Investigation of ligand-binding sites of the acetylcholine receptor using photoactivatable derivatives of neurotoxin II from Naja naja oxiana.

Several photoaffinity derivatives of neurotoxin II from the venom of the central Asian cobra Naja naja oxiana have been prepared. After reaction of the 125I-labeled derivatives with the nicotinic acetylcholine receptor from electric organ, the alpha-subunit of the nAChR is almost exclusively labeled by the derivative carrying the photoactivatable group in position Lys46. In contrast to this, a reactive group at Lys26 predominantly labels the gamma- and delta-subunits, while the alpha- and beta-subunits incorporate much less radioactivity. Competition experiments with d-tubocurarine show that the gamma-subunit is labeled when this derivative occupies the high affinity d-tubocurarine-binding site, while the delta-subunit is labeled by the toxin bound at the low-affinity d-tubocurarine site. A model is discussed for the orientation of different loops of the toxin molecules in the binding site for agonists and competitive antagonists.     

On the molecular mechanisms of neutralization of a cobra neurotoxin by specific antibodies

Examination of 76 homologous neurotoxin sequences suggested that the "toxic" domain of these compounds consists of twelve highly conserved residues. Five of these, namely Lys-27, Trp-29, Asp-31, Arg-33 and Glu-38, together with a variant residue at position 36 are organized into a pattern which resembles that of d-tubocurarine. Two lines of experimental evidence are in agreement with the proposed topology of the "toxic" site in Naja nigricollis toxin alpha--Three highly conserved residues (Lys-27, Trp-29 and Lys-47) have been modified individually in toxin alpha. These modifications induce a decrease in binding affinity of toxin alpha for its target, the nicotinic acetylcholine receptor. In contrast, modifications of three residues (Leu-1, Lys-15 and Lys-51) excluded from the "toxic" domain, do not alter the binding properties of toxin alpha.--Five toxin derivatives carrying a nitroxide group at residues 1, 15, 27, 47 or 51 have been prepared. ESR spectra have been recorded for each derivative in both the free state and bound to the receptor. Mobility of the probes of the residues excluded from the "toxic" site is not altered upon receptor binding. In contrast mobility of the nitroxide of the presumed "toxic" Lys-47 becomes markedly reduced after toxin receptor complex formation. Lys-27 nitroxide is immobilized in both the free and bound state. The antigenic structure of N. nigricollis toxin alpha has been partially clarified using two different approaches. --Fifteen antigenically important residues of toxin alpha have been identified by analyzing cross-reactions between toxin alpha and eleven homologous neurotoxins, using polyclonal antibodies.--- One monoclonal antibody (M alpha 1) specific for toxin alpha has been prepared. Competition experiments, made with (3H) toxin alpha, six mono modified toxin derivatives or alpha three homologous neurotoxins, showed that the binding site of (M alpha 1) comprises the N-terminal group, Lys-15, Pro-18 and probably Thr-16. This site is topographically different from the "toxic" domain. (M alpha 1) inhibits the toxicity of toxin alpha under both in vivo and in vitro conditions. In addition, (M alpha 1) is capable of "removing" toxin molecules bound to the receptor, allowing a rapid recovery of the functional properties of the receptor.     

Toxin binding site of the diphtheria toxin receptor: loss and gain of diphtheria toxin binding of monkey and mouse heparin-binding, epidermal growth factor-like growth factor precursors by reciprocal site-directed mutagenesis

The transmembrane precursor of the monkey (Mk) heparin-binding, epidermal growth factor-like growth factor (proHB-EGF) functions as a diphtheria toxin (DT) receptor, whereas the mouse (Ms) precursor does not. Previously, using chimeric Ms/Mk precursors, we have shown that DT resistance of cells bearing Ms proHB-EGF may be accounted for by several amino acid substitutions between residues 122 and 148 within the EGF-like domain and that Glu-141 is an important amino acid residue for DT binding. In this study, reciprocal site-directed mutagenesis was performed on the major non-conserved residues in the region of 122–148, alone or in combination, between Mk and Ms precursors to identify more precisely which amino acid residues are important for DT binding. Two approaches were used. The first, more traditional approach was to destroy DT sensitivity and binding of Mk proHB-EGF by substitution(s) with the corresponding Ms residue(s). From the single mutations, the greatest loss of DT sensitivity was observed with Mk/Glu-141His (approximately 4000-fold) and the next greatest with Mk/Ile-133Lys (approximately fourfold). The double mutations Mk/Leu-127Phe/Glu-141His, Mk/Ile-133Lys/Glu-141His and Mk/His-135Leu/Glu-141His resulted in complete toxin resistance (> 100 000-fold). The second approach, both novel and complementary, was to gain DT binding and sensitivity of Ms proHB-EGF by substitution(s) with the corresponding Mk residue(s). Surprisingly, the single mutation Ms/His-141Glu resulted in the gain of moderate DT sensitivity (> 260-fold). The double mutation Ms/Lys-133Ile/His-141Glu and the triple mutation Ms/Lys-133Ile/Leu-135His/His-141Glu resulted in a progressive gain in toxin sensitivity (> 4700-fold and > 16 000-fold respectively) and affinity. This triple mutant cell line is essentially as sensitive (IC₅₀ = 3.1 ng ml^?1) as the highly toxin-sensitive monkey Vero cell line (IC₅₀ = 4 ng ml^?1), indicating that these three Mk residues enable the Ms proHB-EGF to act as a fully functional DT receptor. Taken together, these results indicate that Glu-141 plays the most critical role in DT binding and sensitivity and that two additional amino acid residues, Ile-133 and His-135, also play significant roles.     

