Chemical state of the cysteine residues in the Neurospora crassa plasma membrane H(+)-ATPase

Functioning of Yeast Pma1 H+-ATPase under Changing Charge: Role of Asp739 and Arg811 Residues

The plasma membrane Pma1 H+-ATPase of the yeast Saccharomyces cerevisiae contains conserved residue Asp739 located at the interface of transmembrane segment M6 and the cytosol. Its replacement by Asn or Val (Petrov et al. (2000) J. Biol. Chem., 275, 15709-15716) or by Ala (Miranda et al. (2011) Biochim. Biophys. Acta, 1808, 1781-1789) caused complete blockage of biogenesis of the enzyme, which did not reach secretory vesicles. It was proposed that a strong ionic bond (salt bridge) could be formed between this residue and positively charged residue(s) in close proximity, and the replacement D739A disrupted this bond. Based on a 3D homology model of the enzyme, it was suggested that the conserved Arg811 located in close proximity to Asp739 could be such stabilizing residue. To test this suggestion, single mutants with substituted Asp739 (D739V, D739N, D739A, and D739R) and Arg811 (R811L, R811M, R811A, and R811D) as well as double mutants carrying charge-neutralizing (D739A/R811A) or charge-swapping (D739R/R811D) substitutions were used. Expression of ATPases with single substitutions R811A and R811D were 38-63%, and their activities were 29-30% of the wild type level; ATP hydrolysis and H+ transport in these enzymes were essentially uncoupled. For the other substitutions including the double mutations, the biogenesis of the enzyme was practically blocked. These data confirm the important role of Asp739 and Arg811 residues for the biogenesis and function of the enzyme, suggesting their importance for defining H+ transport determinants but ruling out, however, the existence of a strong ionic bond (salt bridge) between these two residues and/or importance of such bridge for structure-function relationships in Pma1 H+-ATPase.

Polyglutamine fibrils are formed using a simple designed beta-hairpin model

Polyglutamine repeats are found in proteins associated with many neurodegenerative diseases. These repeats are responsible for intracellular protein aggregation that resemble amyloid plaques and contain the hallmarks of cross-beta fibrillar structures. Recent work has suggested that the glutamines are involved in aggregation through two possible mechanisms: one involving only side-chain hydrogen bonding and a second involving interdigitation of the glutamines with tight van der Waal's packing (steric zipper model). We are interested in determining which interactions are particularly involved in early assembly processes and have developed a beta-hairpin model system to address this problem. Our model system is designed to stabilize a putative high-energy nucleating structure to provide a window to view early assembly processes. We have applied spectroscopy tools (circular dichroism, infrared, and dynamic light scattering) to probe the self-assembly of beta-sheet fibrils. These experiments established the conditions to study fibrillar morphology using atomic force microscopy. We show that fibrils are short with minimal lateral growth, suggesting that this may be a good model system for studying early assembly steps.

Functional characterization and purification of an intracellular vitamin D-binding protein in vitamin D-resistant new world primate cells. Amino acid sequence homology with proteins in the hsp-70 family

Most genera of New World primates exhibit resistance to vitamin D. These monkeys harbor high circulating concentrations of the prohormone 25-hydroxyvitamin D and the active vitamin D hormone 1, 25-dihydroxyvitamin D. Previous work from this laboratory indicated that resistance is associated with the overexpression of a 60-65-kDa intracellular protein that binds vitamin D metabolites competitively. In the current studies 25-[3H]hydroxyvitamin D3 (25-OHD3) was used as a competitive ligand to investigate the ability of a number of small lipid molecules to interact with this intracellular vitamin D-binding protein (IDBP) in post-nuclear extracts of a prototypical lymphoblast cell line from the common marmoset, a vitamin D-resistant New World primate. Only those vitamin D metabolites with a hydroxyl moiety in the C-25 position were bound by IDBP. Disruption of the C-25 hydroxyl obviated binding, whereas more proximal alterations in the vitamin D side chain did not. Modifications in the A-ring of 25-hydroxylated vitamin D metabolites, most specifically hydroxylation of C-1, diminished but did not abolish ligand binding. Of more than two dozen other small lipid molecules examined, only the C-19 17-hydroxysteroids, 17beta-estradiol and testosterone, and the C-21 steroid progesterone were found to be capable of binding specifically to IDBP. Using a combination of physical and serial chromatographic techniques, we enriched IDBP 25-OHD3 binding activity 17,588-fold in extracts of B95-8 cells. Two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis of this purified fraction demonstrated a predominant 65-kDa molecular species with a pI approximately 4.5. Seven different peptide fragments were isolated from the 65-kDa protein, each possessing sequence similarity to the hsp-70 family of proteins. Ligand binding analyses confirmed that human inducibly expressed hsp-70-bound 25-OHD3 with approximately similar affinity ( approximately 10(-7) M) as did purified IDBP. In summary, these results suggest a novel action for the hsp-70 family of proteins as intracellular vitamin D- and gonadal steroid hormone-binding molecules.

