Mutational analysis of the role of Glu309 in the sarcoplasmic reticulum Ca(2+)-ATPase of frog skeletal muscle

Deciphering the Mechanism of Inhibition of SERCA1a by Sarcolipin Using Molecular Simulations

SERCA1a is an ATPase calcium pump that transports Ca²⁺ from the cytoplasm to the sarco/endoplasmic reticulum lumen. Sarcolipin (SLN), a transmembrane peptide, regulates the activity of SERCA1a by decreasing its Ca²⁺ transport rate, but its mechanism of action is still not well-understood. To decipher this mechanism, we have performed normal mode analysis in the all-atom model, with the SERCA1a-SLN complex, or the isolated SERCA1a, embedded in an explicit membrane. The comparison of the results allowed us to provide an explanation at the atomic level for the action of SLN that is in good agreement with experimental observations. In our analyses, the presence of SLN locally perturbs the TM6 transmembrane helix and as a consequence modifies the position of D800, one of the key metal-chelating residues. Additionally, it reduces the flexibility of the gating residues, V304, and E309 in TM4, at the entrance of the Ca²⁺ binding sites, which would decrease the affinity for Ca²⁺. Unexpectedly, SLN has also an effect on the ATP binding site more than 35 Å away, due to the straightening of TM5, a long helix considered as the spine of the protein. The straightening of TM5 modifies the structure of the P-N linker that sits above it, and which comprises the ³⁵¹DKTG³⁵⁴ conserved motif, resulting in an increase of the distance between ATP and the phosphorylation site. As a consequence, the turn-over rate could be affected. All this gives SERCA1a the propensity to go toward a Ca²⁺ low-affinity E2-like state in the presence of SLN and toward a Ca²⁺ high-affinity E1-like state in the absence of SLN. In addition to a general mechanism of inhibition of SERCA1a regulatory peptides, this study also provides an insight into the conformational transition between the E2 and E1 states.

Use of glycerol-containing media to study the intrinsic fluorescence properties of detergent-solubilized native or expressed SERCA1a

Rapid irreversible inactivation of Ca (2+)-free states of detergent-solubilized SERCA1a (sarco-endoplasmic reticulum calcium ATPase 1a) has so far prevented the use of Trp fluorescence for functional characterization of this ATPase after its solubilization in various detergents. Here we show that using 20-40% glycerol for protection makes this fluorescence characterization possible. Most of the ligand-induced Trp fluorescence changes previously demonstrated to occur for SERCA1a embedded in native sarcoplasmic reticulum membranes were observed in the combined presence of glycerol and detergent, although the results greatly depended on the detergent used, namely, octaethylene glycol mono- n-dodecyl ether (C 12E 8) or dodecyl maltoside (DDM). In particular, at pH 6, we found a C 12E 8-dependent unexpectedly huge reduction in SERCA1a affinity for Ca (2+). We suggest that a major reason for the different effects of the two detergents is that high concentrations of C 12E 8, but not of DDM, slow down the E2 to E1 transition in solubilized and delipidated SERCA1a. Independently of the characterization of the specific effects of various detergents on SR vesicles, our results open the way to functional characterization by Trp fluorescence of heterologously expressed and purified mutants of SERCA1a in the presence of detergent, without their preliminary reconstitution into liposomes. As an example, we used the E309Q mutant to demonstrate our previous suspicion that Ca (2+) binding to Site I of SERCA1a in fact slightly reduces Trp fluorescence, and consequently that the rise in this fluorescence generally observed when two Ca (2+) ions bind to WT SERCA1a mainly reflects Ca (2+) binding at Site II of SERCA1a.

The dimeric form of Ca2+-ATPase is involved in Ca2+ transport in the sarcoplasmic reticulum

To identify the functional unit of Ca2+-ATPase in the sarcoplasmic reticulum, we assessed Ca2+-transport activities occurring on sarcoplasmic reticulum membranes with different combinations of active and inactive Ca2+-ATPase molecules. We prepared heterodimers, consisting of a native Ca2+-ATPase molecule and a Ca2+-ATPase molecule inactivated by FITC labelling, by fusing vesicles loaded with each type of Ca2+-ATPase. The heterodimers exhibited neither Ca2+ transport nor ATP hydrolysis, suggesting that Ca2+ transport by the Ca2+-ATPase requires an interaction between functional Ca2+-ATPase monomers. This finding implies that the functional unit of the Ca2+-ATPase is a dimer.

