Structural information on a membrane transport protein from nuclear magnetic resonance spectroscopy using sequence-selective nitroxide labeling

Paramagnetic Chemical Probes for Studying Biological Macromolecules

Paramagnetic chemical probes have been used in electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopy for more than four decades. Recent years witnessed a great increase in the variety of probes for the study of biological macromolecules (proteins, nucleic acids, and oligosaccharides). This Review aims to provide a comprehensive overview of the existing paramagnetic chemical probes, including chemical synthetic approaches, functional properties, and selected applications. Recent developments have seen, in particular, a rapid expansion of the range of lanthanoid probes with anisotropic magnetic susceptibilities for the generation of structural restraints based on residual dipolar couplings and pseudocontact shifts in solution and solid state NMR spectroscopy, mostly for protein studies. Also many new isotropic paramagnetic probes, suitable for NMR measurements of paramagnetic relaxation enhancements, as well as EPR spectroscopic studies (in particular double resonance techniques) have been developed and employed to investigate biological macromolecules. Notwithstanding the large number of reported probes, only few have found broad application and further development of probes for dedicated applications is foreseen.

Structural studies of proteins by paramagnetic solid-state NMR spectroscopy

Paramagnetism-based nuclear pseudocontact shifts and spin relaxation enhancements contain a wealth of information in solid-state NMR spectra about electron-nucleus distances on the ∼20 Å length scale, far beyond that normally probed through measurements of nuclear dipolar couplings. Such data are especially vital in the context of structural studies of proteins and other biological molecules that suffer from a sparse number of experimentally-accessible atomic distances constraining their three-dimensional fold or intermolecular interactions. This perspective provides a brief overview of the recent developments and applications of paramagnetic magic-angle spinning NMR to biological systems, with primary focus on the investigations of metalloproteins and natively diamagnetic proteins modified with covalent paramagnetic tags.

How to investigate interactions between membrane proteins and ligands by solid-state NMR

Solid-state NMR is an established method for biophysical studies of membrane proteins within the lipid bilayers and an emerging technique for structural biology in general. In particular magic angle sample spinning has been found to be very useful for the investigation of large membrane proteins and their interaction with small molecules within the lipid bilayer. Using a number of examples, we illustrate and discuss in this chapter, which information can be gained and which experimental parameters need to be considered when planning such experiments. We focus especially on the interaction of diffusive ligands with membrane proteins.

Solid-state nuclear magnetic resonance structural studies of proteins using paramagnetic probes

Determination of three-dimensional structures of biological macromolecules by magic-angle spinning (MAS) solid-state NMR spectroscopy is hindered by the paucity of nuclear dipolar coupling-based restraints corresponding to distances exceeding 5 Å. Recent MAS NMR studies of uniformly (13)C,(15)N-enriched proteins containing paramagnetic centers have demonstrated the measurements of site-specific nuclear pseudocontact shifts and spin relaxation enhancements, which report on electron-nucleus distances up to ~20 Å. These studies pave the way for the application of such long-distance paramagnetic restraints to protein structure elucidation and analysis of protein-protein and protein-ligand interactions in the solid phase. Paramagnetic species also facilitate the rapid acquisition of high resolution and sensitivity multidimensional solid-state NMR spectra of biomacromolecules using condensed data collection schemes, and characterization of solvent-accessible surfaces of peptides and proteins. In this review we discuss some of the latest applications of magic-angle spinning NMR spectroscopy in conjunction with paramagnetic probes to the structural studies of proteins in the solid state.

