Arg-52 in the melibiose carrier of Escherichia coli is important for cation-coupled sugar transport and participates in an intrahelical salt bridge

Hyperthermia and Heat Stress as Risk Factors for Sudden Infant Death Syndrome: A Narrative Review

BACKGROUND AND OBJECTIVES

Heat stress and hyperthermia are common findings in sudden infant death syndrome (SIDS) victims. It has been suggested that thermal stress can increase the risk of SIDS directly via lethal hyperthermia or indirectly by altering autonomic functions. Major changes in sleep, thermoregulation, cardiovascular function, and the emergence of circadian functions occur at the age at which the risk of SIDS peaks-explaining the greater vulnerability at this stage of development. Here, we review the literature data on (i) heat stress and hyperthermia as direct risk factors for SIDS, and (ii) the indirect effects of thermal loads on vital physiological functions.

RESULTS

Various situations leading to thermal stress (i.e., outdoors temperatures, thermal insulation from clothing and bedding, the prone position, bed-sharing, and head covering) have been analyzed. Hyperthermia mainly results from excessive clothing and bedding insulation with regard to the ambient thermal conditions. The appropriate amount of clothing and bedding thermal insulation for homeothermia requires further research. The prone position and bed-sharing do not have major thermal impacts; the elevated risk of SIDS in these situations cannot be explained solely by thermal factors. Special attention should be given to brain overheating because of the head's major role in body heat losses, heat production, and autonomic functions. Thermal stress can alter cardiovascular and respiratory functions, which in turn can lead to life-threatening events (e.g., bradycardia, apnea with blood desaturation, and glottal closure). Unfortunately, thermal load impairs the responses to these challenges by reducing chemosensitivity, arousability, and autoresuscitation. As a result, thermal load (even when not lethal directly) can interact detrimentally with vital physiological functions.

CONCLUSIONS

With the exception of excessive thermal insulation (which can lead to lethal hyperthermia), the major risk factors for SIDS appears to be associated with impairments of vital physiological functions when the infant is exposed to thermal stress.

Structural Insights into the Transport Mechanism of the Human Sodium-dependent Lysophosphatidylcholine Transporter MFSD2A

Major facilitator superfamily domain containing 2A (MFSD2A) was recently characterized as a sodium-dependent lysophosphatidylcholine transporter expressed at the blood-brain barrier endothelium. It is the primary route for importation of docosohexaenoic acid and other long-chain fatty acids into fetal and adult brain and is essential for mouse and human brain growth and function. Remarkably, MFSD2A is the first identified major facilitator superfamily member that uniquely transports lipids, implying that MFSD2A harbors unique structural features and transport mechanism. Here, we present three three-dimensional structural models of human MFSD2A derived by homology modeling using MelB- and LacY-based crystal structures and refined by biochemical analysis. All models revealed 12 transmembrane helices and connecting loops and represented the partially outward-open, outward-partially occluded, and inward-open states of the transport cycle. In addition to a conserved sodium-binding site, three unique structural features were identified as follows: a phosphate headgroup binding site, a hydrophobic cleft to accommodate a hydrophobic hydrocarbon tail, and three sets of ionic locks that stabilize the outward-open conformation. Ligand docking studies and biochemical assays identified Lys-436 as a key residue for transport. It is seen forming a salt bridge with the negative charge on the phosphate headgroup. Importantly, MFSD2A transported structurally related acylcarnitines but not a lysolipid without a negative charge, demonstrating the necessity of a negatively charged headgroup interaction with Lys-436 for transport. These findings support a novel transport mechanism by which lysophosphatidylcholines are "flipped" within the transporter cavity by pivoting about Lys-436 leading to net transport from the outer to the inner leaflet of the plasma membrane.