Side-chain protonation and mobility in the sarcoplasmic reticulum Ca2+-ATPase: implications for proton countertransport and Ca2+ release

Protonation of acidic residues in the sarcoplasmic reticulum Ca(2+)-ATPase (SERCA 1a) was studied by multiconformation continuum electrostatic calculations in the Ca(2+)-bound state Ca(2)E1, in the Ca(2+)-free state E2(TG) with bound thapsigargin, and in the E2P (ADP-insensitive phosphoenzyme) analog state with MgF(4)(2-) E2(TG+MgF(4)(2-)). Around physiological pH, all acidic Ca(2+) ligands (Glu(309), Glu(771), Asp(800), and Glu(908)) were unprotonated in Ca(2)E1; in E2(TG) and E2(TG+MgF(4)(2-)) Glu(771), Asp(800), and Glu(908) were protonated. Glu(771) and Glu(908) had calculated pK(a) values larger than 14 in E2(TG) and E2(TG+MgF(4)(2-)), whereas Asp(800) titrated with calculated pK(a) values near 7.5. Glu(309) had very different pK(a) values in the Ca(2+)-free states: 8.4 in E2(TG+MgF(4)(2-)) and 4.7 in E2(TG) because of a different local backbone conformation. This indicates that Glu(309) can switch between a high and a low pK(a) mode, depending on the local backbone conformation. Protonated Glu(309) occupied predominantly two main, very differently orientated side-chain conformations in E2(TG+MgF(4)(2-)): one oriented inward toward the other Ca(2+) ligands and one oriented outward toward a protein channel that seems to be in contact with the cytoplasm. Upon deprotonation, Glu(309) adopted completely the outwardly orientated side-chain conformation. The contact of Glu(309) with the cytoplasm in E2(TG+MgF(4)(2-)) makes this residue unlikely to bind lumenal protons. Instead it might serve as a proton shuttle between Ca(2+)-binding site I and the cytoplasm. Glu(771), Asp(800), and Glu(908) are proposed to take part in proton countertransport.

SERCA2a in heart failure: role and therapeutic prospects

Ca(2+) is a key molecule controlling several cellular processes, from fertilization to cell death, in all cell types. In excitable and contracting cells, such as cardiac myocytes, Ca(2+) controls muscle contractility. The spatial and temporal segregation of Ca(2+) concentrations are central to maintain its concentration gradients across the cells and the cellular compartments for proper function. SERCA2a is a cornerstone molecule for maintaining a balanced concentration of Ca(2+) during the cardiac cycle, since it controls the transport of Ca(2+) to the sarcoplasmic reticulum (SR) during relaxation. Alterations of the activity of this pump have been widely investigated, emphasizing its central role in the control of Ca(2+) homeostasis and consequently in the pathogenesis of the contractile defect seen with heart failure. This review focuses on the molecular characteristics of the pump, its role during the cardiac cycle and the prospects derived from the manipulation of SERCA2a for heart failure treatment.

Cooperative setting for long-range linkage of Ca(2+) binding and ATP synthesis in the Ca(2+) ATPase

High-affinity and cooperative binding of two Ca(2+) per ATPase (SERCA) occurs within the membrane-bound region of the enzyme. Direct measurements of binding at various Ca(2+) concentrations demonstrate that site-directed mutations within this region interfere selectively with Ca(2+) occupancy of either one or both binding sites and with the cooperative character of the binding isotherms. A transition associated with high affinity and cooperative binding of the second Ca(2+) and the engagement of N796 and E309 are both required to form a phosphoenzyme intermediate with ATP in the forward direction of the cycle and also to form ATP from phosphoenzyme intermediate and ADP in the reverse direction of the cycle. This transition, defined by equilibrium and kinetic characterization of the partial reactions of the enzyme cycle, extends from transmembrane helices to the catalytic site through a long-range linkage and is the mechanistic device for interconversion of binding and phosphorylation potentials.

