Predicting allosteric mutants that increase activity of a major antibiotic resistance enzyme.

The CTX-M family of beta lactamases mediate broad-spectrum antibiotic resistance and are present in the majority of drug-resistant Gram-negative bacterial infections worldwide. Allosteric mutations that increase catalytic rates of these drug resistance enzymes have been identified in clinical isolates but are challenging to predict prospectively. We have used molecular dynamics simulations to predict allosteric mutants increasing CTX-M9 drug resistance, experimentally testing top mutants using multiple antibiotics. Purified enzymes show an increase in catalytic rate and efficiency, while mutant crystal structures show no detectable changes from wild-type CTX-M9. We hypothesize that increased drug resistance results from changes in the conformational ensemble of an acyl intermediate in hydrolysis. Machine-learning analyses on the three top mutants identify changes to the binding-pocket conformational ensemble by which these allosteric mutations transmit their effect. These findings show how molecular simulation can predict how allosteric mutations alter active-site conformational equilibria to increase catalytic rates and thus resistance against common clinically used antibiotics.

Conformation of the gamma subunit at the gamma-epsilon-c interface in the complete Escherichia coli F(1)-ATPase complex by site-directed spin labeling.

Structure-function relationships of the gamma-epsilon-c subunit interface of F(O)F(1) ATP synthase, a region of subunit interactions important in coupling between catalysis and transport, were investigated by site-directed spin labeling and electron paramagnetic resonance (EPR) spectroscopy. The EPR line widths and collision accessibilities of 18 spin-labeled, unique cysteine F(1) mutants from gammaLeu198 to gammaLeu215 indicate an alternating pattern in the mobility and accessibility parameters for positions gamma201-209, which is reminiscent of a beta-strand. Labels at positions gamma204 and gamma210 show tertiary contact upon F(1) binding to F(O) and gammaD210C has reduced coupling efficiency. gammaE208C could not be spin labeled, but the uncoupling effects of gammaE208K are suppressed by second-site mutations in the polar loop of subunit c [Ketchum, C. J. and Nakamoto, R. K. (1998) J. Biol. Chem. 273, 22292-22297]. The restricted mobility and accessibility of spin labels in the odd-numbered positions between gamma201 and gamma207 plus the 2-4-fold higher values in k(cat) for ATP hydrolysis of these same mutant F(1) indicate that the interactions of these residues with the epsilon subunit mediate its inhibitory activity. Disrupted interactions with epsilon subunit also cause reduced coupling efficiency. We propose a model for the gamma-epsilon-c interface of Escherichia coli F(O)F(1) ATP synthase in which side chains from the odd-numbered residues of the gammaLys201-gammaTyr207 beta-strand directly and functionally interact with the epsilon subunit, while the even-numbered, acidic residues gammaAsp204, gammaGlu208, and gammaAsp210 interact with the F(O) sector, probably with subunit c. gamma Subunit interactions with both subunits in this region are important for coupling efficiency.

Regulation by GDI of RhoA/Rho-kinase-induced Ca2+ sensitization of smooth muscle myosin II.

We characterized the role of guanine nucleotide dissociation inhibitor (GDI) in RhoA/Rho-kinase-mediated Ca2+ sensitization of smooth muscle. Endogenous contents (approximately 2-4 microM) of RhoA and RhoGDI were near stoichiometric, whereas a supraphysiological GDI concentration was required to relax Ca2+ sensitization of force by GTP and guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS). GDI also inhibited Ca2+ sensitization by GTP. G14V RhoA, by alpha-adrenergic and muscarinic agonists, and extracted RhoA from membranes. GTPgammaS translocated Rho-kinase to a Triton X-114-extractable membrane fraction. GTP. G14V RhoA complexed with GDI also induced Ca2+ sensitization, probably through in vivo dissociation of GTP. RhoA from the complex, because it was reversed by addition of excess GDI. GDI did not inhibit Ca2+ sensitization by phorbol ester. Constitutively active Cdc42 and Rac1 inhibited Ca2+ sensitization by GTP. G14V RhoA. We conclude that 1) the most likely in vivo function of GDI is to prevent perpetual "recycling" of GDP. RhoA to GTP. RhoA; 2) nucleotide exchange (GTP for GDP) on complexed GDP. RhoA/GDI can precede translocation of RhoA to the membrane; 3) activation of Rho-kinase exposes a hydrophobic domain; and 4) Cdc42 and Rac1 can inhibit Ca2+ sensitization by activated GTP. RhoA.