Reduced Na+ affinity increases turnover of Salmonella enterica serovar Typhimurium MelB

The melibiose permease of Salmonella enterica serovar Typhimurium (MelB(St)) catalyzes symport of melibiose with Na(+), Li(+), or H(+). Bioinformatics and mutational analyses indicate that a conserved Gly117 (helix IV) is a component of the Na(+)-binding site. In this study, Gly117 was mutated to Ser, Asn, or Cys. All three mutations increase the maximum rate (V(max)) for melibiose transport in Escherichia coli DW2 and greatly decrease Na(+) affinity, indicating that intracellular release of Na(+) is facilitated. Rapid melibiose transport, particularly by the G117N mutant, triggers osmotic lysis in the lag phase of growth. The findings support the previous conclusion that Gly117 plays an important role in cation binding and translocation. Furthermore, a spontaneous second-site mutation (P148L between loop(4-5) and helix V) in the G117C mutant prevents cell lysis. This mutation significantly decreases V(max) with little effect on cosubstrate binding in G117C, G117S, and G117N mutants. Thus, the P148L mutation specifically inhibits transport velocity and thereby blocks the lethal effect of elevated melibiose transport in the Gly117 mutants.

Role of Gly117 in the cation/melibiose symport of MelB of Salmonella typhimurium

The melibiose permease of Salmonella typhimurium (MelB(St)) catalyzes symport of melibiose with Na(+), Li(+), or H(+), and bioinformatics analysis indicates that a conserved Gly117 (helix IV) is part of the Na(+)-binding site. We mutated Gly117 to Ala, Pro, Trp, or Arg; the effects on melibiose transport and binding of cosubstrates depended on the physical-chemical properties of the side chain. Compared with WT MelB(St), the Gly117 → Ala mutant exhibited little difference in either cosubstrate binding or stimulation of melibiose transport by Na(+) or Li(+), but all other mutations reduced melibiose active transport and efflux, and decreased the apparent affinity for Na(+). The bulky Trp at position 117 caused the greatest inhibition of melibiose binding, and Gly117 → Arg yielded less than a 4-fold decrease in the apparent affinity for melibiose at saturating Na(+) or Li(+) concentration. Remarkably, the mutant Gly117 → Arg catalyzed melibiose exchange in the presence of Na(+) or Li(+), but did not catalyze melibiose translocation involving net flux of the coupling cation, indicating that sugar is released prior to release of the coupling cation. Taken together, the findings are consistent with the notion that Gly117 plays an important role in cation binding and translocation.

A haploid genetic screen identifies the major facilitator domain containing 2A (MFSD2A) transporter as a key mediator in the response to tunicamycin

Tunicamycin (TM) inhibits eukaryotic asparagine-linked glycosylation, protein palmitoylation, ganglioside production, proteoglycan synthesis, 3-hydroxy-3-methylglutaryl coenzyme-A reductase activity, and cell wall biosynthesis in bacteria. Treatment of cells with TM elicits endoplasmic reticulum stress and activates the unfolded protein response. Although widely used in laboratory settings for many years, it is unknown how TM enters cells. Here, we identify in an unbiased genetic screen a transporter of the major facilitator superfamily, major facilitator domain containing 2A (MFSD2A), as a critical mediator of TM toxicity. Cells without MFSD2A are TM-resistant, whereas MFSD2A-overexpressing cells are hypersensitive. Hypersensitivity is associated with increased cellular TM uptake concomitant with an enhanced endoplasmic reticulum stress response. Furthermore, MFSD2A mutant analysis reveals an important function of the C terminus for correct intracellular localization and protein stability, and it identifies transmembrane helical amino acid residues essential for mediating TM sensitivity. Overall, our data uncover a critical role for MFSD2A by acting as a putative TM transporter at the plasma membrane.