Involvement of the cytoplasmic loop L6-7 in the entry mechanism for transport of Ca2+ through the sarcoplasmic reticulum Ca2+-ATPase

We previously found that mutants of conserved aspartate residues of sarcoplasmic reticulum Ca(2+)-ATPase in the cytosolic loop, connecting transmembrane segments M6 and M7 (L6-7 loop), exhibit a strongly reduced sensitivity toward Ca(2+) activation of the transport process. In this study, yeast membranes, expressing wild type and mutant Ca(2+)-ATPases, were reacted with Cr small middle dotATP and tested for their ability to occlude (45)Ca(2+) by HPLC analysis, after cation resin and C(12)E(8) treatment. We found that the D813A/D818A mutant that displays markedly low calcium affinity was capable of occluding Ca(2+) to the same extent as wild type ATPase. Using NMR and mass spectrometry we have analyzed the conformational properties of the synthetic L6-7 loop and demonstrated the formation of specific 1:1 cation complexes of the peptide with calcium and lanthanum. All three aspartate Asp(813)/Asp(815)/Asp(818) were required to coordinate the trivalent lanthanide ion. Overall these observations suggest a dual function of the loop: in addition to mediating contact between the intramembranous Ca(2+)-binding sites and the cytosolic phosphorylation site (Zhang, Z., Lewis, D., Sumbilla, C., Inesi G., and Toyoshima, C. (2001) J. Biol. Chem. 276, 15232-15239), the L6-7 loop, in a preceding step, participates in the formation of an entrance port, before subsequent high affinity binding of Ca(2+) inside the membrane.

Direct demonstration of Ca2+ binding defects in sarco-endoplasmic reticulum Ca2+ ATPase mutants overexpressed in COS-1 cells transfected with adenovirus vectors

Single mutations of specific amino acids within the membrane-bound region of the sarco-endoplasmic reticulum Ca2+ (SERCA)-1 ATPase interfere with Ca2+ inhibition of ATPase phosphorylation by Pi (1), suggesting that these residues may be involved in complexation of two Ca2+ that are known to bind to the enzyme. However, direct measurements of Ca2+ binding in the absence of ATP have been limited by the low quantities of available mutant protein. We have improved the transfection efficiency by means of recombinant adenovirus vectors, yielding sufficient expression of wild type and mutant SERCA-1 ATPase for measurements of Ca2+ binding to the microsomal fraction of the transfected cells. We find that in the presence of 20 microM Ca2+ and in the absence of ATP, the Glu771 --> Gln, Thr799 --> Ala, Asp800 --> Asn, and Glu908 --> Ala mutants exhibit negligible binding, indicating that the oxygen functions of Glu771, Thr799, Asp800, and Glu908 are involved in interactions whose single disruption causes major changes in the highly cooperative "duplex" binding. Total loss of Ca2+ binding is accompanied by loss of Ca2+ inhibition of the Pi reaction. We also find that, at pH 7.0, the Glu309 --> Gln and the Asn796 --> Ala mutants bind approximately half as much Ca2+ as the wild type ATPase and do not interfere with Ca2+ inhibition of the Pi reaction. At pH 6.2, the Glu309 --> Gln mutant does not bind any Ca2+, and its phosphorylation by Pi is not inhibited by Ca2+. On the contrary, the Asn796 --> Ala mutant retains the behavior displayed at pH 7.0. This suggests that in the Glu309 --> Gln mutant, ionization of acidic functions in other amino acids (e.g. Glu771 and Asp800) occurs as the pH is shifted, thereby rendering Ca2+ binding possible. In the Asn796 --> Ala mutant, on the other hand, the Glu309 carboxylic function allows binding of inhibitory Ca2+ even at pH 6.2. In all cases mutational interference with the inhibition of the Pi reaction by Ca2+ can be overcome by raising the Ca2+ concentration to the mM range, consistent with a general effect of mutations on the affinity of the ATPase for Ca2+.

