Membrane transport proteins

After more than a decade of systematic work of M.H. Saier Jr. and his co-workers, and after some five years of discussions with members of the Nomenclature Committee of IUBMB, headed by A. Kotyk, a list of over 400 families of membrane transport proteins is now accessible on the Web at The document contains three sections:

(1) List of families and subfamilies of the transport classification (TC) system,

(2) Properties of families of transport proteins included in the TC system,

(3) Classification of transport proteins according to specificity.

The system uses a five-character code: the first character (a number) designates the class of proteins, the second character (a letter) designates the subclass, both of these referring to the mechanism of translocation or the source of energy used for the process; the third character (a number) specifies the transporter family, the fourth character (a number) the subfamily. These hierarchical levels are defined on the basis of their primary structure. The fifth characer (a number) then refers to a particular transport protein. In outline, the transport proteins are classified as follows:

1. Pores and channels
1.A alpha-Helical channels36 families
1.B beta-Strand porins34 families
1.C Pore-forming toxins58 families
1.D Non-ribosomally synthesized channels10 families
1.E Holins19 families
2. Electrochemical-potential-driven transporters
2.A Transporters (uniporters, symporters and antiporters80 families
2.B Non-ribosomally-synthesized transporters16 families
3. Primary active transporters
3.A P-P-bond-hydrolysis-driven transporters14 families
3.B Decarboxylation-driven transporters1 family
3.C Methyl-transfer-driven transporters1 family
3.D Oxidoreduction-driven transporters9 families
3.E Light-driven transporters2 families
4. Group translocators
4.A. Phosphotransferases6 families
5. Transmembrane electron carriers
5.A Two-electron carriers2 families
5.B One-electron carriers1 family
8. Accessory factors involved in transport
8.A Auxiliary transport proteins13 families
9. Incompletely characterized transport proteins
9.A Transporters of unknown classification23 families

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