Mechanism of melibiose/cation symport of the melibiose permease of Salmonella typhimurium

The MelB permease of Salmonella typhimurium (MelB-ST) catalyzes the coupled symport of melibiose and Na(+), Li(+), or H(+). In right-side-out membrane vesicles, melibiose efflux is inhibited by an inwardly directed gradient of Na(+) or Li(+) and stimulated by equimolar concentrations of internal and external Na(+) or Li(+). Melibiose exchange is faster than efflux in the presence of H(+) or Na(+) and stimulated by an inwardly directed Na(+) gradient. Thus, sugar is released from MelB-ST externally prior to the release of cation in agreement with current models proposed for MelB of Escherichia coli (MelB-EC) and LacY. Although Li(+) stimulates efflux, and an outwardly directed Li(+) gradient increases exchange, it is striking that internal and external Li(+) with no gradient inhibits exchange. Furthermore, Trp → dansyl FRET measurements with a fluorescent sugar (2'-(N-dansyl)aminoalkyl-1-thio-β-D-galactopyranoside) demonstrate that MelB-ST, in the presence of Na(+) or Li(+), exhibits (app)K(d) values of ∼1 mM for melibiose. Na(+) and Li(+) compete for a common binding pocket with activation constants for FRET of ∼1 mM, whereas Rb(+) or Cs(+) exhibits little or no effect. Taken together, the findings indicate that MelB-ST utilizes H(+) in addition to Na(+) and Li(+). FRET studies also show symmetrical emission maximum at ∼500 nm with MelB-ST in the presence of 2'-(N-dansyl)aminoalkyl-1-thio-β-D-galactopyranoside and Na(+), Li(+), or H(+), which implies a relatively homogeneous distribution of conformers of MelB-ST ternary complexes in the membrane.

Alteration of sugar-induced conformational changes of the melibiose permease by mutating Arg141 in loop 4-5

The melibiose permease (MelB) from Escherichia coli couples the uptake of melibiose to that of Na+, Li+, or H+. In this work, we applied attenuated total reflection Fourier transform infrared (ATR-FTIR) difference spectroscopy to obtain information about the structural changes involved in substrate interaction with the R141C mutant and with the wild-type MelB reacted with N-ethylmaleimide (NEM). These modified permeases have the ability to bind the substrates but fail to transport them. It is shown that the sugar-induced ATR-FTIR difference spectra of the R141C mutant are different from those corresponding to the Cys-less permease from which it is derived. There are alterations of peaks assigned to turns and beta-structures located most likely in loop 4-5. In addition, and quite notably, a peak at 1659 cm(-1), assigned to changes at the level of one alpha-helix subpopulation, disappears in the melibiose-induced difference spectrum in the presence of Na+, suggesting a reduction of the conformational change capacity of the mutated MelB. These helices may involve structural components that couple the cation- and sugar-binding sites. On the other hand, MelB-NEM difference spectra are proportionally less disrupted than the R141C ones. Hence, the transport cycle of these two permeases, modified at two different loops, is most likely impaired at a different stage. It is proposed that the R141C mutant leads to the generation of a partially defective ternary complex that is unable to catalyze the subsequent conformational change necessary for substrate translocation.

A 3D structure model of the melibiose permease of Escherichia coli represents a distinctive fold for Na+ symporters

The melibiose permease of Escherichia coli (MelB) catalyzes the coupled stoichiometric symport of a galactoside with a cation (either Na(+), Li(+), or H(+)), using free energy from the downhill translocation of one cosubstrate to catalyze the accumulation of the other. Here, we present a 3D structure model of MelB threaded through a crystal structure of the lactose permease of E. coli (LacY), manually adjusted, and energetically minimized. The model contains 442 consecutive residues ( approximately 94% of the polypeptide), including all 12 transmembrane helices and connecting loops, with no steric clashes and superimposes well with the template structure. The electrostatic surface potential calculated from the model is typical for a membrane protein and exhibits a characteristic ring of positive charges around the periphery of the cytoplasmic side. The 3D model indicates that MelB consists of two pseudosymmetrical 6-helix bundles lining an internal hydrophilic cavity, which faces the cytoplasmic side of the membrane. Both sugar and cation binding sites are proposed to lie within the internal cavity. The model is consistent with numerous previous mutational, biochemical/biophysical characterizations as well as low-resolution structural data. Thus, an alternating access mechanism with sequential binding is discussed. The proposed overall fold of MelB is different from the available crystal structures of other Na(+)-coupled transporters, suggesting a distinctive fold for Na(+) symporters.