Scanning mutagenesis reveals a similar pattern of mutation sensitivity in transmembrane sequences M4, M5, and M6, but not in M8, of the Ca2+-ATPase of sarcoplasmic reticulum (SERCA1a)

Scanning mutagenesis was performed on all amino acids in transmembrane sequences M5, M6, and M8, which, together with M4, make up the Ca2+ binding domain of the Ca2+-ATPase of sarcoplasmic reticulum (SERCA1a). When these transmembrane sequences were displayed on a helical net, examination of the effects of 101 novel point mutations and 95 prior mutations carried out on 92 transmembrane amino acids revealed "patches" of sensitivity to mutation in M4, M5, and M6 but not in M8. The patches of mutation-sensitive residues spanned 6 of the 7 tiers of the helical net and covered about 240 degrees at their widest point in tiers 3 or 4 and 140 degrees in tiers 2 and 5. A contiguous column of mutation-insensitive hydrophobic amino acids was found in M4 and M6 and in tiers 4 to 7 of M5. A six-residue motif, (E/D)GLPA(T/V) in tiers 3 and 4 of M4 and M6 with Ca2+-binding residues Glu309 and Asp800 as the first residue, was highlighted by mutation sensitivity. Elements of the motif could also be discerned in M5, but reading in the C-terminal to N-terminal direction. Mutation sensitivity in tier 5 of M4 mirrored mutation sensitivity of tier 5 in M6, although the amino acid sequences were not similar. The motif or its counterpart was found in a region in M4, M5, and M6 that is made up of tiny or small amino acids but is bounded by tiers with a larger percentage of bulky amino acids. Tiers 3, 4, and 5 of M4, M5, and M6 contain Ca2+ binding and affinity mutations, E1P to E2P block mutations and E2P dephosphorylation mutations, indicating an important role for these central tiers in Ca2+ binding and in the conformational changes that accompany Ca2+ translocation. Analysis of M8 revealed only a single mutation-sensitive residue, the Ca2+-binding amino acid, Glu908. This residue and a mutation-insensitive residue, Ala912, were the only vestiges of the motif that was found in M4 and M6. Additional mutations to Glu908 provided further evidence for its role in Ca2+ binding. Since mutation of M8 failed to identify residues involved in blocking conformational changes or altering Ca2+ affinity, it is apparent that M8 plays a peripheral role in Ca2+ binding and translocation in comparison with M4, M5, and M6.

Short and long range functions of amino acids in the transmembrane region of the sarcoplasmic reticulum ATPase. A mutational study

Mutational analysis of several amino acids in the transmembrane region of the sarcoplasmic reticulum ATPase was performed by expressing wild type ATPase and 32 site-directed mutants in COS-1 cells followed by functional characterization of the microsomal fraction. Four different phenotype characteristics were observed in the mutants: (a) functions similar to those sustained by the wild type ATPase; (b) Ca2+ transport inhibited to a greater extent than ATPase hydrolytic activity; (c) inhibition of transport and hydrolytic activity in the presence of high levels of phosphorylated enzyme intermediate; and (d) total inhibition of ATP utilization by the enzyme while retaining the ability to form phosphoenzyme by utilization of P(i). Analysis of experimental observations and molecular models revealed short and long range functions of several amino acids within the transmembrane region. Short range functions include: (a) direct involvement of five amino acids in Ca2+ binding within a channel formed by clustered transmembrane helices M4, M5, M6, and M8; (b) roles of several amino acids in structural stabilization of the helical cluster for optimal channel function; and (c) a specific role of Lys297 in sealing the distal end of the channel, suggesting that the M4 helix rotates to allow vectorial flux of Ca2+ upon enzyme phosphorylation. Long range functions are related to the influence of several transmembrane amino acids on phosphorylation reactions with ATP or P(i), transmitted to the extramembranous region of the ATPase in the presence or in the absence of Ca2+.