Site-specific phosphorylation and point mutations of telokin modulate its Ca2+-desensitizing effect in smooth muscle.

Forskolin and 8-bromoguanosine 3'-5'-cyclic monophosphate (8-Br-cGMP) induce phosphorylation of Ser-13 of telokin and relaxation of smooth muscle at constant calcium. Comparison with the effect of wild type with aspartate (D; to mimic phosphorylation) and alanine (A; non-phosphorylatable) mutants of telokin showed that the S13D mutant was more effective than wild type in relaxing smooth muscle at constant calcium. The efficacy of the Ser-13A, S12A, and S12D mutants was not significantly different from that of wild-type telokin. The effect of neither S13D nor Ser-13A was affected by 8-Br-cGMP, whereas the effect of wild type, S12A, and S12D was enhanced by 8-Br-cGMP, indicating the specificity of Ser-13 charge modification. Mutation of Ser-19 (a mitogen-activated protein kinase site) showed the S19A to be more effective than, and S19D to be not different from, wild-type telokin. The effect of both mutants was slightly enhanced by 8-Br-cGMP. A truncated (residues 1-142) form lacking the acidic C terminus had the same relaxant effect as wild-type telokin, whereas the C-terminal peptide (residues 142-155) had no effect. We conclude that site-specific modification of the N terminus modulates the Ca2+ -desensitizing effect of telokin on force.

Catalytic control and coupling efficiency of the Escherichia coli FoF1 ATP synthase: influence of the Fo sector and epsilon subunit on the catalytic transition state.

The rate-limiting transition state of steady-state ATP hydrolysis and synthesis reactions in the F(o)F(1) ATP synthase involves the rotation of the gamma, epsilon, and c subunits. To probe the role of the transport and coupling mechanisms in controlling catalysis, kinetic and thermodynamic parameters of ATP hydrolysis were determined for enzymes in the presence of the detergent lauryldimethylamine oxide (LDAO), which uncouples active transport and disables the inhibitory effect of the epsilon subunit. At 5 mM LDAO or greater, the inhibitory effects of epsilon subunit are abrogated in both purified F(1) and membranous F(o)F(1). In these conditions, LDAO solubilized F(o)F(1) has a higher k(cat) for ATP hydrolysis than F(1). These results indicate an influence of F(o) on F(1) even though catalysis is uncoupled from transport. The alpha(3)beta(3)gamma complex free of the epsilon subunit is activated at a lower concentration of 0.5 mM LDAO. Significantly, the gammaY205C mutant enzyme is similarly activated at 0.5 mM LDAO, suggesting that the mutant enzyme lacks epsilon inhibition. The gammaY205C F(o)F(1), which has a k(cat) for ATP hydrolysis 2-fold higher than wild type, has an ATP synthesis rate 3-fold lower than wild type, showing that coupling is inefficient. Arrhenius and isokinetic analyses indicate that enzymes that are free of epsilon subunit inhibition have a different transition-state structure from those under the influence of the epsilon subunit. We propose that the epsilon subunit is one of the factors that determines the proper transition-state structure, which is essential for efficient coupling.

Phosphorylation of telokin by cyclic nucleotide kinases and the identification of in vivo phosphorylation sites in smooth muscle.

The Ca(2+)-independent acceleration of dephosphorylation of the regulatory light chain of smooth muscle myosin and relaxation of smooth muscle by telokin are enhanced by cyclic nucleotide-activated protein kinase(s) [Wu et al. (1998) J. Biol. Chem. 273, 11362-113691. The purpose of this study was to determine the in vivo site(s) and in vitro rates of telokin phosphorylation and to evaluate the possible effects of sequential phosphorylation by different kinases. The in vivo site(s) of phosphorylation of telokin were determined in rabbit smooth muscles of longitudinal ileum and portal vein. Following stimulation of ileum with forskolin (20 microM) the serine at position 13 was the only amino acid to exhibit increased phosphorylation. Rabbit portal vein telokin was phosphorylated on both Ser-13 and -19 as a result of forskolin and GTPgammaS stimulation in vivo. Point mutation of Ser-13 (to Ala or Asp) abolished in vitro phosphorylation by cyclic nucleotide-dependent protein kinases.

