Chemical modification of the mitochondrial ornithine/citrulline carrier by SH reagents: effects on the transport activity and transition from carrier to pore-like function

The transport activity of the purified and reconstituted ornithine/citrulline carrier from rat liver mitochondria was correlated to modification of its sulfhydryl groups by various reagents. Both the ornithine/ornithine (antiport) and the ornithine/H(+) (unidirectional) transport modes catalysed by the ornithine/citrulline carrier were inhibited by methanethiosulfonates, mercurial reagents, N-ethylmaleimide (NEM) and 5,5'-dithiobis(2-nitrobenzoate) (DTNB). The treatment of the ornithine/citrulline carrier with mercurial reagents, at concentrations above 5 microM, caused the induction of an additional (pore-like) transport mode, characterized by loss of substrate specificity and a transport activity higher than that of the unmodified carrier. The S-S forming reagent Cu(2+)-phenanthroline inhibited the transport catalysed by the carrier, indicating the presence of close sulfhydryl groups. The effect of consecutive addition of the various reagents revealed a peculiar aspect of the ornithine/citrulline carrier, i.e. the presence of three distinct populations of sulfhydryl groups. The first was responsible for the inhibition of the physiological transport modes by methanethiosulfonates, NEM and DTNB and low concentrations (<5 microM) of mercurials; the second population was responsible for the transition to the pore-like activity induced by higher concentrations (>5 microM) of mercurials; the third population was involved in S-S bridge formation.     

Pore-like and carrier-like properties of the mitochondrial aspartate/glutamate carrier after modification by SH-reagents: evidence for a performed channel as a structural requirement of carrier-mediated transport.

Upon modification of the reconstituted aspartate/glutamate carrier by mercury reagents the antiporter was converted into a unidirectional efflux carrier (Dierks, T., Salentin, A., Heberger, C. and Kr?mer, R. (1990) Biochim. Biophys. Acta 1028, 268). In addition to this basic change in the mechanism, the mercurials, reacting with exofacial cysteines, also affected the internal binding site of the carrier leading to an unmeasurable high Km and to a drastically reduced substrate specificity. The spectrum of efflux substrates comprised small anions from chloride to glutamate, but not cationic amino acids and ATP, hence resembling pore-like properties. However, in the efflux state important carrier properties were also observed. The activation energy (86 kJ/mol) was as high as for the antiport. Furthermore, efflux was inhibited by the presence of external substrate. This trans-inhibition strongly suggests that the external binding site of the carrier, prerequisite in the antiport mechanism, also is involved in conformational transitions during efflux function. However, antiport no longer is catalyzed after switching to the efflux state. Reversion of the induced efflux carrier to the antiport state was achieved using dithioerythritol, thereby further restoring substrate specificity and saturation kinetics. A model for antiport-efflux interconversion is presented suggesting that two reactive cysteines have to be modified in order to uncouple the inward and outward directed component of antiport. The pore-type characteristics of efflux are taken as evidence that a channel-like structure determines the selectivity of unidirectional transport. This intrinsic channel of the protein then is required for substrate translocation also during antiport function.     

Identification and purification of the carnitine carrier from rat liver mitochondria

The carnitine carrier from rat liver mitochondria, solubilized in Triton X-100 and partially purified on hydroxyapatite, was identified and completely purified by specific elution from celite in the presence of cardiolipin. On SDS-gel electrophoresis, the purified celite fraction consisted of a single band with an apparent Mr of 32,500. When reconstituted into liposomes the carnitine transport protein catalyzed an N-ethylmaleimide-sensitive carnitine/carnitine exchange. It was purified 970-fold with a recovery of 43% and a protein yield of 0.04% with respect to the mitochondrial extract. The properties of the reconstituted carrier, i.e., requirement for a countersubstrate, substrate specificity and inhibitor sensitivity, were similar to those of the carnitine transport system as characterized in intact mitochondria.     

