A functional-phylogenetic classification system for transmembrane solute transporters

A comprehensive classification system for transmembrane molecular transport ers has been developed and recently approved by the transport panel of the nomenclature committee of the International Union of Biochemistry and Molec ular Biology This system is hared on (i) transporter class and subclass (mo de of transport and energy coupling mechanism), (ii) protein phylogenetic f amily and subfamily and (iii) substrate specificity. Almost all of the more than,250 identified families of transporters include members that function exclusively in transport. Channels (115 families), secondary active transp orters (uniporters, symporters and antiporters) (78 families), primary acti ve transporters (23 families), group translocators (6 families), and transp ort proteins of ill-defined function or of unknown mechanism (51 families) constitute distinct categories. Transport mode and energy coupling prove to be relatively immutable characteristics and therefore provide primary base s for classification. Phylogenetic grouping reflects structure, function, m echanism, and often substrate specificity and therefore provides a reliable secondary basis for classification. Substrate specificity and polarity of transport prove to be more readily altered during evolutionary history and therefore provide a tertiary basis for classification. With very few except ions, a phylogenetic family of transporters includes members that function by a single transport mode and energy coupling mechanism, although a variet y of substrates may be transported sometimes with either inwardly or outwar dly directed polarity. In this review, I provide cross-referencing of well- characterized constituent transporters according to (i) transport mode, (ii ) energy coupling mechanism, (iii) phylogenetic grouping, and (iv) substrat es transported. The structural features and distribution of recognized fami ly members throughout the living world are also evaluated. The tabulations should facilitate familial and functional assignments of newly sequenced tr ansport proteins that will result from future genome sequencing projects.