Evaluation of the influence of intermolecular electron-nucleus couplings and intrinsic metal binding sites on the measurement of 15N longitudinal paramagnetic relaxation enhancements in proteins by solid-state NMR

Magic-angle spinning solid-state NMR measurements of (15)N longitudinal paramagnetic relaxation enhancements (PREs) in (13)C,(15)N-labeled proteins modified with Cu(2+)-chelating tags can yield multiple long-range electron-nucleus distance restraints up to ~20 Å (Nadaud et al. in J Am Chem Soc 131:8108-8120, 2009). Using the EDTA-Cu(2+) K28C mutant of B1 immunoglobulin binding domain of protein G (GB1) as a model, we investigate the effects on such measurements of intermolecular electron-nucleus couplings and intrinsic metal binding sites, both of which may potentially complicate the interpretation of PRE data in terms of the intramolecular protein fold. To quantitatively assess the influence of intermolecular (15)N-Cu(2+) interactions we have determined a nearly complete set of longitudinal (15)N PREs for a series of microcrystalline samples containing ~10, 15 and 25 mol percent of the (13)C,(15)N-labeled EDTA-Cu(2+)-tagged protein diluted in a matrix of diamagnetic natural abundance GB1. The residual intermolecular interactions were found to be minor on the whole and account for only a fraction of the relatively small but systematic deviations observed between the experimental (15)N PREs and corresponding values calculated using protein structural models for residues furthest removed from the EDTA-Cu(2+) tag. This suggests that these deviations are also caused in part by other factors not related to the protein structure, such as the presence in the protein of intrinsic secondary sites capable of binding Cu(2+) ions. To probe this issue we performed a Cu(2+) titration study for K28C-EDTA GB1 monitored by 2D (15)N-(1)H solution-state NMR, which revealed that while for Cu(2+):protein molar ratios of ≤ 1.0 Cu(2+) binds primarily to the high-affinity EDTA tag, as anticipated, at even slightly super-stoichiometric ratios the Cu(2+) ions can also associate with side-chains of aspartate and glutamate residues. This in turn is expected to lead to enhanced PREs for residues located in the vicinity of the secondary Cu(2+) binding sites, and indeed many of these residues were ones found to display the elevated longitudinal (15)N PREs in the solid phase.

NMR and EPR studies of membrane transporters

In order to fulfill their function, membrane transport proteins have to cycle through a number of conformational and/or energetic states. Thus, understanding the role of conformational dynamics seems to be the key for elucidation of the functional mechanism of these proteins. However, membrane proteins in general are often difficult to express heterologously and in sufficient amounts for structural studies. It is especially challenging to trap a stable energy minimum, e.g., for crystallographic analysis. Furthermore, crystallization is often only possible by subjecting the protein to conditions that do not resemble its native environment and crystals can only be snapshots of selected conformational states. Nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy are complementary methods that offer unique possibilities for studying membrane proteins in their natural membrane environment and for investigating functional conformational changes, lipid interactions, substrate-lipid and substrate-protein interactions, oligomerization states and overall dynamics of membrane transporters. Here, we review recent progress in the field including studies from primary and secondary active transporters.

Investigating transport proteins by solid state NMR

Transporters form an interesting and complex class of membrane proteins. Many of them are potential drug targets due to their role in translocation of ions, small molecules and peptides across the membrane or due to their role in multidrug resistance. Hence elucidating their structure and mechanism is of great importance and may lead to a host of new drugs and methods to alter or inhibit their function. Solid state NMR is an emerging technique for investigating transport proteins. Along with other biochemical and biophysical techniques solid state NMR can provide data on drug binding, protein dynamics and structure at the interface between structural biology and functional analysis. Here, we review solid state NMR applications to primary active and secondary transporters involved in translocation of small molecules. We discuss current experimental limitations and give an overall perspective on how the technique may be used to address some pertinent questions relevant to transporters.

Solid-state NMR in drug design and discovery for membrane-embedded targets

Observing drugs and ligands at their site of action in membrane proteins is now possible through the use of a development in biomolecular NMR spectroscopy known as solid-state NMR. Even large, functionally active complexes are being examined using this method, with structural details being resolved at super-high subnanometre resolution. This is supplemented by detailed dynamic and electronic information about the surrounding ligand environment, and gives surprising new insights into the way in which ligands bind, which can aid drug design.