Molecular mechanisms of rotational catalysis in the F(0)F(1) ATP synthase.

Rotation of the F(0)F(1) ATP synthase gamma subunit drives each of the three catalytic sites through their reaction pathways. The enzyme completes three cycles and synthesizes or hydrolyzes three ATP for each 360 degrees rotation of the gamma subunit. Mutagenesis studies have yielded considerable information on the roles of interactions between the rotor gamma subunit and the catalytic beta subunits. Amino acid substitutions, such as replacement of the conserved gammaMet-23 by Lys, cause altered interactions between gamma and beta subunits that have dramatic effects on the transition state of the steady state ATP synthesis and hydrolysis reactions. The mutations also perturb transmission of specific conformational information between subunits which is important for efficient conversion of energy between rotation and catalysis, and render the coupling between catalysis and transport inefficient. Amino acid replacements in the transport domain also affect the steady state catalytic transition state indicating that rotation is involved in coupling to transport.

Intragenic and intergenic suppression of the Escherichia coli ATP synthase subunit a mutation of Gly-213 to Asn: functional interactions between residues in the proton transport site.

Subunit a of the ATP synthase F(o) sector contains a transmembrane helix that interacts with subunit c and is critical for H(+) transport activity. From a cysteine scan in the region around the essential subunit a residue, Arg-210, we found that the replacement of aGly-213 greatly attenuated ATP hydrolysis, ATP-dependent proton pumping and Delta mu(H)+-dependent ATP synthesis. Various amino acid substitutions caused similar effects, suggesting that functional perturbations were caused by altering the environment or conformation of aArg-210. aG213N, which was particularly severe in effect, was suppressed by two second-site mutations, aL251V and cD61E. These mutations restored efficient coupling; the latter also increased ATP-dependent proton transport rates. These results were consistent with the proposed functional interaction between aArg-210 and cAsp-61, the likely carrier of the transported proton. From Arrhenius analysis of steady-state ATP hydrolytic activity, the transport mutants had large increases in the transition-state enthalpic and entropic parameters. Linear isokinetic relationships demonstrate that the transport mechanism is coupled to the rate-limiting catalytic transition-state step, which we have previously shown to involve the rotation of the gamma subunit in multi-site, co-operative catalysis.

Functional role of charged residues in the transmembrane segments of the yeast plasma membrane H+-ATPase.

As defined by hydropathy analysis, the membrane-spanning segments of the yeast plasma membrane H(+)-ATPase contain seven negatively charged amino acids (Asp and Glu) and four positively charged amino acids (Arg and His). To explore the functional role of these residues, site-directed mutants at all 11 positions and at Glu-288, located near the cytoplasmic end of M3, have been constructed and expressed in yeast secretory vesicles. Substitutions at four of the positions (Glu-129, Glu-288, Asp-833, and Arg-857) had no significant effect on ATP hydrolysis or ATP-dependent proton pumping, substitutions at five additional positions (Arg-695, His-701, Asp-730, Asp-739, and Arg-811) led to misfolding of the ATPase and blockage at an early stage of biogenesis, and substitutions of Asp-143 allowed measurable biogenesis but nearly abolished ATP hydrolysis and proton transport. Of greatest interest were mutations of Glu-703 in M5 and Glu-803 in M8, which altered the apparent coupling between hydrolysis and transport. Three Glu-703 mutants (E703Q, E703L, E703D) showed significantly reduced pumping over a wide range of hydrolysis values and thus appeared to be partially uncoupled. At Glu-803, by contrast, one mutant (E803N) was almost completely uncoupled, while another (E803Q) pumped protons at an enhanced rate relative to the rate of ATP hydrolysis. Both Glu-703 and Glu-803 occupy positions at which amino acid substitutions have been shown to affect transport by mammalian P-ATPases. Taken together, the results provide growing evidence that residues in membrane segments 5 and 8 of the P-ATPases contribute to the cation transport pathway and that the fundamental mechanism of transport has been conserved throughout the group.

Use of chemical chaperones in the yeast Saccharomyces cerevisiae to enhance heterologous membrane protein expression: high-yield expression and purification of human P-glycoprotein.

