The lactose permease of Escherichia coli: evidence in favor of a dimer

The monomeric state of the proton-coupled folate transporter represents the functional unit in the plasma membrane

Folic acid is an essential vitamin required for de novo biosynthesis of nucleotides and amino acids. The proton-coupled folate transporter (PCFT; SLC46A1) has been identified as the major contributor for intestinal folate uptake. It is also involved in folate transport across the blood-brain barrier and into solid tumors. PCFT belongs to the major facilitator superfamily. Major facilitator superfamily members can exist in either monomeric or homo-oligomeric form. Here, we utilized blue native polyacrylamide gel electrophoresis (BN/PAGE) and crosslinking with bi-functional chemicals to investigate the quaternary structure of human PCFT after heterologous expression in Xenopus laevis oocytes and CHO cells. PCFT was expressed in the plasma membrane in both expression systems. The functionality of the utilized PCFT construct was confirmed in oocytes by folic acid induced currents at acidic pH. For both the oocyte and CHO expression system [(3)H]folic acid uptake studies indicated that PCFT was functional. To analyze the oligomeric state of PCFT in the plasma membrane, plasma membranes were isolated by polymerization with colloidal silica and polyacrylic acid and subsequent centrifugation. The digitonin-solubilized non-denatured PCFT migrated during BN/PAGE as a monomer, as judged by comparison with a membrane protein (5-HT(3A) receptor) of known pentameric assembly that was used to create a molecular sizing ladder. The chemical crosslinkers glutaraldehyde and dimethyl adipimidate were not able to covalently link potential higher order PCFT structures to form oligomers that were stable following SDS treatment. Together, our results demonstrate that plasma-membrane PCFT functions as a monomeric protein.

Surface-exposed positions in the transmembrane helices of the lactose permease of Escherichia coli determined by intermolecular thiol cross-linking

Intermolecular thiol cross-linking was used to determine surface-exposed positions in 250 lactose permease mutants containing single-Cys replacements in each transmembrane helix. Significant cross-linking of monomers to produce homodimers is observed in nine mutants with a 5-A-long cross-linking agent containing bis-methane thiosulfonate reactive groups [position 78 (helix III); positions 185, 186, and 187 (helix VI); positions 263, 275, and 278 (helix VIII); and positions 308 (helix IX) and 398 (helix XII)]. The results are consistent with a current helix-packing model of the permease. Seven of the nine mutants that exhibit intermolecular cross-linking are located at or near the cytoplasmic ends of transmembrane helices; two are near periplasmic ends. The results suggest that only those Cys replacements accessible from the aqueous phase and not from the hydrophobic core of the membrane are susceptible to cross-linking because of the much higher reactivity of the thiolate anion relative to the thiol. Single-Cys mutants at positions 278 (helix VIII) and 398 (helix XII), which are located in opposite sides of the 12-helix bundle, exhibit similar rates of cross-linking with sigmoid kinetics. Furthermore, cross-linking is markedly decreased at 0 degrees C, suggesting that lateral diffusion of the permease within the plane of the membrane is important for intermolecular cross-linking. The findings confirm previous observations indicating that intermolecular cross-linking is a stochastic process resulting from random collisions and support a number of other lines of evidence that lactose permease is a monomer.

Complete removal and exchange of sodium dodecyl sulfate bound to soluble and membrane proteins and restoration of their activities, using ceramic hydroxyapatite chromatography

Up to now, removal of sodium dodecyl sulfate (SDS) from proteins in terms of restoration of their activity was an unsolved problem. A general procedure using ceramic hydroxyapatite (HAP) chromatography was developed for the complete removal of SDS bound to soluble or membrane proteins. This procedure involves (i) the binding of the SDS-protein complexes onto the ceramic hydroxyapatite column, (ii) extensive washing of bound proteins with phosphate buffer containing a mild detergent to exchange SDS, (iii) elution of the retained protein by increasing the phosphate concentration. Using this approach, complete exchange of [35S]SDS into a nonionic detergent such as dodecyl maltoside was achieved with a 90-100% protein recovery. The efficiency of protein-bound SDS removal is very likely due to the combined effect of phosphate ions and the hydrophobic tail of nonionic detergent: acting together, they are able to displace SDS molecules from their protein-binding sites. The advantages of this HAP-mediated SDS removal method include high efficiency, rapidity, simplicity and general applicability to a wide variety of detergents and soluble or membrane proteins. Of utmost importance, SDS-treated P-glycoprotein, glutamate dehydrogenase, and lysozyme fully recovered their enzymatic activities after HAP chromatography, including lysozyme electroeluted from SDS-polyacrylamide gel electrophoresis. This demonstrates that reactivation of SDS-treated protein can be achieved, provided that SDS is completely removed under mild conditions.