Structural modeling of dual-affinity purified Pho84 phosphate transporter

The phosphate transporter Pho84 of Saccharomyces cerevisiae is predicted to contain 12 transmembrane (TM) regions, divided into two partially duplicated parts of 6 TM segments. The three-dimensional (3D) organization of the Pho84 protein has not yet been determined. However, the 3D crystal structure of the Escherichia coli MFS glycerol-3-phosphate/phosphate antiporter, GlpT, and lactose transporter, LacY, has recently been determined. On the basis of extensive prediction and fold recognition analyses (at the MetaServer), GlpT was proposed as the best structural template on which the arrangement of TM segments of the Pho84 transporter was fit, using the comparative structural modeling program MODELLER. To initiate an evaluation of the appropriateness of the Pho84 model, we have performed two direct tests by targeting spin labels to putative TM segments 8 and 12. Electron paramagnetic resonance spectroscopy was then applied on purified and spin labeled Pho84. The line shape from labels located at both positions is consistent with the structural environment predicted by the template-generated model, thus supporting the model.

Amino acid type selective isotope labelling of the multidrug ABC transporter LmrA for solid-state NMR studies

The ABC transporter LmrA in Lactococcus lactis confers resistance to a wide range of antibiotics and cytotoxic drugs and is a functional homologue of P-glycoprotein. Recently, solid-state NMR methods have shown potential for structural- and non-perturbing, site directed functional studies. These experiments require isotopic labelling of selected sites. We have developed a strategy to produce large quantities of selectively labelled LmrA reconstituted at a high density in lipid membranes. This makes the 64 kDa integral membrane protein LmrA and therefore the ABC transporter superfamily accessible to NMR analysis.

Loop X/XI, the largest cytoplasmic loop in the membrane-bound melibiose carrier of Escherichia coli, is a functional re-entrant loop

The melibiose carrier of Escherichia coli is a membrane-bound sugar-cation cotransporter consisting of 12 transmembrane helices connected by cytoplasmic and periplasmic loops, with both N- and C-terminus on the cytoplasmic side. Using a functional cysteine-less carrier, cysteine was substituted individually for residues 347-378 that comprise the largest cytoplasmic loop X/XI. The majority of the cysteine mutants have good protein expression levels. The cysteine mutants were studied for their transport activities, and the inhibitory effects of two sulfhydryl reagents, PCMBS (7-A long) and BM (29-A long). Cysteine substitution resulted in substantial loss of transport in 12 mutants. While PCMBS caused significant inhibition in only two mutants, T373C and V376C, from the periplasmic side (in a substrate-protective manner), more extensive inhibition pattern was observed from the cytoplasmic side, in seven mutants: V353C, Y358C, V371C, Q372C, T373C, V376C and G378C, suggesting that these residues are along the sugar pathway in the aqueous channel, close to the cytoplasmic side. Furthermore, the inhibitory effect of BM on the inside-out vesicles of the above mutants was clearly less than that of PCMBS, suggesting channel space limitation to large molecules, consistent with those residues being inside the channel. Three second-site revertants (A350C/F268L, A350C/I22S, and A350C/I22N) were selected. They may suggest proximities between loop X/XI and helices I and VIII, in agreement with a re-entrant loop structure. Self thiol cross-linkings of the cysteine mutants on loop X/XI failed to form dimers, suggesting that most of the loop is not surface-exposed from cytoplasmic side. Together, these results strongly indicated a functional re-entrant loop mechanistically important in Na+-coupled transporters.

Development of a new method, based on a bioreactor coupled with an L-lactate biosensor, toward the determination of a nonspecific inhibition of L-lactic acid production during milk fermentation

The development and characteristics of a bioreactor employing bacteria (Streptococcus thermophilus) encapsulated in Ca-alginate beads coupled with an L-lactate biosensor are reported. The biosensor comprises a carbon paste electrode modified with enzymes HRP (horseradish peroxidase), LOD (lactate oxidase), and FcH (ferrocene) as redox mediator. The measurement of L-lactate is based on the signal produced by H(2)O(2), the product of the enzymatic oxidation of L-lactate by LOD. The detection of H(2)O(2) is performed at the electrode surface via HRP/FcH at low operating potential (-100mV vs Ag/AgCl). Optimization studies were performed using the bioreactor in conjunction with an L-lactate electrode operating in a flow injection system to assess the ability of encapsulated bacteria to ferment carbohydrate solutions. The possibility of using the developed method to assess the fermentation capability of milk samples was evaluated. Bronopol (2-bromo-2-nitro propane-1,3-diol) was chosen to simulate the effect of an inhibitory agent of milk fermentation. The obtained results indicated that the evaluation of the amount of L-lactate amount produced through the bioreactor could be used as a measure of inhibition of lactic acid production in milk samples.