Changes in secondary structures and acidic side chains of melibiose permease upon cosubstrates binding

Infrared difference spectroscopy analysis of the purified melibiose permease of Escherichia coli reconstituted into liposomes was carried out as a function of the presence of the two symporter substrates (Na(+), melibiose) in either H(2)O or in D(2)O media. Essentially, the data first show that addition of Na(+) induces appearance of peaks assigned to changes in the environment and/or orientation of alpha-helical domains of purified melibiose permease. Likewise, melibiose addition in the presence of Na(+) produces peaks corresponding to additional changes of alpha-helix environment or tilt. In addition to these changes, a pair of peaks (1599 (+) cm(-1)/1576 (-) cm(-1)) appearing in the Na(+)-induced difference spectrum is assigned to the antisymmetric stretching of COO(-) groups, since they show practically no shift upon H/D exchange. It is proposed that these acidic groups participate in Na(+) co-ordination. A corresponding pair of peaks, again fairly insensitive to H/D substitution (1591 (-) cm(-1)/1567 (+) cm(-1)), appear in the melibiose-induced difference spectra, and may again be assigned to COO(-) groups. The latter carboxyl groups may correspond to part or all of the acidic residues interacting with Lys or Arg in the resting state that become free upon melibiose binding.

Identification of the dimer interface of the lactose transport protein from Streptococcus thermophilus

The lactose transporter from Streptococcus thermophilus catalyses the symport of galactosides and protons. The carrier domain of the protein harbours the contact sites for dimerization, and the individual subunits in the dimer interact functionally during the transport reaction. As a first step towards the elucidation of the mechanism behind the cooperation between the subunits, regions involved in the dimer interface were determined by oxidative and chemical cross-linking of 12 cysteine substitution mutants. Four positions in the protein were found to be susceptible to intermolecular cross-linking. To ensure that the observed cross-links were not the result of randomly colliding particles, the cross-linking was studied in samples in which either the concentration of LacS in the membrane was varied or the oligomeric state was manipulated. These experiments showed that the cross-links were formed specifically within the dimer. The four regions of the protein located at the dimer interface are close to the extracellular ends of transmembrane segments V and VIII and the intracellular ends of transmembrane segments VI and VII.

Functional analysis of an inosine-guanosine transporter from Leishmania donovani. The role of conserved residues, aspartate 389 and arginine 393

Equilibrative nucleoside transporters encompass two conserved, charged residues that occur within predicted transmembrane domain 8. To assess the role of these "signature" residues in transporter function, the Asp389 and Arg393 residues within the LdNT2 nucleoside transporter from Leishmania donovani were mutated and the resultant phenotypes evaluated after transfection into Delta ldnt2 parasites. Whereas an R393K mutant retained transporter activity similar to that of wild type LdNT2, the R393L, D389E, and D389N mutations resulted in dramatic losses of transport capability. Tagging the wild type and mutant ldnt2 proteins with green fluorescent protein demonstrated that the D389N and D389E mutants targeted properly to the parasite cell surface and flagellum, whereas the expression of R393L at the cell surface was profoundly compromised. To test whether Asp389 and Arg393 interact, a series of mutants was generated, D389R/R393R, D389D/R393D, and D389R/R393D, within the green fluorescent protein-tagged LdNT2 construct. Although all of these ldnt2 mutants were transport-deficient, D389R/R393D localized properly to the plasma membrane, while neither D389R/R393R nor D389D/R393D could be detected. Moreover, a transport-incompetent D389N/R393N double ldnt2 mutant also localized to the parasite membrane, whereas a D389L/R393L ldnt2 mutant did not, suggesting that an interaction between residues 389 and 393 may be involved in LdNT2 membrane targeting. These studies establish genetically that Asp389 is critical for optimal transporter function and that a positively charged or polar residue at Arg393 is essential for proper expression of LdNT2 at the plasma membrane.