Erythrocyte Lii-Nao countertransport system. Inhibition by N-ethylmaleimide probes for a conformational change of the transport system

Human erythrocytes were treated by a series of SH-reagents, including maleimides, iodo compounds, mercurials and oxidizing agents. Rates of Li efflux into Na-rich medium, Li leak and Lii-Nao countertransport were then determined. Of the 13 different reagents studied, only N-ethylmaleimide, iodoacetamide and iodoacetate inhibited selectively the countertransport activity. The effect of the various reagents indicates that the sensitive SH-groups of the countertransport system are not externally exposed. N-Ethylmaleimide was used to probe for changes elicited by substrate cations in Lii-Nao countertransport. In Na- and Li-free medium, inhibition of Lii-Nao countertransport by N-ethylmaleimide of 35% was reached within 2 s. In Na or Li medium, maximal inhibition was twice as great, but was attained much more slowly, within 10 min. Kinetic data and Hill plot analysis indicate the involvement of two classes of SH-groups: one expressed in the various media with and without substrate cations, and an additional one, which becomes specifically available to N-ethylmaleimide in the presence of external Na or Li. The affinity of Na to the site promoting inhibition by N-ethylmaleimide (apparent Km = 12 mM) is higher than the affinity of Na to its external countertransport site (apparent Km = 25 mM, as reported by Sarakadi, B., Alifimoff, J.K., Gunn, R.B. and Tosteson, D.C. (1978) J. Gen. Physiol. 72, 249-265). Reactivity of N-ethyl[14C]maleimide was not modified by the media tested. It is concluded that external Na and Li cause a conformational change in the protein(s) of the countertransport system in human erythrocytes.     

The mitochondrial tricarboxylate carrier of silver eel: chemical modification by sulfhydryl reagents

The tricarboxylate (or citrate) carrier was purified from eel liver mitochondria and functionally reconstituted into liposomes. Incubation of the proteoliposomes with various sulfhydryl reagents led to inhibition of the reconstituted citrate transport activity. Preincubation of the proteoliposomes with reversible SH reagents, such as mercurials and methanethiosulfonates, protected the eel liver tricarboxylate carrier against inactivation by the irreversible reagent N-(1-pyrenyl)maleimide (PM). Citrate and L-malate, two substrates of the tricarboxylate carrier, protected the protein against inactivation by sulfhydryl reagents and decreased the fluorescent PM bound to the purified protein. These results suggest that the eel liver tricarboxylate carrier requires a single population of free cysteine(s) in order to manifest catalytic activity. The reactive cysteine(s) is most probably located at or near the substrate binding site of the carrier protein.     

Citrate uniport by the mitochondrial tricarboxylate carrier: a basis for a new hypothesis for the transport mechanism

The tricarboxylate transport system located in the inner mitochondrial membrane was studied as an isolated protein reconstituted in proteoliposomes. The effects on the transport of citrate by various reagents, specific for different aminoacid residues, were analyzed. In the group of SH reagents, it was found that N-ethylmaleimide is an irreversible inhibitor of the citrate–citrate exchange, while HgCl₂ and the mercurial mersalyl cause a rapid unidirectional efflux of citrate from liposomes. It was demonstrated that NEM and mercurials act on different SH groups. Dithioerythritol is not able to reverse the effect of mersalyl unless another reagent, pyridoxalphosphate, is present. Pyridoxalphosphate itself, a reagent specific for NH₂ residues, is an effective inhibitor of citrate exchange transport, as measured in both influx and efflux, but it has no effect on the mercurial-induced efflux. The same behavior was observed with diethylpyrocarbonate, a reagent specific for histidine and tyrosine residues. Interestingly, a slow basic efflux of internal citrate, in the absence of countersubstrate, was observed in proteoliposomes. Because it is inhibited by the same reagents acting on the exchange process, it is deduced that it is catalyzed by the tricarboxylate carrier. The ability of the carrier to perform a uniport of the substrate suggests the presence of a single substrate binding site on the carrier protein. A preliminary kinetic approach indicates that such a transport model is compatible with this theory.     

The mitochondrial aspartate/glutamate and ADP/ATP carrier switch from obligate counterexchange to unidirectional transport after modification by SH-reagents