Site-directed mutagenesis of the cysteine residues in the Neurospora crassa plasma membrane H(+)-ATPase

A high-yield yeast expression system for site-directed mutagenesis of the Neurospora crassa plasma membrane H(+)-ATPase has recently been reported (Mahanty, S. K., Rao, U. S., Nicholas, R. A., and Scarborough, G. A. (1994) J. Biol. Chem. 269, 17705-17712). Using this system, each of the eight cysteine residues in the ATPase was changed to a serine or an alanine residue, producing strains C148S and C148A, C376S and C376A, C409S and C409A, C472S and C472A, C532S and C532A, C545S and C545A, C840S and C840A, and C869S and C869A, respectively. With the exception of C376S and C532S, all of the mutant ATPases are able to support the growth of yeast cells to different extents, indicating that they are functional. The C376S and C532S enzymes appear to be non-functional. After solubilization of the functional mutant ATPase molecules from isolated membranes with lysolecithin, all behaved similar to the native enzyme when subjected to glycerol density gradient centrifugation, indicating that they fold in a natural manner. The kinetic properties of these mutant enzymes were also similar to the native ATPase with the exception of C409A, which has a substantially higher Km. These results clearly indicate that none of the eight cysteine residues in the H(+)-ATPase molecule are essential for ATPase activity, but that Cys376, Cys409, and Cys532 may be in or near important sites. They also demonstrate that the previously described disulfide bridge between Cys148 and Cys840 or Cys869 plays no obvious role in the structure or function of this membrane transport enzyme.

Site-directed mutagenesis of the yeast PMA1 H(+)-ATPase. Structural and functional role of cysteine residues

The yeast plasma-membrane H(+)-ATPase contains nine cysteines, three in presumed transmembrane segments (Cys-148, Cys-312, and Cys-867) and the rest in hydrophilic regions thought to be exposed at the cytoplasmic surface (Cys-221, Cys-376, Cys-409, Cys-472, Cys-532, and Cys-569). To gather new functional and structural information, we have studied the yeast ATPase by cysteine mutagenesis. It proved possible to replace seven of the nine cysteines by alanine, one at a time, without any significant decrease in ATP hydrolysis or ATP-dependent proton pumping. In the remaining two cases (Cys-409 and Cys-472), there were small but reproducible effects; the results clearly indicated, however, that no single Cys is required for activity and that, if a disulfide bridge is formed in the yeast ATPase, it does not play an obligatory structural or functional role. Next, multiple mutants were constructed to ask how many Cys residues could be replaced simultaneously while leaving a fully functional enzyme. After substitution of all "membrane" Cys (Cys-148, Cys-312, and Cys-867) together with two non-conserved Cys located in hydrophilic regions (Cys-221 and Cys-569), there were no significant abnormalities in expression (87%) or activity (89% ATP hydrolysis/93% H+ pumping) of the mutant protein. Replacement of two additional cysteines (Cys-376 near the phosphorylation site and Cys-532, in or near the ATP-binding site) caused a drop in expression (to 54%), although the corrected hydrolytic and H+ pumping activities were still normal. When Cys-472 was also mutated, the corrected activity fell to 44% hydrolysis/47% pumping; finally, substitution of Cys-409 to give a "cysteine-free" ATPase led to a very poorly expressed and poorly active enzyme. Brief exposure of the "one-cysteine" and "two-cysteine" ATPases to trypsin revealed a normal pattern of degradation, but there was a slight impairment in the ability of vanadate to protect against proteolysis. Thus, although single Cys replacements are tolerated well by the yeast ATPase, multiple replacements are progressively more harmful, suggesting that they cause small but additive perturbations of protein folding.