Cytoplasmic loop connecting helices IV and V of the melibiose permease from Escherichia coli is involved in the process of Na+-coupled sugar translocation

Previous photolabeling and limited proteolysis studies suggested that one of the four basic residues (Arg-141) of the N-terminal cytoplasmic loop connecting helices IV and V (loop 4-5) of the melibiose permease (MelB) from Escherichia coli has a potential role in its symport function (Ambroise, Y., Leblanc, G., and Rousseau, B. (2000) Biochemistry 39, 1338-1345). A mutagenesis study of Arg-141 and of the other three basic residues of loop 4-5 was undertaken to further examine this hypothesis. Cys replacement analysis indicated that Arg-141 and Arg-149, but not Lys-138 and Arg-139, are essential for MelB transport activity. Replacement of Arg-141 by neutral residues (Cys or Gln) inactivated transport and energy-independent carrier-mediated flows of substrates (counterflow, efflux), whereas it had a limited effect on co-substrate binding. R141C sugar transport was partially rescued on reintroducing a positive charge with a charged and permeant thiol reagent. Whereas R149C was completely inactive, R149K and R149Q remained functional. Strikingly, introduction of an additional mutation in the C-terminal helix X (Gly for Val-343) of R149C restored sugar transport. Impermeant thiol reagents inhibited R149C/V343G transport activity in right-side-out membrane vesicles and prevented sugar binding in a sugar-protected manner. All these data suggest that MelB loop 4-5 is close to the sugar binding site and that the charged residue Arg-141 is involved in the reaction of co-substrate translocation or substrate release in the inner compartment.

The proximity between helix I and helix XI in the melibiose carrier of Escherichia coli as determined by cross-linking

The melibiose carrier of Escherichia coli is a transmembrane protein that comprises 12 transmembrane helices connected by periplasmic and cytoplasmic loops, with both the N- and C-termini located on the cytoplasmic side. Our previous studies of second-site revertants suggested proximity between several helices, including helices XI and I. In this study, we constructed six double cysteine mutants, each having one cysteine in helix I and the other in helix XI: three mutants, K18C/S380C, D19C/S380C, and F20C/S380C, have their cysteine pairs near the cytoplasmic side of the carrier, and the other three, T34C/G395C, D35C/G395C, and V36C/G395C, have their cysteine pairs near the periplasmic side. In the absence of substrate, disulfide formations catalyzed by iodine and copper-(1,10-phenanthroline)(3) indicate that helix I and helix XI are in immediate proximity to each other on the periplasmic side but not on the cytoplasmic side, as shown by protease cleavage analyses. We infer that the two helices are tilted with respect to each other, with the periplasmic sides in close proximity.

The effect of modifications of the charged residues in the transmembrane helices on the transport activity of the melibiose carrier of Escherichia coli

The melibiose transport carrier of Escherichia coli (coded by melB gene) is a cotransport system which couples the transport of a-galactosides to protons, sodium, or lithium ions. The charged amino acid residues in membrane-spanning helices are of considerable interest because many of them have important function in substrate recognition. In most cases changing these charged residue to an uncharged residue (cysteine) results in total loss of activity. In this communication we describe experiments in which the cysteine substitution for a charged residue was chemically changed by sulfhydryl reagents (MTSEA and MTSET to restore a positive charge and MTSES a negative charge) or by iodoacetic acid or through oxidation by hydrogen peroxide so as to regain the original negative charge. In two cases (D55C and D124C) the reconstructed negative charges via the oxidation of the thiol to the sulfinic and/or sulfonic acid resulted in partial recovery of transport: D55C up to 27% of the normal and D124C up to 4% of the normal in melibiose accumulation; D55C up to 36% of the normal and D124 up to 4.5% of the normal in downhill transport. Sulfhydryl reagents and iodoacetic acid failed to recover transport in all cases. We infer that the configurations of the charges as well as the structure of the side chains that carry them are critical in the maintenance of the transport.