The influence of various SH-reagents on the aspartate/glutamate carrier was investigated in the reconstituted system. When liposomes carrying partially purified carrier protein were treated with 5,5'-dithiobis(2-nitrobenzoic acid) or N-ethylmaleimide, antiport activity was strongly reduced. Several mercury compounds exerted a dual effect. They completely blocked the antiport and, in addition, induced an efflux pathway for internal aspartate. The maximum rate of this unidirectional flux was comparable to the original antiport activity. Induction of efflux always was coupled to inhibition of antiport. Efflux was neither due to unspecific leakage of proteoliposomes nor to a possible contamination by porin, but depended on active carrier protein, as elucidated by the sensitivity to proteinases and protein-modifying reagents. Besides efflux of aspartate, HgCl2 and mersalyl also induced a slow efflux of ATP from liposomes carrying coreconstituted aspartate/glutamate and ADP/ATP carrier. The two efflux activities could be discriminated taking advantage of the differential effectiveness of several inhibitors and proteinases. Although basic carrier properties were changed by the applied mercurials (Dierks, T., Salentin, A. and Kr?mer, R. (1990) Biochim. Biophys. Acta 1028, 281), aspartate and ATP efflux could clearly be correlated with the aspartate/glutamate and the ADP/ATP carrier, respectively. When purifying these two translocators the respective efflux activity copurified with the antiporter, thus elucidating that the two different transport functions are mediated by the same protein. These results argue for a participation of the aspartate/glutamate and the ADP/ATP carrier in the generally observed increase of mitochondrial permeability after treatment with SH-reagents.     

Reconstitution into liposomes and functional characterization of the carnitine transporter from renal cell plasma membrane

The carnitine transporter was solubilized from rat renal apical plasma membrane (brush-border membrane) with C12E8 and reconstituted into liposomes by removing the detergent from mixed micelles by hydrophobic chromatography on Amberlite XAD-4. The reconstitution was optimised with respect to the protein concentration, the detergent/phospholipid ratio and the number of passages through a single Amberlite column. The reconstituted carnitine transporter catalysed a first-order antiport reaction (carnitine/carnitine or carnitine/substrate) stimulated by external, not internal, Na+, with a positive cooperativity. Na+ was co-transported with carnitine. Optimal activity was found between pH 5.5 and pH 6.0. The sulfhydryl reagents MTSES, MTSET and mercurials strongly inhibited the transport. Substrate analogues inhibited the transport; the most effective were acylcarnitines and betaine, followed by dimethylglicine, tetraethylammonium and arginine. Besides carnitine, only acylcarnitines and betaine were efficiently translocated. The Km for carnitine on the external and internal side of the transporter was 0.08 and 1.2 mM, respectively. The transporter is asymmetrical and it is unidirectionally inserted into the proteoliposomal membrane with an orientation corresponding to that of the native membrane. The reconstituted carnitine transporter corresponds, very probably, to the OCTN2 protein.     

Reconstitution into liposomes and functional characterization of the carnitine transporter from renal cell plasma membrane

The carnitine transporter was solubilized from rat renal apical plasma membrane (brush-border membrane) with C₁₂E₈ and reconstituted into liposomes by removing the detergent from mixed micelles by hydrophobic chromatography on Amberlite XAD-4. The reconstitution was optimised with respect to the protein concentration, the detergent/phospholipid ratio and the number of passages through a single Amberlite column. The reconstituted carnitine transporter catalysed a first-order antiport reaction (carnitine/carnitine or carnitine/substrate) stimulated by external, not internal, Na⁺, with a positive cooperativity. Na⁺ was co-transported with carnitine. Optimal activity was found between pH 5.5 and pH 6.0. The sulfhydryl reagents MTSES, MTSET and mercurials strongly inhibited the transport. Substrate analogues inhibited the transport; the most effective were acylcarnitines and betaine, followed by dimethylglicine, tetraethylammonium and arginine. Besides carnitine, only acylcarnitines and betaine were efficiently translocated. The Km for carnitine on the external and internal side of the transporter was 0.08 and 1.2 mM, respectively. The transporter is asymmetrical and it is unidirectionally inserted into the proteoliposomal membrane with an orientation corresponding to that of the native membrane. The reconstituted carnitine transporter corresponds, very probably, to the OCTN2 protein.     

Site-directed mutagenesis and chemical modification of the six native cysteine residues of the rat mitochondrial carnitine carrier: implications for the role of cysteine-136