Utilizing human P-glycoprotein (P-gp), we investigated methods to enhance the heterologous expression of ATP-binding cassette transporters in Saccharomyces cerevisiae. Human multidrug resistance gene MDR1 cDNA was placed in a high-copy 2 mu yeast expression plasmid under the control of the inducible GAL1 promoter or the strong constitutive PMA1 promoter from which P-gp was expressed in functional form. Yeast cells expressing P-gp were valinomycin resistant. Basal ATPase activity of P-gp in yeast membranes was 0. 4-0.7 micromol/mg/min indicating excellent functionality. P-glycoprotein expressed in the protease-deficient strain BJ5457 was found in the plasma membrane and was not N-glycosylated. By use of the PMA1 promoter, P-gp could be expressed at 3% of total membrane protein. The expression level could be further enhanced to 8% when cells were grown in the presence of 10% glycerol as a chemical chaperone. Similarly, glycerol enhanced protein levels of P-gp expressed under control of the GAL1 promoter. Glycerol was demonstrated to enhance posttranslational stability of P-gp. Polyhistidine-tagged P-gp was purified by metal affinity chromatography and reconstituted into proteoliposomes in milligram quantities and its ATPase activity was characterized. Turnover numbers as high as 12 s(-1) were observed. The kinetic parameters K(MgATP)(M), V(max), and drug activation were dependent on the lipid composition of proteoliposomes and pH of the assay and were similar to P-gp purified from mammalian sources. In conclusion, we developed a system for cost-effective, high-yield, heterologous expression of functional P-gp useful in producing large quantities of normal and mutant P-gp forms for structural and mechanistic studies.

Escherichia coli ATP synthase alpha subunit Arg-376: the catalytic site arginine does not participate in the hydrolysis/synthesis reaction but is required for promotion to the steady state.

The three catalytic sites of the F(O)F(1) ATP synthase interact through a cooperative mechanism that is required for the promotion of catalysis. Replacement of the conserved alpha subunit Arg-376 in the Escherichia coli F(1) catalytic site with Ala or Lys resulted in turnover rates of ATP hydrolysis that were 2 x 10(3)-fold lower than that of the wild type. Mutant enzymes catalyzed hydrolysis at a single site with kinetics similar to that of the wild type; however, addition of excess ATP did not chase bound ATP, ADP, or Pi from the catalytic site, indicating that binding of ATP to the second and third sites failed to promote release of products from the first site. Direct monitoring of nucleotide binding in the alphaR376A and alphaR376K mutant F(1) by a tryptophan in place of betaTyr-331 (Weber et al. (1993) J. Biol. Chem. 268, 20126-20133) showed that the catalytic sites of the mutant enzymes, like the wild type, have different affinities and therefore, are structurally asymmetric. These results indicate that alphaArg-376, which is close to the beta- or gamma-phosphate group of bound ADP or ATP, respectively, does not make a significant contribution to the catalytic reaction, but coordination of the arginine to nucleotide filling the low-affinity sites is essential for promotion of rotational catalysis to steady-state turnover.

Human RhoA/RhoGDI complex expressed in yeast: GTP exchange is sufficient for translocation of RhoA to liposomes.

The human small GTPase, RhoA, expressed in Saccharomyces cerevisiae is post-translationally processed and, when co-expressed with its cytosolic inhibitory protein, RhoGDI, spontaneously forms a heterodimer in vivo. The RhoA/RhoGDI complex, purified to greater than 98% at high yield from the yeast cytosolic fraction, could be stoichiometrically ADP-ribosylated by Clostridium botulinum C3 exoenzyme, contained stoichiometric GDP, and could be nucleotide exchanged fully with [3H]GDP or partially with GTP in the presence of submicromolar Mg2+. The GTP-RhoA/RhoGDI complex hydrolyzed GTP with a rate constant of 4.5 X 10(-5) s(-1), considerably slower than free RhoA. Hydrolysis followed pseudo-first-order kinetics indicating that the RhoA hydrolyzing GTP was RhoGDI associated. The constitutively active G14V-RhoA mutant expressed as a complex with RhoGDI and purified without added nucleotide also bound stoichiometric guanine nucleotide: 95% contained GDP and 5% GTP. Microinjection of the GTP-bound G14V-RhoA/RhoGDI complex (but not the GDP form) into serum-starved Swiss 3T3 cells elicited formation of stress fibers and focal adhesions. In vitro, GTP-bound-RhoA spontaneously translocated from its complex with RhoGDI to liposomes, whereas GDP-RhoA did not. These results show that GTP-triggered translocation of RhoA from RhoGDI to a membrane, where it carries out its signaling function, is an intrinsic property of the RhoA/RhoGDI complex that does not require other protein factors or membrane receptors.