The distribution of lipid attached spin probes in bilayers: application to membrane protein topology

The distribution of the lipid-attached doxyl electron paramagnetic resonance (EPR) spin label in 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine membranes has been studied by (1)H and (13)C magic angle spinning nuclear magnetic resonance relaxation measurements. The doxyl spin label was covalently attached to the 5th, 10th, and 16th carbons of the sn-2 stearic acid chain of a 1-palmitoyl-2-stearoyl-(5/10/16-doxyl)-sn-glycero-3-phosphocholine analog. Due to the unpaired electron of the spin label, (1)H and (13)C lipid relaxation rates are enhanced by paramagnetic relaxation. For all lipid segments the influence of paramagnetic relaxation is observed even at low probe concentrations. Paramagnetic relaxation rates provide a measure for the interaction strength between lipid segments and the doxyl group. Plotted along the membrane director a transverse distribution profile of the EPR probe is obtained. The chain-attached spin labels are broadly distributed in the membrane with a maximum at the approximate chain position of the probe. Both (1)H and (13)C relaxation measurements show these broad distributions of the doxyl group in the membrane indicating that (1)H spin diffusion does not influence the relaxation measurements. The broad distributions of the EPR label result from the high degree of mobility and structural heterogeneity in liquid-crystalline membranes. Knowing the distribution profiles of the EPR probes, their influence on relaxation behavior of membrane inserted peptide and protein segments can be studied by (13)C magic angle spinning nuclear magnetic resonance. As an example, the location of Ala residues positioned at three sites of the transmembrane WALP-16 peptide was investigated. All three doxyl-labeled phospholipid analogs induce paramagnetic relaxation of the respective Ala site. However, for well ordered secondary structures the strongest relaxation enhancement is observed for that doxyl group in the closest proximity to the respective Ala. Thus, this approach allows study of membrane insertion of protein segments with respect to the high molecular mobility in liquid-crystalline membranes.

Biomolecular solid state NMR: advances in structural methodology and applications to peptide and protein fibrils

Solid state nuclear magnetic resonance (NMR) methods can provide atomic-level structural constraints on peptides and proteins in forms that are not amenable to characterization by other high-resolution structural techniques, owing to insolubility, high molecular weight, noncrystallinity, or other characteristics. Important examples include peptide and protein fibrils and membrane-bound peptides and proteins. Recent advances in solid state NMR methodology aimed at structural problems in biological systems are reviewed. The power of these methods is illustrated by experimental results on amyloid fibrils and other protein fibrils.

Cysteine residues in the D-galactose-H+ symport protein of Escherichia coli: effects of mutagenesis on transport, reaction with N-ethylmaleimide and antibiotic binding

The galactose-H(+) membrane-transport protein, GalP, of Escherichia coli is similar in substrate specificity and susceptibility to cytochalasin B and forskolin, to the human GLUT1 sugar-transport protein; furthermore, they are about 30% identical in amino acid sequence. Transport activities of both GalP and GLUT1 are inhibited by the thiol-group-specific reagent, N-ethylmaleimide. GalP contains only three cysteine residues at positions 19, 374 and 389, each of which we have mutated, singly and in combination, to serine. Each single change of Cys-->Ser has only a minor effect on transport activity, whereas alteration of all three simultaneously profoundly diminishes V(max) for transport. The high level of expression of the GalP protein facilitates measurements of the reactivity of each mutant with N-ethylmaleimide or eosin 5-maleimide, which conclusively demonstrate that Cys(374) is the site of covalent modification by the reagents. By comparing the reactivity of Cys(374) in right-side-out and inside-out vesicles it appears that Cys(374) is located on the cytoplasmic face of the GalP protein. Although impaired in transport activity, the 'Cys-free' mutant, with all three cysteine residues mutated into serine, binds cytochalasin B and forskolin with wild-type affinities. All these results are interpreted in terms of a 12-helix model of the folding of the protein, in which the relative orientations of helix 10, containing the reactive Cys(374) residue, and helix 11, containing the unreactive Cys(389) residue, can now be defined.

