugars60-950B1HPr of Escherichia coli
8.A.9rBATcationic and neutral amino acids (uptake)500-7004E(An)2rBAT of Oryctolagus cuniculus
8.A.10MinKK1100-2001E(An)2MinK of Rattus norvegicus
8.A.11PLBCa2+c. 501E(An)2PLB of Homo sapiens
8.A.12BEAbacteriocins100-2001B(G+)0BrcD of Brochothrix campestris
8.A.13Tpr1K+, alkali metal ionsc. 10401A, B, E2Tpr1 of Schizosaccharomyces pombe

9. Incompletely Characterized Transport Systems

9.A Transporters of unknown classification

Transport protein families of unknown classification are grouped here, awaiting their final placing once their transport mode and energy coupling have been established. These families include at least one member for which a transport function has been described but either the mode of transport or the energy coupling is not known.

TC no. a Family Substrates b Size range c Number of transmembrane segmentsd Distributione nf Examples
9.A.1PSTpolysaccharides (export)400-50012B2lipopolysaccharide exporter RfbX1 of Escherichia coli
9.A.2MerTPHg2+ (uptake)c. 2003B2mercuric ion transporter MerTP encoded on the IncJ plasmid pMERPH of Shewanella putrefaciens
9.A.3MerCHg2+ (uptake)c. 1404B1mercuric ion uptake transporter MerC encoded on the IncJ plasmid pMERPH of Shewanella putrefaciens
9.A.4PnuCnicotinamide mononucleotide (uptake)c. 3207B1nicotinamide mononucleotide uptake transporter PnuC of Salmonella typhimurium
9.A.5PPIperoxisomal proteinsmultimeric subunits3-5(10p) + 2-4(12p) + 1-2(13p) + 0-1(14p) + 0-1(5p) + 0-2(7p) + 0-1(4p)E1PEX of Homo sapiens
9.A.6INTnucleosides200-300E (An)2intracellular nucleoside transporter MTP of Mus musculus
9.A.7MerFHg2+c. 802B2MerF importer of Pseudomonas aeruginosa plasmid
9.A.8FeoBFe2+ (uptake)c. 8008-13A, B2Fe2+ uptake transporter FeoB of Escherichia coli
9.A.9FeTFe2+ (Co2+, Cd2+) (uptake)c. 5506E (Y)0Fe2+ transporter Fet4p of Saccharomyces cerevisia
9.A.10OFeTFe2+ (uptake)c. 4006A, B, E2oxidase-dependent Fe2+ transporter Ftr1p of Saccharomyces cerevisiae
9.A.11Ctr1Cu2+ (uptake)c. 4002-3E (Y)1copper transporter Ctr1p of Saccharomyces cerevisiae
9.A.12Ctr2Cu2+ (uptake)150-2003E1copper transporter Ctr2p of Saccharomyces cerevisiae
9.A.13Cjlcolicins Jsc. 650B0Cjl of Shigella sonnei
9.A.14NPCRNA; proteins, small molecules, etc.30-50 proteinsmultipleE1nuclear pore complex of Saccharomyces cerevisiae
9.A.15YhaGtryptophan150-2006B0YhaG of Bacillus subtilis
9.A.16LPIproteinc. 4002E1LAMP of Homo sapiens
9.A.17PbrTPb2+400-6507B1PbrT of Ralstonia metallidurans
9.A.18PUPpeptides, microcins, antibiotics (uptake)c. 4007B1microbin uptake porter SbmA of Escherichia coli
9.A.19MgtEMg2+, Co2+ (uptake)300-5004-5A, B2Mg2+-transporter MgtE of Bacillus firmus
9.A.20LCTunivalent cationsc. 5708-10E (Pl)0low-affinity cation transporter LCT1 of Triticum aestivum
9.A.21ComCDNA, proteins1000-1250multipleB(G-)1pilus assembly protein of Neisseria meningitidis
9.A.22NhaENa+, K+c. 2507-9BNhaE of Bacillus subtilis
9.A.23FPFe2+; H+400-8008-10E(An,Pl)Mus musculus ferroportin IREG1

a Number of the family according to the transport protein classification system.

b Substrates of single transporters within a family are separated by commas; substrates transported by different protein members of the family are separated by semicolons. When various solutes serve as transported substrates, they are separated by a slash; when two different solutes are transported in a symport fashion, they are separated by a comma; when two or more substrates are transported in an antiport fashion they are separated by a colon.

c Size range (in number of amino acid residues) when a single type of subunit is present, or for the entire complex when several types of subunits are present. In some cases the individual subunits in multisubunit systems are indicated separately.

d Number of (putative) transmembrane a-helical segments, TMS, (or b-strands in section 1.B) in a polypeptide chain. Underlined numbers indicate that the number is established by X-ray chrystallographic data or that substantial experimental evidence suggests the proposed topology, usually as a result of the use of gene fusion technology. If not underlined, numbers indicate the number of TMS predicted on the basis of hydropathy analysis using available programs such as WHAT and AveHAS see the Web site at ( In some cases, the numbers of predicted TMS is zero, and hence a "0" is entered. In many such cases, the actual TMS is (are) amphipathic, and hence the program does not predict the number correctly. Subscripts refer to the number of polypeptide chains in the complex when known; n indicates an oligomeric structure of unknown or poorly defined number of subunits. If alternative structures are found for different transporters within a single family, these are separated by semicolons.

e Abbreviations used for types of organisms, organelles and viruses are as follows: B, Bacteria; A, Archaea; E, Eucarya; G-, Gram-negative bacteria; G+, Gram-positive bacteria; Y, yeasts; Fu, fungi; Pr, protozoans; Pl, plants; An, animals; Mito, mitochondria; Chloro, chloroplasts; ER, endoplasmic reticulum; Bp, bacteriophage; V, virus

f The numbers represent the order of magnitude of members in this family as of November 2001. 0: between 1 and 5; 1: between 6 and 49; 2: between 50 and 499; 3: more than 500.

Continued with Class 1
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