Properties of permease dimer, a fusion protein containing two lactose permease molecules from Escherichia coli

An engineered fusion protein containing two tandem lactose permease molecules (permease dimer) exhibits high transport activity and is used to test the phenomenon of negative dominance. Introduction of the mutation Glu-325-->Cys into either the first or the second half of the dimer results in a 50% decrease in activity, whereas introduction of the mutation into both halves of the dimer abolishes transport. Lactose transport by permease dimer is completely inactivated by N-ethylmaleimide; however, 40-45% activity is retained after N-ethylmaleimide treatment when either the first or the second half of the dimer is replaced with a mutant devoid of cysteine residues. The observations demonstrate that both halves of the fusion protein are equally active and suggest that each half may function independently. To test the possibility that oligomerization between dimers might account for the findings, a permease dimer was constructed that contains two different deletion mutants that complement functionally when expressed as untethered molecules. Because this construct does not catalyze lactose transport to any extent whatsoever, it is unlikely that the two halves of the dimer interact or that there is an oligomeric interaction between dimers. The approach is consistent with the contention that the functional unit of lactose permease is a monomer.

Functional complementation of internal deletion mutants in the lactose permease of Escherichia coli

Using the lactose permease of Escherichia coli, a well-characterized membrane protein with 12 transmembrane domains, we demonstrated that certain paired in-frame deletion constructs complement each other functionally. Although cells expressing the deletion mutants individually are unable to catalyze active lactose accumulation, cells simultaneously expressing specific pairs of deletions catalyze transport up to 60% as do cells expressing wild-type permease. Moreover, complementation clearly does not occur at the level of DNA but probably occurs at the protein level. Remarkably, functional complementation is observed only with pairs of permease molecules containing large deletions and is not observed with missense mutations or point deletions. Although the mechanism of functional complementation is obscure, the findings indicate that certain pairs of permease molecules containing specific internal deletions can interact to form a functional complex in the same way phenomenologically as do independently expressed polypeptides corresponding to different N- and C-terminal portions of the permease.

The use of gel chromatography for the determination of sizes and relative molecular masses of proteins. Interpretation of calibration curves in terms of gel-pore-size distribution

The separation of proteins by gel-exclusion chromatography has been explained in terms of partitioning of the macromolecules within the gel by a distribution of pores of various radii. The assumption that the distribution of pore sizes is Gaussian has led to the prediction of a linear relationship between the molecular Stokes radius (RS) of the protein and the function erf-1 (1-KD), where KD is the partition coefficient [Ackers (1967) J. Biol. Chem. 242, 3237-3238]. Since careful calibrations of classical (agarose and dextran) gels and h.p.l.c. gels have shown that such a linear relationship is not verified experimentally over a wide range of native protein sizes, we have reinvestigated the model of Ackers (above reference). We show that Ackers' (above reference) derivation is not valid except for a particular Gaussian distribution of pore sizes centred at the origin. Relaxation of this restriction to allow for other types of Gaussian distributions cannot account for the non-linear calibration curves that we have obtained. Instead we show that the pore-size distribution can be calculated from the experimentally determined function KD = f(RS) and that this distribution is bimodal (non-Gaussian). One distribution is centred below 2 nm, whereas the mean value of the second one is around 6-8 nm. The minimum in this bimodal distribution corresponds, for some gels, to a region of poor resolution, which needs to be appreciated for the proper use of gel chromatography in the determination of molecular size.

The use of high-performance liquid chromatography for the determination of size and molecular weight of proteins: a caution and a list of membrane proteins suitable as standards

We propose a list of 15 water-soluble globular proteins and 13 forms of detergent-soluble membrane proteins of known Stokes radii for the calibration of high-performance liquid chromatography columns. It is shown that it is advisable to use different sets of standards for these two types of proteins as the detergent-solubilized membrane proteins may behave differently, being excluded or retarded, depending upon the gel support. A smooth, although nonlinear, relationship between Stokes radii and erf-1(1--KD) is observed while a large scatter of points exists if the calibration is expressed as the molecular weight as a function of KD.

[Protein-protein interactions in membranes: concepts and the example of calcium ATPase from the sarcoplasmic reticulum]

The organization of polypeptide chains in the membrane has attracted widespread interest. This is particularly true for transport proteins: indeed, the existence of a quaternary structure may obviously have important implications for the mechanism of solute transport through the membrane. The problem arises from the fact that it is extremely difficult to demonstrate unambiguously that a protein is truly oligomeric in the membrane. In this paper various techniques are considered, either direct methods of investigation such as X-ray or neutron scattering, ESR, and radiation inactivation, or indirect methods (primarily the solubilization of the protein by non-denaturing detergents). In very few cases the existence of a 'structural' oligomer has been demonstrated. However, the question remains whether the oligomer also has a functional role, i.e., is it directly necessary for example to form a hydrophilic pathway for an ion, or indirectly to stabilize the enzyme structure or to allow a control to take place at a certain defined step of the transport cycle?.

[Secondary active transport]

Secondary active transport is defined as the transport of a solute in the direction of its increasing electrochemical potential coupled to the facilitated diffusion of a second solute (usually an ion) in the direction of its decreasing electrochemical potential. The coupling agents are membrane proteins (carriers), each of which catalyzes simultaneously the facilitated diffusion of the driving ion and the active transport of a given solute. The review starts with some considerations on the energetics followed by a presentation of the kinetics of secondary active transport. Examples of information which may be gained by such studies are discussed. In the second part, some examples of secondary transport are given; we also describe the characteristics of the corresponding carriers. The various transport systems presented are: the D-glucose/Na+ symport in brush-border membranes, the lactose/H+ symport in E. coli, the Na+/H+ antiport, the different transport systems in the inner mitochondrial membrane.