Selective NMR observation of inhibitor and sugar binding to the galactose-H(+) symport protein GalP, of Escherichia coli

The binding of the transport inhibitor forskolin, synthetically labelled with (13)C, to the galactose-H(+) symport protein GalP, overexpressed in its native inner membranes from Escherichia coli, was studied using cross-polarization magic angle spinning (13)C NMR. (13)C-Labelled D-galactose and D-glucose were displaced from GalP with the singly labelled [7-OCO(13)CH(3)]forskolin and were not bound to any alternative site within the protein, demonstrating that any multiple sugar binding sites are not simultaneously accessible to these sugars and the inhibitor within GalP. The observation of singly (13)C-labelled forskolin was hampered by interference from natural abundance (13)C in the membranes and so the effectiveness of double-quantum filtration was assessed for the exclusive detection of (13)C spin pairs in sugar (D-[1,2-(13)C(2)]glucose) and inhibitor ([7-O(13)CO(13)CH(3)]forskolin) bound to the GalP protein. The solid state NMR methodology was not effective in creating double-quantum selection of ligand bound with membranes in the 'fluid' state (approx. 2 degrees C) but could be applied in a straightforward way to systems that were kept frozen. At -35 degrees C, double-quantum filtration detected unbound sugar that was incorporated into ice structure within the sample, and was not distinguished from protein-bound sugar. However, the method detected doubly labelled forskolin that is selectively bound only to the transport system under these conditions and provided very effective suppression of interference from natural abundance (13)C background. These results indicate that solid state NMR methods can be used to resolve selectively the interactions of more hydrophobic ligands in the binding sites of target proteins.

Quaternary structure of the lactose transport protein of Streptococcus thermophilus in the detergent-solubilized and membrane-reconstituted state

The quaternary structure of LacS, the lactose transporter of Streptococcus thermophilus, has been determined for the detergent-solubilized and the membrane-reconstituted state of the protein. The quaternary structure of the n-dodecyl-beta-d-maltoside-solubilized state was studied using a combination of sedimentation velocity and equilibrium centrifugation analysis. From these measurements it followed that the detergent-solubilized LacS undergoes reversible self-association with a monomer to dimer mode of association. The association constants were 5.4 +/- 3.6 and 4.4 +/- 1.0 ml mg(-1) as determined from the velocity and equilibrium sedimentation measurements, respectively. The experiments did not indicate significant changes in the shape of the protein-detergent complex or the amount of detergent bound in going from the monomeric to dimeric state of LacS. Importantly, a single Cys mutant of LacS is labeled by 2-(4'-maleimidylanilino)naphthalene-6-sulfonic acid in a substrate-dependent manner, indicating that the detergent-solubilized protein exhibits ligand binding activity. The quaternary structure of membrane-reconstituted LacS was determined by freeze-fracture electron microscopy analysis. Recent developments in the analysis of freeze-fracture images (Eskandari, S. P., Wright, E. M., Freman, M., Starace, D. M., and Zampighi, G. A. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 11235-11240) allowed us to directly correlate the cross-sectional area of the transmembrane segment to a dimeric state of the functionally membrane-reconstituted LacS protein. The cross-sectional area of the LacS protein was calibrated using the membrane-reconstituted transmembrane domain of the mannitol transporter enzyme II, an intramembrane particle for which the cross-sectional area was obtained from maps of two-dimensional crystals. The consequences of the determined quaternary structure for the transport function and regulation of LacS are discussed.

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.