Glucose transporter function is controlled by transporter oligomeric structure. A single, intramolecular disulfide promotes GLUT1 tetramerization

The human erythrocyte glucose transporter is an allosteric complex of four GLUT1 proteins whose structure and substrate binding properties are stabilized by reductant-sensitive, noncovalent subunit interactions [Hebert, D. N., & Carruthers, A. (1992) J. Biol. Chem. 267, 23829-23838]. In the present study, we use biochemical and molecular approaches to isolate specific determinants of transporter oligomeric structure and transport function. When unfolded in denaturant, each subunit (GLUT1 protein) of the transporter complex exposes two sulfhydryl groups. Four additional thiol groups are accessible following subunit exposure to reductant. Assays of subunit disulfide bridge content suggest that two inaccessible sulfhydryl groups form an internal disulfide bridge. Differential alkylation/peptide mapping/N-terminal sequence analyses show that a GLUT1 carboxyl-terminal peptide (residues 232-492) contains three inaccessible sulfhydryl groups and that an N-terminal GLUT1 peptide (residues 147-261/299) contains two accessible thiols. The carboxyl-terminal peptide most likely contains the intramolecular disulfide bridge since neither its yield nor its electrophoretic mobility is altered by addition of reductant. Each GLUT1 cysteine was changed to serine by oligonucleotide-directed, in vitro mutagenesis. The resulting transport proteins were expressed in CHO cells and screened by immunofluorescence microscopy for their ability to expose tetrameric GLUT1-specific epitopes. Serine substitution at cysteine residues 133, 201, 207, and 429 does not inhibit exposure of tetrameric GLUT1-specific epitopes. Serine substitution at cysteines 347 or 421 prevents exposure of tetrameric GLUT1-specific epitopes. Hydrodynamic analysis of GLUT1/GLUT4 chimeras expressed in and subsequently solubilized from CHO cells indicates that GLUT1 residues 1-199 promote chimera dimerization and permit GLUT1/chimera heterotetramerization. This GLUT1 N-terminal domain is insufficient for chimera tetramerization which additionally requires GLUT1 residues 200-463. Extracellular reductants (dithiothreitol, beta-mercaptoethanol, or glutathione) reduce erythrocyte 3-O-methylglucose uptake by up to 15-fold. This noncompetitive inhibition of sugar uptake is reversed by the cell-impermeant, oxidized glutathione. Reductant is without effect on sugar exit from erythrocytes. Dithiothreitol doubles the cytochalasin B binding capacity of erythrocyte-resident glucose transporter, abolishes allosteric interactions between substrate binding sites on adjacent subunits, and occludes tetrameric GLUT1-specific GLUT1 epitopes in situ. CHO cell-resident GLUT1 structure and transport function are similarly affected by extracellular reductant. We conclude that each subunit of the glucose transporter contains an extracellular disulfide bridge (Cys347 and Cys421) that stabilizes transporter oligomeric structure and thereby accelerates transport function.

Fourier transform infrared spectroscopy study of the secondary structure of the reconstituted Neurospora crassa plasma membrane H(+)-ATPase and of its membrane-associated proteolytic peptides

We reconstituted purified plasma membrane H(+)-ATPase from Neurospora crassa into soybean phospholipid vesicles (lipid/ATPase ratio of 5:1 w/w). The proteoliposomes contained an active ATPase, oriented inside-out. They were subjected to proteolysis by using Pronase, proteinase K, trypsin, and carboxypeptidase Y. Fourier transform infrared attenuated total reflection spectroscopy indicates that the amount of protein remaining after hydrolysis and elimination of the extramembrane domain of ATPase represents about 43% of the intact protein. The secondary structure of intact ATPase and of the membrane-associated domain of ATPase was determined by infrared spectroscopy. The membrane domain shows a typical alpha-helix and beta-sheet absorption. Polarized infrared spectroscopy reveals that the orientation of the helices is about perpendicular to the membrane. Amide hydrogen/deuterium exchange kinetics performed for the intact H(+)-ATPase and for the membrane-associated domain demonstrate that this part of ATPase shows less accessibility to the solvent than the entire protein but remains much more accessible to the solvent than bacteriorhodopsin membrane segments.