Dissection of the functional domains of the sarcoplasmic reticulum Ca(2+)-ATPase by site-directed mutagenesis

The results of site-directed mutagenesis studies of the sarcoplasmic reticulum Ca(2+)-ATPase are reviewed. More than 250 different point mutants have been expressed in cell culture and analysed by a panel of functional assays. Thereby, 40-50 important amino acid residues have been pinpointed, and the mutants have been assigned to functional classes: the Ca(2+)-affinity mutants, the phosphorylation-negative mutants, the ATP-affinity mutants, the E1P mutants, the E2P mutants, and the uncoupled mutants. Moreover, regions important to the specific inhibition by thapsigargin have been identified by analysis of Ca(2+)-ATPase/Na+,K(+)-ATPase chimeric constructs.

Glutamic acid 327 in the sheep alpha 1 isoform of Na+,K(+)-ATPase is a pivotal residue for cation-induced conformational changes

The cation binding characteristics of the mutant E327A formed in the sheep alpha 1 isoform of the Na+,K(+)-ATPase were examined using [3H]ouabain binding as a function of monovalent cation concentrations. Equilibrium competition binding assays in the presence of Mg2+, inorganic phosphate and various amounts of unlabelled ouabain indicated that both wild-type sheep alpha 1 protein and the E327A mutant expressed in 3T3 cells had similar affinities for ouabain (KD = 1.53 and 1.31 nM respectively). Sodium inhibition of ouabain binding appeared competitive in both enzymes. However, binding of three Na+ ions was required to explain the steep character of the Na+ inhibition curve for the wild-type Na+,K(+)-ATPase (Ki = 12.8 +/- 1.6 mM), whereas the binding of two Na+ ions was detected for the mutant E327A (Ki = 19.2 +/- 2.5 mM). Potassium binding of [3H]ouabain binding displayed a partially competitive nature with Hill coefficients of 2 for both wild-type sheep alpha 1 (Ki = 0.743 +/- 0.044 mM) and E327A (Ki = 0.875 +/- 0.067 mM). At concentrations of K+ above 10 mM, the sheep alpha 1 competition curve levelled off whereas the inhibition curve for E327A displayed a stimulation in ouabain binding. This stimulation in [3H]ouabain binding also occurred with Rb+, Cs+ and Li+, but was never observed with choline or Na+, suggesting that this effect was not due to ionic strength. From these [3H]ouabain-binding studies, it is obvious that the mutant enzyme E327A in the presence of Mg2+, Pi and ouabain, interacts with monovalent cations in a unique fashion. One interpretation of these data is that the glutamic acid residue at position 327 is involved in a conformational transition induced by the binding of monovalent cations to the Na+,K+-ATPase and that this transition is inhibited by the mutation of E327A.

Structure-function relationships of cation translocation by Ca(2+)- and Na+, K(+)-ATPases studied by site-directed mutagenesis

Site-directed mutagenesis studies of the sarcoplasmic reticulum Ca(2+)-ATPase have pinpointed five amino acid residues that are essential to Ca2+ occlusion, and these residues have been assigned to different parts of a Ca2+ binding pocket with channel-like structure. Three of the homologous Na+, K(+)-ATPase residues have been shown to be important for binding of cytoplasmic Na+ at transport sites. In addition, three of the above mentioned Ca(2+)-ATPase residues appear to participate in the countertransport of H+, and two of the Na+, K(+)-ATPase residues to participate in the countertransport of K+. Residues involved in energy transducing conformational changes have also been identified by mutagenesis. In the Ca(2+)-ATPase, ATP hydrolysis is uncoupled from Ca2+ transport following mutation of a tyrosine residue located at the top of transmembrane segment M5. This tyrosine, present also in the Na+, K(+)-ATPase, may play a critical role in closing the gate to a transmembrane channel.