Rotational mobility and orientational stability of a transport protein in lipid membranes

A single-cysteine mutant of the lactose transport protein LacS(C320A/W399C) from Streptococcus thermophilus was selectively labeled with a nitroxide spin label, and its mobility in lipid membranes was studied as a function of its concentration in the membrane by saturation-transfer electron spin resonance. Bovine rhodopsin was also selectively spin-labeled and studied to aid the interpretation of the measurements. Observations of spin-labeled proteins in macroscopically aligned bilayers indicated that the spin label tends to orient so as to reflect the transmembrane orientation of the protein. Rotational correlation times of 1-2 micros for purified spin-labeled bovine rhodopsin in lipid membranes led to viscosities of 2.2 poise for bilayers of dimyristoylphosphatidylcholine (28 degrees C) and 3.0 poise for the specific mixture of lipids used to reconstitute LacS (30 degrees C). The rotational correlation time for LacS did not vary significantly over the range of low concentrations in lipid bilayers, where optimal activity was seen to decrease sharply and was determined to be 9 +/- 1 micros (mean +/- SD) for these samples. This mobility was interpreted as being too low for a monomer but could correspond to a dimer if the protein self-associates into an elongated configuration within the membrane. Rather than changing its oligomeric state, LacS appeared to become less ordered at the concentrations in aligned membranes exceeding 1:100 (w/w) with respect to the lipid.

A spectroscopic assay for the analysis of carbohydrate transport reactions

A carbohydrate-transport assay was developed that does not require isotopically labelled substrates, but allows transport reactions to be followed spectrophotometrically. It makes use of a membrane system (hybrid membranes or proteoliposomes) bearing the transport system of interest, and a pyrroloquinoline quinone-dependent aldose dehydrogenase [soluble glucose dehydrogenase (sGDH)] and the electron acceptor 2,6-dichloroindophenol (Cl2Ind) enclosed in the vesicle lumen. After transport across the vesicular membrane, the sugar is oxidized by sGDH. The accompanying reduction of Cl2Ind results in a decrease in A600. The assay was developed and optimized for the lactose carrier (LacS) of Streptococcus thermophilus, and both solute/H+ symport and exchange types of transport could be measured with high sensitivity in crude membranes as well as in proteoliposomes. To observe exchange transport, the membranes were preloaded with a nonoxidizable substrate analogue and diluted in assay buffer containing an oxidizable sugar. Transport rates measured with this assay are comparable with those obtained with the conventional assay using isotopically labelled substrates. The method is particularly suited for determining transport reactions that are not coupled to any form of metabolic energy such as uniport reactions, or for characterizing mutant proteins with a defective energy-coupling mechanism or systems with high-affinity constants for sugars.

Substrate recognition at the cytoplasmic and extracellular binding site of the lactose transport protein of Streptococcus thermophilus

The lactose transport protein (LacS) of Streptococcus thermophilus catalyzes the uptake of lactose in an exchange reaction with intracellularly formed galactose. The interactions between the substrate and the cytoplasmic and extracellular binding site of LacS have been characterized by assaying binding and transport of a range of sugars in proteoliposomes, in which the purified protein was reconstituted with a unidirectional orientation. Specificity for galactoside binding is given by the spatial configuration of the C-2, C-3, C-4, and C-6 hydroxyl groups of the galactose moiety. Except for a C-4 methoxy substitution, replacement of the hydroxyl groups for bulkier groups is not tolerated at these positions. Large hydrophobic or hydrophilic substitutions on the galactose C-1 alpha or beta position did not impair transport. In fact, the hydrophobic groups increased the binding affinity but decreased transport rates compared with galactose. Binding and transport characteristics of deoxygalactosides from either side of the membrane showed that the cytoplasmic and extracellular binding site interact differently with galactose. Compared with galactose, the IC(50) values for 2-deoxy- and 6-deoxygalactose at the cytoplasmic binding site were increased 150- and 20-fold, respectively, whereas they were the same at the extracellular binding site. From these and other experiments, we conclude that the binding sites and translocation pathway of LacS are spacious along the C-1 to C-4 axis of the galactose moiety and are restricted along the C-2 to C-6 axis. The differences in affinity at the cytoplasmic and extracellular binding site ensure that the transport via LacS is highly asymmetrical for the two opposing directions of translocation.