How RhoGDI binds Rho.

Like all Rho (Ras homology) GTPases, RhoA functions as a molecular switch in cell signaling, alternating between GTP- and GDP-bound states, with its biologically inactive GDP-bound form maintained as a cytosolic complex with RhoGDI (guanine nucleotide-exchange inhibitor). The crystal structures of RhoA-GDP and of the C-terminal immunoglobulin-like domain of RhoGDI (residues 67-203) are known, but the mechanism by which the two proteins interact is not known. The functional human RhoA-RhoGDI complex has been expressed in yeast and crystallized (P6(5)22, unit-cell parameters a = b = 139, c = 253 A, two complexes in the asymmetric unit). Although diffraction from these crystals extends to 3.5 A and is highly anisotropic, the experimentally phased (MAD plus MIR) electron-density map was adequate to reveal the mutual disposition of the two molecules. The result was validated by molecular-replacement calculations when data were corrected for anisotropy. Furthermore, the N-terminus of RhoGDI (the region involved in inhibition of nucleotide exchange) can be identified in the electron-density map: it is bound to the switch I and switch II regions of RhoA, occluding an epitope which binds Dbl-like nucleotide-exchange factors. The entrance of the hydrophobic pocket of RhoGDI is 25 A from the last residue in the RhoA model, with its C-terminus oriented to accommodate the geranylgeranyl group without conformational change in RhoA.

Rotational coupling in the F0F1 ATP synthase.

The F0F1 ATP synthase is a large multisubunit complex that couples translocation of protons down an electrochemical gradient to the synthesis of ATP. Recent advances in structural analyses have led to the demonstration that the enzyme utilizes a rotational catalytic mechanism. Kinetic and biochemical evidence is consistent with the expected equal participation of the three catalytic sites in the alpha 3 beta 3 hexamer, which operate in sequential, cooperative reaction pathways. The rotation of the core gamma subunit plays critical roles in establishing the conformation of the sites and the cooperative interactions. Mutational analyses have shown that the rotor subunits are responsible for coupling and in doing so transmit specific conformational information between transport and catalysis.

A mutation in the Escherichia coli F0F1-ATP synthase rotor, gammaE208K, perturbs conformational coupling between transport and catalysis.

Cross-linking studies on the Escherichia coli F0F1-ATP synthase indicated a site of interaction involving gamma and epsilon subunits in F1 and subunit c in F0 (Watts, S. D., Tang, C., and Capaldi, R. A. (1996) J. Biol. Chem. 271, 28341-28347). To assess the function of these interactions, we introduced random mutations in this region of the gamma subunit (gamma194-213). One mutation, gammaGlu-208 to Lys (gammaE208K), caused a temperature-sensitive defect in oxidative phosphorylation-dependent growth. ATP hydrolytic rates of the gammaE208K F0F1 enzyme became increasingly uncoupled from H+ pumping above 28 degreesC. In contrast, Arrhenius plot of steady-state ATP hydrolysis of the mutant enzyme was linear from 20 to 50 degreesC. Analysis of this plot revealed a significant increase in the activation energy of the catalytic transition state to a value very similar to soluble, epsilon subunit-inhibited F1 and suggested that the mutation blocked normal release of epsilon inhibition of ATP hydrolytic activity upon binding of F1 to F0. The difference in temperature dependence suggested that the gammaE208K mutation perturbed release of inhibition via a different mechanism than it did energy coupling. Suppressor mutations in the polar loop of subunit c restored ATP-dependent H+ pumping and transition state thermodynamic parameters close to wild-type values indicating that interactions between gamma and c subunits mediate release of epsilon inhibition and communication of coupling information.

Identification of an uncoupling mutation affecting the b subunit of F1F0 ATP synthase in Escherichia coli.

A specific b subunit arginine, b(Arg-36) in Escherichia coli, displays evolutionary conservation among bacterial F1F0 ATP synthases. Site-directed mutagenesis was used to generate a collection of mutations affecting b(Arg-36). The phenotype differed depending upon the substitution, and the b(Arg-36-Glu) and b(Arg-36-Ile) substitutions virtually abolished enzyme function. Although the total amounts of F1F0 ATP synthase present in the membranes prepared from mutant strains were reduced, the primary effect of the b(Arg-36) substitutions was on the activities of the intact enzyme complexes. The most interesting result was that the b(Arg-36-Glu) substitution results in the uncoupling of a functional F0 from F1 ATP hydrolysis activity.