Modeling a conformationally sensitive region of the membrane sector of the fungal plasma membrane proton pump

A molecular model for transmembrane segments 1 and 2 from the fungal proton pumping ATPase has been developed, and this structure is predicted to form a helical hairpin loop structure in the membrane. This region was selected because it is highly conformationally active and is believed to be an important site of action for clinically important therapeutics in related animal cell enzymes. The hairpin loop is predicted to form an asymmetric tightly packed structure that is stabilized by an N-cap between D140 and V142, by hydrogen bonding between residues in the turn region and the helices, and by pi-pi interactions between closely apposed aromatic residues. A short four-residue S-shaped turn is stabilized by hydrogen bonding but is predicted to be conformationally heterogeneous. The principal effect of mutations within the hairpin head region is to destabilize the local close packing of side groups which disrupts the pattern of hydrogen bonding in and around the turn region. Depending on the mutation, this causes either a localized or a more global distortion of the primary structure in the hairpin region. These altered structures may explain the effects of mutations in transmembrane segments 1 and 2 on ATP hydrolysis, sensitivity to vanadate, and electrogenic proton transport. The conformational sensitivity of the hairpin structure around the S-turn may also account for the effects of SCH28080 and possibly ouabain in blocking ATPase function in related animal cell enzymes. Finally, the model of transmembrane segments 1 and 2 serves as a template to position transmembrane segments 3 and 8. This model provides a new view of the H(+)-ATPase that promotes novel structure/function experimentation and could serve as the basis for a more detailed model of the membrane sector of this enzyme.

Mercaptoethanol and dithiothreitol decrease the difference of electrochemical proton potentials across the yeast plasma and vacuolar membranes and activate their H(+)-ATPases

Mercaptoethanol and dithiothreitol (DTT) inhibited the acidification of external medium by Saccharomyces carlsbergensis cells and protoplasts during glucose oxidation. The inhibition was also observed when cells were incubated with mercaptoethanol or when mercaptoethanol and DTT were used to prepare protoplasts. Experiments with S. carlsbergensis plasma membrane vesicles and vacuoles showed these thiol reagents to inhibit ATP-dependent generation of delta pH and Em across plasma membrane vesicles and vacuoles but to activate their H(+)-ATPases. Mercaptoethanol and DTT are suggested to de-energize plasmalemma as well as tonoplast by increasing their H(+)-permeability and to disturb the cell ion homeostasis.

Cytoplasmic location of amino acids 359-440 of the Neurospora crassa plasma membrane H(+)-ATPase

The topographic location of the region comprising amino acids 359-440 of the Neurospora crassa plasma membrane H(+)-ATPase has been elucidated using reconstituted proteoliposomes and protein chemical techniques. Proteoliposomes containing H(+)-ATPase molecules oriented predominantly with their cytoplasmic surface facing outward were cleaved with trypsin and the resulting digest was subjected to centrifugation on a glycerol step gradient to separate the released and liposome-bound peptides. The released peptides were recovered in the upper regions of the step gradient, whereas the liposome-bound peptides were recovered near the 40% glycerol interface. The released peptides present in the upper fractions were reduced, 14C-carboxy-methylated, and then separated by high performance liquid chromatography. Two radioactive cysteine-containing peptides with retention times of about 162 and 182 min were identified as H(+)-ATPase peptides comprising residues Leu363-Lys379 and Leu388-Arg414, respectively, by comparison to standards prepared from the purified ATPase. This information thus establishes a cytoplasmic location for residues 359-418 in the H(+)-ATPase polypeptide chain. It also infers a cytoplasmic location for residues 419-440, since this stretch of amino acids is too short to cross the membrane and return between regions known to be cytoplasmically located. These results and the results of other recent experiments establish the topographical location of nearly all of the 919 residues in the H(+)-ATPase molecule.