Transmembrane segment 11 of UhpT, the sugar phosphate carrier of Escherichia coli, is an alpha-helix that carries determinants of substrate selectivity

In Escherichia coli, transport of hexose 6-phosphates is mediated by the P(i)-linked antiport carrier, UhpT, a member of the major facilitator superfamily. We showed earlier that Lys(391), a member of an intrahelical salt bridge (Asp(388)/Lys(391)) in the eleventh transmembrane segment (TM11) of this transporter, can function as a determinant of substrate selectivity (Hall, J. A., Fann, M.-C., and Maloney, P. C. (1999) J. Biol. Chem. 274, 6148-6153). Here, we examine in detail the role of TM11 in setting substrate preference. Derivatives having an uncompensated cationic charge at either position 388 or 391 (the D388C, D388V, or D388K/K391C variants) are gain-of-function mutants in which phosphoenolpyruvate, not sugar 6-phosphate, is the preferred organic substrate. By contrast, when an uncompensated anionic charge is placed at position 388 (K391C), we observed behavior consistent with an increased preference for monovalent rather than divalent sugar 6-phosphate. Because positions 388 and 391 lie deep within the UhpT hydrophobic sector, these findings suggested that an extended length of TM11 may be accessible to external substrates and probes. To explore this issue, we used a panel of TM11 single cysteine variants to examine the transport of glucose 6-phosphate in the presence and absence of the membrane-impermeant, thiol-reactive agent p-chloromercuribenzosulfonate (PCMBS). Accessibility to PCMBS, together with the pattern of substrate protection against PCMBS inhibition, leads us to conclude that TM11 spans the membrane as an alpha-helix, with approximately two-thirds of its surface lining a substrate translocation pathway. We suggest that this feature is a general property of carrier proteins in the major facilitator superfamily and that for this reason residues in TM11 will serve to carry determinants of substrate selectivity.

Sodium-substrate cotransport in bacteria

A variety of sodium-substrate cotransport systems are known in bacteria. Sodium enters the cell down an electrochemical concentration gradient. There is obligatory coupling between the entry of the ion and the entry of substrate with a stoichiometry (in the cases studied) of 1:1. Thus, the downhill movement of sodium ion into the cell leads to the accumulation of substrate within the cell. The melibiose carrier of Escherichia coli is perhaps the most carefully studied of the sodium cotransport systems in bacteria. This carrier is of special interest because it can also use protons or lithium ions for cotransport. Other sodium cotransport carriers that have been studied recently are for proline, glutamate, serine-threonine, citrate and branched chain amino acids.

Physiological evidence for an interaction between helices II and XI in the melibiose carrier of Escherichia coli

The melibiose carrier from Escherichia coli is a cation-substrate cotransporter that catalyzes the accumulation of galactosides at the expense of H(+), Na(+), or Li(+) electrochemical gradients. Charged residues on transmembrane domains in the amino-terminal portion of this carrier play an important role in the recognition of cations, while the carboxyl portion of the protein seems to be important for sugar recognition. In the present study, we substituted Lys-377 on helix XI with Val. This mutant carrier, K377V, had reduced melibiose transport activity. We subsequently used this mutant for the isolation of functional second-site revertants. Revertant strains showed the additional substitutions of Val or Asn for Asp-59 (helix II), or Leu for Phe-20 (helix I). Isolation of revertant strains where both Lys-377 and Asp-59 are substituted with neutral residues suggested the possibility that a salt bridge exists between helix II and helix XI. To further test this idea, we constructed three additional site-directed mutants: Asp-59-->Lys (D59K), Lys-377-->Asp (K377D), and a double mutant, Asp-59-->Lys/Lys-377-->Asp (D59K/K377D), in which the position of these charges was exchanged. K377D accumulated melibiose only marginally while D59K could not accumulate. However, the D59K/K377D double mutant accumulated melibiose to a modest level although this activity was no longer stimulated by Na(+). We suggest that Asp-59 and Lys-377 interact via a salt bridge that brings helix II and helix XI close to one another in the three-dimensional structure of the carrier.