By use of site-directed mutagenesis in combination with chemical modification of mutated proteins, the role of the six Cys residues in the transport function of the rat mitochondrial carnitine carrier (CAC) was studied. Several CAC mutants, in which one or more Cys residues had been replaced with Ser, were overexpressed in Escherichia coli, purified, and reconstituted in liposomes. The efficiency of incorporation into liposomes of the reconstituted proteins was lower for all constructs lacking Cys-23. Single, double, and quadruple replacement mutants showed V(max) comparable to that of the wild type. On the basis of the values of internal and external transport affinities (K(m)) for carnitine and of their comparison with those measured in mitochondria, the recombinant CAC is oriented unidirectionally in the liposomes, right side out compared to mitochondria. Substitution of Cys-136 with Ser caused a nearly complete loss of sensitivity of the CAC to N-ethylmaleimide, (2-aminoethyl)methanethiosulfonate hydrobromide (MTSES), and other hydrophilic SH reagents but not to the very hydrophobic N-phenylmaleimide. The wild-type CAC and the mutants containing Cys-136 showed substrate protection against NEM and MTSES inhibition and against NEM labeling. The data show that none of the native cysteines is essential for the transport mechanism and that Cys-136 is the major target of SH reagents and raise the hypothesis that Cys-136 is accessible from the external medium and is located at, or near, the substrate binding site. A model of the CAC is proposed in which the matrix hydrophilic loop containing Cys-136 protrudes into the membrane between the transmembrane domains of the protein.     

Conformation-dependent accessibility of Cys-136 and Cys-155 of the mitochondrial rat carnitine/acylcarnitine carrier to membrane-impermeable SH reagents

During substrate translocation mitochondrial carriers cycle between the cytoplasmic-state (c-state) with substrate-binding site open to the intermembrane space and matrix-state (m-state) with the binding site open to the mitochondrial matrix. Here, the accessibility of Cys-58, Cys-136 and Cys-155 of the rat mitochondrial carnitine/acylcarnitine carrier (CAC) to membrane-impermeable SH reagents was examined as a function of the conformational state. Reconstituted mutant CACs containing the combinations Cys-58/Cys-136, Cys-58/Cys-155, and Cys-136/Cys-155 transport carnitine with a ping-pong mechanism like the wild-type, since increasing substrate concentrations on one side of the membrane decreased the apparent affinity for the substrate on the other side. In view of this mechanism, the effect of SH reagents on the transport activity of mutant CACs was tested by varying the substrate concentration inside or outside the proteoliposomes, keeping the substrate concentration on the opposite side constant. The reagents MTSES, MTSEA and fluorescein-5-maleimide did not affect the carnitine/carnitine exchange activity of the mutant carrier with only Cys-58 in contrast to mutant carriers with Cys-58/Cys-136, Cys-58/Cys-155 or Cys-136/Cys-155. In the latter, the inhibitory effect of the reagents was more pronounced when the intraliposomal carnitine concentration was increased, favouring the m-state of the carrier, whereas the effect was less when the concentration of carnitine was increased in the external compartment of the proteoliposomes, favouring the c-state. Moreover, the mutant carrier proteins with Cys-136/Cys-155, Cys-58/Cys-136 or Cys-58/Cys-155 were more fluorescent when extracted from fluorescein-5-maleimide-treated proteoliposomes containing 15 mM internal carnitine as compared to 2.5 mM. These results are discussed in terms of conformational changes of the carrier occurring during substrate translocation.     

Effect of thiol-specific reagents on the activity of PaeI and PaeII restrictases from Pseudomonas aeruginosa

5,5'-dithiobis-2-nitrobenzoic acid, N-ethylmaleimide, and parachloromercuribenzoate have been demonstrated to inhibit the activity of restrictases PaeI and PaeII from Ps. aeruginosa bacterial cells. Restrictase PaeII was more sensitive to the action of thiol-specific reagents, as compared to PaeI. The minimal concentration of reagents for SH-groups that completely inhibited the activity of restrictases PaeI and PaeII was determined. The protective effect against the inhibitory action of 5,5'-dithiobis-2-nitrobenzoic acid on the activity of PaeII was observed after preincubation of these enzymes with phage lambda DNA and Mg2+ cations. It is suggested that restrictase PaeI and PaeII molecules contain SH-groups, essential for the enzymatic activity. They are believed responsible for restrictase binding with DNA substrate.     

