The major facilitator superfamily, discovered in the late 1980s, is currently composed of thousands of characterized integral membrane transporters. These transport systems of the superfamily have an extremely diverse array of substrates and organismal origins, but share similarities in their molecular evolution, modes of energization (passive and secondary active), mechanisms of transport (uniport, symport and antiport), and overall two- and three-dimensional protein structures. Extensive collections of transporter protein sequences have been deposited in sequence databases since the first sequences were reported from gene cloning studies, and these early studies of a relatively few number of transporter sequences have revealed several conserved sequence motifs that have been studied at the structure-function level. This review article summarizes the evidence produced to assign the functional roles for these conserved amino acid residues and sequence motifs of the major facilitator superfamily. The implications of these functional roles are considered, with emphasis placed on their usefulness in biomedical science and medicine.
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