Functional consequences of alterations to amino acids at the M5S5 boundary of the Ca(2+)-ATPase of sarcoplasmic reticulum. Mutation Tyr763-->Gly uncouples ATP hydrolysis from Ca2+ transport

The roles of the hydrophobic side chains of residues Phe760, Ile761, Tyr763, Leu764, and Ile765 located at the M5S5 boundary of the Ca(2+)-ATPase of sarcoplasmic reticulum were analyzed by site-directed mutagenesis. Substitution of Tyr763 with glycine resulted in a new phenotypic variant of the Ca(2+)-ATPase that catalyzed a high rate of Ca(2+)-activated ATP hydrolysis without net accumulation of Ca2+ in the microsomal vesicles. The ATPase activity of the Tyr763-->Gly mutant displayed characteristics similar to the ATPase activity of the wild-type enzyme measured in the presence of calcium ionophore, and the mutant was able to form the ADP-insensitive phosphoenzyme intermediate. Mutants Phe760-->Gly, Ile761-->Gly, Leu764-->Gly, and Ile765-->Gly were able to accumulate Ca2+. In mutants Leu764-->Gly and Ile765-->Gly, the turnover rate was low due to inhibition of dephosphorylation of the ADP-insensitive phosphoenzyme intermediate. On the other hand, mutant Leu764-->Lys dephosphorylated rapidly. Mutants Phe760-->Gly and Leu764-->Lys displayed apparent Ca2+ affinities that were reduced two and three orders of magnitude, respectively, relative to that of the wild-type.

Expression of the sarcoplasmic reticulum Ca(2+)-ATPase in yeast

We describe here an easy system for the production of mg amounts of the rabbit Ca(2+)-ATPase SERCA 1a in the yeast S. cerevisiae. The protein is present in several membranes, including the plasma membrane of the yeast, in a native conformation. It can be purified by immunoprecipitation and can be phosphorylated from ATP in a Ca(2+)-dependent manner. Using a temperature-sensitive secretion mutant strain, the fully active protein can also be obtained in secretory vesicles.

Mutational analysis of Glu771 of the Ca(2+)-ATPase of sarcoplasmic reticulum. Effect of positive charge on dephosphorylation

The glutamic acid residue Glu771 in the fifth transmembrane segment M5 of the Ca(2+)-ATPase of rabbit fast twitch muscle sarcoplasmic reticulum was substituted with lysine, alanine, and glycine by site-directed mutagenesis. Mutant Glu771-->Lys was unable to occlude Ca2+, and Ca2+ did not inhibit phosphorylation from P(i) or activate phosphorylation from ATP of this mutant. Mutants Glu771-->Ala and Glu771-->Gly were likewise unable to occlude Ca2+, but Ca2+ in the millimolar concentration range activated phosphorylation from ATP and inhibited phosphorylation from P(i) of these mutants. The dephosphorylation of the ADP-insensitive E2P phosphoenzyme intermediate of mutants Glu771-->Ala and Glu771-->Gly was found to be blocked, whereas the dephosphorylation proceeded rapidly for mutant Glu771-->Lys. This finding suggests a role of the positive charge of the lysine in induction of dephosphorylation, supporting the hypothesis that the side chain of Glu771 participates in the countertransport of two protons per Ca(2+)-ATPase cycle.

Site-directed mutagenesis of the Na,K-ATPase: consequences of substitutions of negatively-charged amino acids localized in the transmembrane domains

Site-directed mutagenesis was used to examine the importance of five carboxyl-containing amino acids localized in the putative membrane-spanning regions of the Na,K-ATPase (i.e., E327, E778, D803, D807, and D925 of the rat alpha 2 isoform). The substitutions were introduced into a cDNA encoding a ouabain-resistant isoform (i.e., rat alpha 2* which was mutated to encode a ouabain-resistant isoform), and the effect of these substitutions on Na,K-ATPase function was assessed by screening the altered enzymes for their ability to confer ouabain resistance when expressed in otherwise ouabain-sensitive cells. The expression of the alpha isoform containing certain substitutions at positions 327 and 925 was able to confer ouabain resistance to HeLa cells while the expression of rat alpha 2* containing substitutions at positions 778, 803, and 807 was not. In particular, amino acids in each of these positions were substituted with leucine to evaluate the importance of the carboxyl-containing side chain. The ability of rat alpha 2* containing E327L and D925L to confer ouabain resistance to HeLa cells indicates that neither the negative charge nor the oxygen-containing side chain is absolutely essential for overall function in this position. In contrast, the inability of rat alpha 2* carrying E778L, D803L, and D807L to confer ouabain resistance suggests that the naturally occurring amino acid may be more critical structurally and/or functionally for the Na,K-ATPase. Other more conservative substitutions introduced to further characterize the role of particular amino acid side chains include E327D, E327Q, D803N, D803E, and D925N.(ABSTRACT TRUNCATED AT 250 WORDS)