Kinetic characterization of the reconstituted carnitine carrier from rat liver mitochondria

The carnitine carrier was purified from rat liver mitochondria and reconstituted into liposomes by removing the detergent from mixed micelles by Amberlite. Optimal transport activity was obtained with 1 microgram/ml and 12.5 mg/ml of protein and phospholipid concentration, respectively, with a Triton X-100/phospholipid ratio of 1.8 and with 16 passages through the same Amberlite column. The activity of the carrier was influenced by the phospholipid composition of the liposomes, being increased in the presence of cardiolipin and decreased in the presence of phosphatidylinositol. In the reconstituted system the incorporated carnitine carrier catalyzed a carnitine/carnitine exchange which followed a first-order reaction. The maximum transport rate of external [3H]carnitine was 1.7 mmol/min per g protein at 25 degrees C and was independent of the type of countersubstrate. The half-saturation constant (Km) for carnitine was 0.51 mM. The affinity of the carrier for acylcarnitines was in the microM range and depended on the carbon chain length. The activation energy of the carnitine/carnitine exchange was 133 kJ/mol. The carrier function was independent of the pH in the range between 6 and 8 and was inhibited at pH below 6.     

Solubilization and reconstitution of rat liver mitochondrial carnitine acylcarnitine translocase

Carnitine acylcarnitine translocase has been solubilized from inverted inner membrane vesicles of rat liver mitochondria with octyl glucoside and reconstituted into asolectin liposomes. For both processes, optimization of the detergent to phospholipid ratio was found crucial for obtaining reconstitutively active liposomes. Reassembly of the solubilized carrier into asolectin liposomes was achieved either by the octyl glucoside dilution method or by Extracti-Gel D column chromatography. The reconstituted system catalyzed exchange diffusion of carnitine, exhibited the expected inhibitor and temperature sensitivity, and discriminated between stereoisomers of octanoylcarnitine. The activity of unidirectional import of carnitine was low compared to exchange diffusion. It showed high-temperature sensitivity and a loss of activity on prolonged sonication that was regained by an appropriate freeze-thaw step subsequently.     

Conformation-dependent accessibility of Cys-136 and Cys-155 of the mitochondrial rat carnitine/acylcarnitine carrier to membrane-impermeable SH reagents

During substrate translocation mitochondrial carriers cycle between the cytoplasmic-state (c-state) with substrate-binding site open to the intermembrane space and matrix-state (m-state) with the binding site open to the mitochondrial matrix. Here, the accessibility of Cys-58, Cys-136 and Cys-155 of the rat mitochondrial carnitine/acylcarnitine carrier (CAC) to membrane-impermeable SH reagents was examined as a function of the conformational state. Reconstituted mutant CACs containing the combinations Cys-58/Cys-136, Cys-58/Cys-155, and Cys-136/Cys-155 transport carnitine with a ping-pong mechanism like the wild-type, since increasing substrate concentrations on one side of the membrane decreased the apparent affinity for the substrate on the other side. In view of this mechanism, the effect of SH reagents on the transport activity of mutant CACs was tested by varying the substrate concentration inside or outside the proteoliposomes, keeping the substrate concentration on the opposite side constant. The reagents MTSES, MTSEA and fluorescein-5-maleimide did not affect the carnitine/carnitine exchange activity of the mutant carrier with only Cys-58 in contrast to mutant carriers with Cys-58/Cys-136, Cys-58/Cys-155 or Cys-136/Cys-155. In the latter, the inhibitory effect of the reagents was more pronounced when the intraliposomal carnitine concentration was increased, favouring the m-state of the carrier, whereas the effect was less when the concentration of carnitine was increased in the external compartment of the proteoliposomes, favouring the c-state. Moreover, the mutant carrier proteins with Cys-136/Cys-155, Cys-58/Cys-136 or Cys-58/Cys-155 were more fluorescent when extracted from fluorescein-5-maleimide-treated proteoliposomes containing 15 mM internal carnitine as compared to 2.5 mM. These results are discussed in terms of conformational changes of the carrier occurring during substrate translocation.     

Purification of the mitochondrial carnitine carrier by chromatography on hydroxyapatite and celite

The carnitine carrier from rat liver mitochondria has been extracted with Triton X-100 ad partially purified by chromatography on hydroxyapatite and celite. During purification the activity of the carrier was monitored by functional reconstitution into liposomes. The purified fraction is 250-fold enriched with respect to the N-ethylmaleimide-sensitive carnitine/carnitine transport activity. The substrate specificity and the inhibitor sensitivity of carnitine transport in liposomes resemble closely those described for the transport of carnitine in mitochondria.     

