In 1998 we updated earlier descriptions of the largest family of secondary transport carriers found in living organisms, the major facilitator superfamily (MFS). Seventeen families of transport proteins were shown to comprise this superfamily. We here report expansion of the MFS to include 29 established families as well as five probable families. Structural, functional, and mechanistic features of the constituent permeases are described, and each newly identified family is shown to exhibit specificity for a single class of substrates. Phylogenetic analyses define the evolutionary relationships of the members of each family to each other, and multiple alignments allow definition of family-specific signature sequences as well as all wellconserved sequence motifs. The work described serves to update previous publications and allows extrapolation of structural, functional and mechanistic information obtained with any one member of the superfamily to other members with limitations determined by the degrees of sequence divergence.

The major facilitator superfamily.

The major facilitator superfamily (MFS) is one of the two largest families of membrane transporters found on Earth. 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. 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. Phylogenetic analyses revealed the occurrence of 17 distinct families within the MFS, each of which generally transports a single class of compounds. 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. Protein members of some MFS families are found exclusively in bacteria or in eukaryotes, but others are found in bacteria, archaea, and eukaryotes. 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. All 17 families are shown to exhibit the common feature of a well-conserved motif present between transmembrane spanners 2 and 3. The analyses reported serve to characterize one of the largest and most diverse families of transport proteins found in living organisms.

Major Facilitator Superfamily

The hypothesis that traits closely associated with fitness will generally possess lower heritabilities than traits more loosely connected with fitness is tested using 1120 narrow sense heritability estimates for wild, outbred animal populations, collected from the published record. Our results indicate that life history traits generally possess lower heritabilities than morphological traits, and that the means, medians, and cumulative frequency distributions of behavioural and physiological traits are intermediate between life history and morphological traits. These findings are consistent with popular interpretations of Fisher's (1930, 1958) Fundamental Theorem of Natural Selection, and Falconer (1960, 1981), but also indicate that high heritabilities are maintained within natural populations even for traits believed to be under strong selection. It is also found that the heritability of morphological traits is significantly lower for ectotherms than it is for endotherms which may in part be a result of the strong correlation between life history and body size for many ectotherms.

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Natural selection and the heritability of fitness components

Membrane transport proteins that transduce free energy stored in electrochemical ion gradients into a concentration gradient are a major class of membrane proteins. We report the crystal structure at 3.5 angstroms of the Escherichia coli lactose permease, an intensively studied member of the major facilitator superfamily of transporters. The molecule is composed of N- and C-terminal domains, each with six transmembrane helices, symmetrically positioned within the permease. A large internal hydrophilic cavity open to the cytoplasmic side represents the inward-facing conformation of the transporter. The structure with a bound lactose homolog, β-D-galactopyranosyl-1-thio-β-D-galactopyranoside, reveals the sugar-binding site in the cavity, and residues that play major roles in substrate recognition and proton translocation are identified. We propose a possible mechanism for lactose/proton symport (co-transport) consistent with both the structure and a large body of experimental data.

/pdf/structure-and-mechanism-of-the-lactose-permease-of-4s1relu1k0.pdf

Structure and mechanism of the lactose permease of Escherichia coli

Proton-dependent multidrug efflux systems.

The Conserved Motif, GXXX(D/E)(R/K)XG[X](R/K)(R/K), in Hydrophilic Loop 2/3 of the Lactose Permease

The uniporter/symporter/antiporter superfamily is an evolutionarily related group of solute transporters. For the entire superfamily, we have used a new predictive program to identify the transmembrane domains. These transmembrane domains were then analyzed with regard to their overall hydrophobicity and amphipathicity. In addition, the lengths of the hydrophilic loops connecting the transmembrane domains were calculated. These data, together with structural information in the literature, were collectively used to produce a general model for the three-dimensional arrangement of the transmembrane domains.

/pdf/structural-features-of-the-uniporter-symporter-antiporter-4ep78hubay.pdf

Robert J. Brooker

Papers

The Conserved Motif, GXXX(D/E)(R/K)XG[X](R/K)(R/K), in Hydrophilic Loop 2/3 of the Lactose Permease

Structural features of the uniporter/symporter/antiporter superfamily

Electrogenic H+ translocation by the plasma membrane ATPase of Neurospora. Studies on plasma membrane vesicles and reconstituted enzyme.

Functional roles of Glu-269 and Glu-325 within the lactose permease of Escherichia coli.

The lactose permease of Escherichia coli.