A Glu329-->Gln variant of the alpha-subunit of the rat kidney Na+,K(+)-ATPase can sustain active transport of Na+ and K+ and Na+,K(+)-activated ATP hydrolysis with normal turnover number

An allelic variant of the ouabain-insensitive rat kidney Na+,K(+)-ATPase alpha 1-isoform was identified by chance in a cDNA library. The variant differed from the wild-type rat kidney Na+,K(+)-ATPase by a single G-to-C base substitution in the cDNA, which on amino acid level gave rise to a glutamine in place of the glutamate residue Glu329 previously suggested as a likely donator of oxygen ligands for Na+ and K+ binding. The variant cDNA was transfected into COS-1 cells and the transfectants expanded with success into stable cell lines that were able to grow in the presence of a concentration of ouabain highly cytotoxic to the parental cells containing only the endogenous COS-1 cell Na+,K(+)-ATPase. Under these conditions, the viability of the cells depended on the cation transport mediated by the ouabain-insensitive Glu329-->Gln variant, whose cDNA was shown by polymerase chain reaction amplification to be stably integrated into the COS-1 cell genome. The maximum specific ATP hydrolysis activity of isolated plasma membranes of the Glu329-->Gln variant did not differ significantly from that of plasma membranes containing the wild type. A method was established for measurement of the phosphorylation capacity of the expressed Glu329-->Gln variant and wild-type enzyme, and it was thereby demonstrated that the variant had a turnover number similar if not identical to that of the wild-type.

Functional consequences of alterations to Pro328 and Leu332 located in the 4th transmembrane segment of the alpha-subunit of the rat kidney Na+,K(+)-ATPase

Site-specific mutagenesis was used to analyse the functional roles of the residues Pro328 and Leu332 located in the conserved PEGLL motif of the predicted transmembrane helix M4 in the alpha 1-subunit of the ouabain resistant rat kidney Na+,K(+)-ATPase. cDNAs encoding either of the Na+,K(+)-ATPase mutants Pro328-->Ala and Leu332-->Ala, and wild type, were cloned into the expression vector pMT2 and transfected into COS-1 cells. Ouabain-resistant clones growing in the presence of 10 microM ouabain were isolated, and the Na+,K+, ATP and pH dependencies of the Na+,K(+)-ATPase activity measured in the presence of 10 microM ouabain were analysed. Under these conditions the exogenous expressed Na+,K(+)-ATPase contributed more than 95% of the Na+,K(+)-ATPase activity. The Pro328-->Ala mutant displayed a reduced apparent affinity for Na+ (K0.5 (Na+) 13.04 mM), relative to the wild type (K0.5 (Na+) 7.13 mM). By contrast, the apparent affinity for Na+ displayed by the Leu332-->Ala mutant was increased (K0.5 (Na+) 3.92 mM). Either of the mutants exhibited lower apparent affinity for K+ relative to the wild type (K0.5 (K+) 2.46 mM for Pro328-->Ala and 1.97 mM for Leu332-->Ala, compared with 0.78 mM for wild type). Both mutants exhibited higher apparent affinity for ATP than the wild type (K0.5 (ATP) 0.086 mM for Pro328-->Ala and 0.042 mM for Leu332-->Ala, compared with 0.287 mM for wild type). The influence of pH was in accordance with an acceleration of the E2 (K)-->E1 transition in the mutants relative to the wild type. These data are consistent with a role of Pro328 and Leu332 in the stabilization of the E2 form and of Pro328 in Na+ binding. The possible role of the mutated residues in K+ binding is discussed.