Identification and purification of the tricarboxylate carrier from rat liver mitochondria

The tricarboxylate carrier from rat liver mitochondria was solubilized with Triton X-100 and purified by chromatography on hydroxyapatite and celite. SDS-gel electrophoresis of the purified fraction showed a single polypeptide band with an apparent Mr of 30,000. When reconstituted into liposomes, the tricarboxylate transport protein catalyzed a 1,2,3-benzenetricarboxylate-sensitive citrate/citrate exchange. We obtained a 1070-fold purification with respect to the mitochondrial extract, the recovery was 22% and the protein yield 0.02%. The properties of the reconstituted carrier, i.e., requirement for a counteranion, substrate specificity and inhibitor sensitivity, were similar to those of the tricarboxylate transport system as characterized in intact mitochondria.     

Relationships of Cysteine and Lysine residues with the substrate binding site of the mitochondrial ornithine/citrulline carrier: an inhibition kinetic approach combined with the analysis of the homology structural model

To gain insights in the relationships of specific amino acid residues with the active site of the mitochondrial ornithine/citrulline carrier, we studied the effect of specific protein modifying reagents on the transport catalysed by the carrier reconstituted into liposomes. It was found that, besides the sulfhydryl reagents NEM, MTSEA, p-hydroxymercuribenzoate, diamide also the lysine reagents PLP, DIDS, SITS, the carboxyl reagents WRK, EDC and the arginine reagent methylglyoxal inhibited the carrier. NEM, MTSEA and PLP inhibited the ornithine/citrulline carrier with a completely competitive type of mechanism. A 1:1 interaction of NEM with the carrier molecule has been demonstrated. The results are in agreement with the localization of one sulfhydryl and at least one amino group in the substrate binding site. On the basis of the interferences between SH reagents and PLP in the transport inhibition, it has been deduced that the distance between the SH and the NH(2) residues of the active site should be comparable to the distance between the gamma-NH(2) and COOH residues of the ornithine molecule. The structural model of the ornithine/citrulline carrier has been obtained by homology modelling using as template the ADP/ATP carrier structure. The combined analysis of the experimental data and the structural model allows to deduce that Cys-132 is located in the substrate binding site, flanked by at least one Lys residue.     

Functional properties of purified and reconstituted mitochondrial metabolite carriers

Eight mitochondrial carrier proteins were solubilized and purified in the authors' laboratories using variations of a general procedure based on hydroxyapatite and Celite chromatography. The molecular mass of all the carriers ranges between 28 and 34 kDa on SDS-PAGE. The purified carrier proteins were reconstituted into liposomes mainly by using a method of detergent removal by hydrophobic chromatography on polystyrene beads. The various carriers were identified in the reconstituted state by their kinetic properties. A complete set of basic kinetic data including substrate specificity, affinity, interaction with inhibitors, and activation energy was obtained. These data closely resemble those of intact mitochondria, as far as they are available from the intact organelle. Mainly on the basis of kinetic data, the asymmetric orientation of most of the reconstituted carrier proteins were established. Several of their functional properties are significantly affected by the type of phospholipids used for reconstitution. All carriers which have been investigated in proteoliposomes function according to a simultaneous (sequential) mechanism of transport; i.e., a ternary complex, made up of two substrates and the carrier protein, is involved in the catalytic cycle. The only exception was the carnitine carrier, where a ping-pong mechanism of transport was found. By reaction of particular cysteine residues with mercurial reagents, several carriers could be reversibly converted to a functional state different from the various physiological transport modes. This unphysiological transport mode is characterized by a combination of channel-type and carrier-type properties.     

Water permeability in human erythrocytes: identification of membrane proteins involved in water transport

The water permeability of human erythrocytes has been monitored by nuclear magnetic resonance (NMR) before and after treatment of the cells with various sulfhydryl reagents. Preincubation of the cells with N-ethylmaleimide (NEM), a non-inhibitory sulfhydryl reagent, results in a faster and more sensitive inhibition of water exchange by mercurials. The inhibition of water exchange by p-chloromercuribenzene sulfonate (PCMBS) was maximal at a binding of approximately 10 nmol PCMBS per mg protein when non-specific sulfhydryl groups are blocked by NEM. Inhibition by PCMBS has been correlated with the binding of 203Hg to erythrocyte membrane proteins. A significant binding of label to band 3 and the polypeptides in band 4.5 occurs, with approximately 1 mol of mercurial bound per mol of protein. Inhibition of water transport by sulfhydryl reagents does not induce major morphological changes in the cells as assessed by freeze-fracture and scanning electron microscopy.