2.A.1 The Major Facilitator Superfamily (MFS)
The MFS is a very old, large and diverse superfamily that includes over 10,000 sequenced members. They catalyze uniport, solute:cation (H+ or Na+) symport and/or solute:H+ or solute:solute antiport. Most are of 400-600 amino acyl residues in length and possess either 12, 14 or 24 putative transmembrane α-helical spanners. The mechanistic principles applicable to all MFS carriers have been summarized by Law et al (2008). The 24 TMS MFS permease, NarK, of Paracoccus pantotrophus has two 12 TMS domains, NarK1 and NarK2, both of which are required for normal nitrate uptake. NarK1 catalyzes NO3-:H+ symport, dependent on the pmf, while NarK2 catalyzes NO3-:NO2- antiport, independent of the pmf (Wood et al., 2002). Thus, the protein is a fusion protein of two homologous but distinct MFS permeases.
MFS permeases exhibit specificity for sugars, polyols, drugs, neurotransmitters, Krebs cycle metabolites, phosphorylated glycolytic intermediates, amino acids, peptides, osmolites, siderophores (efflux), iron-siderophores (uptake), nucleosides, organic anions, inorganic anions, etc. They are found ubiquitously in all three kingdoms of living organisms. One member of the DHA2 family with 14 spanners, the TetL Me2+ · tetracycline:H+ antiporter of B. subtilis (TC #2.A.1.3.16), which also exhibits monovalent ion antiport activity, can be converted to a monovalent cation (Na+, K+, H+) antiporter with no tetracycline transport activity by deletion of TMSs 7 and 8, the two central and extra TMSs (Jin et al., 2001). Genome analyses of MFS permeases have been published (Lorca et al., 2007).
A 6.5 Å resolution structure for the MFS permease, OxlT (TC #2.A.1.11.1) has been obtained (Heymann et al., 2001; Hirai et al., 2002) which shows the positions of the transmembrane α-helices but does not allow assignment of the TMS # to these helices. Molecular modeling (Hirai et al., 2003) led to the suggestion that the 12 TMS protein arose from a 3 TMS element by two successive duplication events. The same suggestion resulted from sequence comparisons showing that the primordial 3 TMS element may have arisen from a VIC family (TC #1.A.1) 2 TMS channel-forming unit Hvorup & Saier, (2002). TMSs 3, 6, 9 and 12 are hydrophobic while TMSs 1, 2, 4, 5, 7, 8, 10 and 11 line the channel. The protein may exhibit 4 fold symmetry.
The high-resolution 3-dimensional structures (3.3 and 3.5 Å resolution) of the glycerol-3-P:P antiporter (GlpT; TC #2.A.1.4.3) and the lactose:H+ symporter (LacY; TC #2.A.1.5.1), respectively (Huang et al., 2003 and Abramson et al., 2003, respectively; see also Locher et al., 2003; Guan et al., 2007) have been determined. These structures reveal the 2-fold symmetry expected, based on sequence similarity of the two halves. However, the 4-fold symmetry seen in the OxlT structure was not observed. The substrate pathway is predicted to exist between the two halves of the permeases using an alternating access mechanism with a single substrate binding site (Huang et al., 2003). This mechanism is termed a 'rocker switch' type of movement.
MFS antiporters operate via a single binding site, alternating-access mechanism that involves a rocker-switch type movement of the two halves of the protein (Law et al., 2008). In the sn-glycerol-3-phosphate transporter (GlpT) from Escherichia coli, the substrate-binding site is formed by several charged residues and a histidine that can be protonated. Salt-bridge formation and breakage are involved in the conformational changes of the protein during transport (Law et al., 2008).
Vesicular glutamate transporters (VGLUTs [2.A.1.14.13 and 2.A.1.14.16]) are responsible for the vesicular storage of L-glutamate and play an essential role in glutamatergic signal transmission in the central nervous system. VGLUT2 facilitates L-glutamate uptake in a membrane potential (ΔΨ)-dependent fashion. Uptake exhibits an absolute requirement for ~4 mM Cl- and was sensitive to Evans blue, but was insensitive to D,L-aspartate. VGLUT2s with mutations in the transmembrane-located residues Arg184, His128, and Glu191 showed a dramatic loss in L-glutamate transport activity, whereas Na+-dependent inorganic phosphate (Pi) uptake remained comparable to that of the wild type. Furthermore, Pi transport did not require Cl- and was not inhibited by Evans blue. Thus, VGLUT2 appears to possess two intrinsic transport machineries that are independent of each other: a ΔΨ-dependent L-glutamate uptake and a Na+-dependent Pi uptake (Juge et al., 2006).
The generalized transport reactions catalyzed by MFS porters are:
(1) Uniport: S (out) ⇌ S (in).
(2) Symport: S (out) + [H+ or Na+] (out) ⇌ S (in) + [H+ or Na+] (in).
(3) Antiport: S1 (out) + S2 (in) ⇌ S1 (in) + S2 (out) (S1 may be H+ or a solute).