Sugar recognition mutants of the melibiose carrier of Escherichia coli: possible structural information concerning the arrangement of membrane-bound helices and sugar/cation recognition site

Melibiose carrier mutants, isolated by growing cells on melibiose plus the non-metabolizable competitive inhibitor thiomethyl-beta-galactoside (TMG), were studied to determine sugar and cation recognition abnormalities. Most of the mutants show good transport of melibiose but have lost the recognition of TMG. In addition, most mutants show little or no transport of lactose. Cation recognition is also affected as all of these mutants have lost the ability to transport protons with melibiose. The amino acids causing these mutations were determined by sequencing the melB gene on the plasmid. The mutations were located on helices I, IV, VII, X and XI. We propose that these five helices are in proximity with each other and that they line the sugar/cation transport channel.

Close approximation of putative alpha -helices II, IV, VII, X, and XI in the translocation pathway of the lactose transport protein of Streptococcus thermophilus

The lactose transport protein (LacS) of Streptococcus thermophilus belongs to a family of transporters in which putative alpha-helices II and IV have been implicated in cation binding and the coupled transport of the substrate and the cation. Here, the analysis of site-directed mutants shows that a positive and negative charge at positions 64 and 71 in helix II are essential for transport, but not for lactose binding. The conservation of charge/side-chain properties is less critical for Glu-67 and Ile-70 in helix II, and Asp-133 and Lys-139 in helix IV, but these residues are important for the coupled transport of lactose together with a proton. The analysis of second-site suppressor mutants indicates an ion pair exists between helices II and IV, and thus a close approximation of these helices can be made. The second-site suppressor analysis also suggests ion pairing between helix II and the intracellular loops 6-7 and 10-11. Because the C-terminal region of the transmembrane domain, especially helix XI and loop 10-11, is important for substrate binding in this family of proteins, we propose that sugar and proton binding and translocation are performed by the joint action of these regions in the protein. Indeed, substrate protection of maleimide labeling of single cysteine mutants confirms that alpha-helices II and IV are directly interacting or at least conformationally involved in sugar binding and/or translocation. On the basis of new and published data, we reason that the helices II, IV, VII, X, and XI and the intracellular loops 6-7 and 10-11 are in close proximity and form the binding sites and/or the translocation pathway in the transporters of the galactosides-pentosides-hexuronides family.

Physiological evidence for an interaction between helix XI and helices I, II, and V in the melibiose carrier of Escherichia coli

In a previous study 23 residues in helix XI of the cysteine-less melibiose carrier were changed individually to cysteine. Several of these cysteine mutants (K377C, A383C, F385C, L391C, G395C) had low transport activity and they were white on melibiose MacConkey fermentation plates. After several days of incubation of these white clones on melibiose MacConkey plates a rare red mutant appeared. The plasmid DNA was then isolated and sequenced. The two second site revertants from K377C were I22S and D59A. This change of aspartic acid to a neutral residue suggests that physiologically there is an interaction between K377 and D59 (possibly a salt bridge). The revertants from A383C were in positions 20 (F20L) and 22 (I22S and I22N). Revertants of F385C were intrahelical changes (I387M and A388G) and a change in C-terminal loop (R441C). Revertants of L391C were in helix I (I22N, I22T and D19E) and helix V (A152S). Revertants of G395C were in helix I (D19E and I22N). We suggest that there is an interaction between helix XI and helices I, II, and V and proximity between these helices.