Acceleration of myosin light chain dephosphorylation and relaxation of smooth muscle by telokin. Synergism with cyclic nucleotide-activated kinase.

Incorporation of 32P into telokin, a smooth muscle-specific, 17-18-kDa, acidic (pI 4.2-4.4) protein, was increased by forskolin (20 microM) in intact rabbit ileum smooth muscle (ileum) and by 8-bromo-cyclic GMP (100 microM) in alpha-toxin-permeabilized ileum. Native telokin (5-20 microM), purified from turkey gizzard, and recombinant rabbit telokin, expressed in Escherichia coli and purified to >90% purity, induced dose-dependent relaxation, associated with a significant decrease in regulatory myosin light chain phosphorylation, without affecting the rate of thiophosphorylation of regulatory myosin light chain of ileum permeabilized with 0.1% Triton X-100. Endogenous telokin was lost from ileum during prolonged permeabilization (>20 min) with 0.1% Triton X-100, and the time course of loss was correlated with the loss of 8-bromo-cyclic GMP-induced calcium desensitization. Recombinant and native gizzard telokins were phosphorylated, in vitro, by the catalytic subunit of cAMP-dependent protein kinase, cGMP-dependent protein kinase, and p42/44 mitogen-activated protein kinase; the recombinant protein was also phosphorylated by calmodulin-dependent protein kinase II. Exogenous cGMP-dependent protein kinase (0.5 microM) activated by 8-bromo-cyclic GMP (50 microM) phosphorylated recombinant telokin (10 microM) when added concurrently to ileum depleted of its endogenous telokin, and their relaxant effects were mutually potentiated. Forskolin (20 microM) also increased phosphorylation of telokin in intact ileum. We conclude that telokin induces calcium desensitization in smooth muscle by enhancing myosin light chain phosphatase activity, and cGMP- and/or cAMP-dependent phosphorylation of telokin up-regulates its relaxant effect.

Stability and functionality of cysteine-less F(0)F1 ATP synthase from Escherichia coli.

All 21 native cysteines in the Escherichia coli F(0)F1 ATP synthase were replaced by alanines. In isolated E. coli membranes, ATP-dependent proton pumping, turnover of ATP hydrolysis and steady-state transition state thermodynamic parameters of the cysteine-less enzyme were similar to wild-type. The cysteine-less enzyme was solubilized in n-octyl beta-D-glucopyranoside, purified by affinity chromatography, and reconstituted into pre-formed liposomes made from E. coli lipids. The properties of the reconstituted, purified enzyme were not significantly different from the membranous enzyme. These data demonstrate that cysteine-less F(0)F1 is biochemically stable and has functionality similar to wild-type.

Substitutions of aspartate 378 in the phosphorylation domain of the yeast PMA1 H+-ATPase disrupt protein folding and biogenesis.

There is strong evidence that Asp-378 of the yeast PMA1 ATPase plays an essential role in ATP hydrolysis by forming a covalent beta-aspartyl phosphate reaction intermediate. In this study, Asp-378 was replaced by Asn, Ser, and Glu, and the mutant ATPases were expressed in a temperature-sensitive secretion-deficient strain (sec6-4) that allowed their properties to be examined. Although all three mutant proteins were produced at nearly normal levels and remained stable for at least 2 h at 37 degrees C, they failed to travel to the vesicles that serve as immediate precursors of the plasma membrane; instead, they became arrested at an earlier step of the secretory pathway. A closer look at the mutant proteins revealed that they were firmly inserted into the bilayer and were not released by washing with high salt, urea, or sodium carbonate (pH 11), treatments commonly used to strip nonintegral proteins from membranes. However, all three mutant ATPases were extremely sensitive to digestion by trypsin, pointing to a marked abnormality in protein folding. Furthermore, in contrast to the wild-type enzyme, the mutant ATPases could not be protected against trypsinolysis by ligands such as MgATP, MgADP, or inorganic orthovanadate. Thus, Asp-378 functions in an unexpectedly complex way during the acquisition of a mature structure by the yeast PMA1 ATPase.