Neurotoxins active on insects: amino acid sequences, chemical modifications, and secondary structure estimation by circular dichroism of toxins from the scorpion Androctonus australis Hector

Two scorpion neurotoxins active only on insects, the insect toxins AaH IT1 and AaH IT2, were purified from the venom of scorpions Androctonus australis Hector collected in Tozeur (Tunisia) and characterized. AaH IT2 was sequenced and found to differ in four amino acid positions from AaH IT, the single previously sequenced insect toxin [Darbon, H., Zlotkin, E., Kopeyan, C., Van Rietschoten, J., & Rochat, H. (1982) Int. J. Pept. Protein Res. 20, 320-330] which possessed an equal potential for paralyzing fly larvae. The basic amino acid residues of AaH IT1, which differs from AaH IT by one amino acid residue, were selectively chemically modified. Six derivatives were characterized. Their toxicity toward fly larvae and cockroach was determined, and their affinity for the AaH IT1 synaptosomal receptor from cockroach nerve cord was measured. Modification of His-30, Lys-34, and Arg-60 showed no significant effect on biological activity. However, the modification of Lys-28 or Lys-51 demonstrated that these two amino acids are important for toxicity. Furthermore, simultaneous modifications of both Lys-28 and Lys-51 led to a cumulative decrease in biological activity. AaH IT1 and AaH IT2 show similar CD spectra. The secondary structures content of AaH IT2 was estimated from circular dichroism data. Results showed that this class of toxin should possess an additional alpha-helical region and a beta-sheet strand, not found in toxins active on mammals. Attempts to localize these secondary structural features in the amino acid sequence of AaH IT2 indicated that these two regions would be located within the last 20 C-terminal amino acid residues. From these studies on secondary structures, it is possible to consider that toxins active on insects are more structurally constrained than those active on mammals; a decreased molecular flexibility may be, at least partially, responsible for the observed specificity of these toxins for the insect sodium channel. Furthermore, the two alpha-helices found in insect toxins enclosed the two conserved Lys-28 and Lys-51 and might thus be implicated in the toxic site of insect toxins.     

Affinity labeling of a previously undetected essential lysyl residue in class I fructose bisphosphate aldolase.

The affinity label N-bromoacetylethanolamine phosphate (BrAcNHEtOP) has been used previously at pH 6.5 to identify His-359 of rabbit muscle aldolase as an active site residue. We now find that the specificity of the reagent is pH-dependent. At pH 8.5, alkylation with 14C-labeled BrAcNHEtOP abolishes both fructose-1,6-P2 cleavage activity and transaldolase activity. The stoichiometry of incorporation, the kinetics of inactivation, and the protection against inactivation afforded by a competitive inhibitor or dihydroxyacetone phosphate are consistent with the involvement of an active site residue. A comparison of 14C profiles obtained from chromatography on the amino acid analyzer of acid hydrolysates of inactivated and protected samples reveals that inactivation results from the alkylation of lysyl residues. The major peptide in tryptic digests of the inactivated enzyme has been isolated. Based on its amino acid composition and the known sequence of aldolase, Lys-146 is the residue preferentially alkylated by the reagent. Aldolase modified at His-359 is still subject to alkylation of lysine; thus Lys-146 and His-359 are not mutually exclusive sites. However, aldolase modified at Lys-146 is not subject to alkylation of histidine. One explanation of these observations is that modification of Lys-146 abolishes the binding capacity of aldolase for substrates and substrate analogs (BrAcNHEtOP), whereas modification of his-359 does not. Consistent with this explanation is the ability of aldolase modified at His-359 to form a Schiff base with substrate and the inability of aldolase modified at Lys-146 to do so. Therefore, Lys-146 could be one of the cationic groups that functions in electrostatic binding of the substrate's phosphate groups.     

Immunochemistry of sea anemone toxins: structure-antigenicity relationships and toxin-receptor interactions probed by antibodies specific for one antigenic region

Two antibody subpopulations directed against Anemonia sulcata toxin I or II have been purified by immunoaffinity chromatography. These antibodies are specific for a single antigenic region and were used in a structure-antigenicity relationship study using homologous toxins and chemically modified derivatives of A. sulcata toxin II. Asp-7 and/or Asp-9 and Gln-47 of toxin II were found to be implicated in the antigenic region recognized by the two antibody subpopulations. On the contrary, Arg-14, Lys-35, -36, and -46, and alpha-NH2 of the glycine residue of A. sulcata toxin II are not involved in the corresponding antigenic region. When assayed for interaction with the sodium channel, the antigenic region of toxin II, including Asp-9 and Gln-47, appeared fully accessible to its specific antibodies, suggesting that it is not involved in the binding of the toxin to its receptor.     

Site-specific charge interactions of alpha-conotoxin MI with the nicotinic acetylcholine receptor

We have tested the importance of charge interactions for alpha-conotoxin MI binding to the nicotinic acetylcholine receptor (AChR). Ionic residues on alpha-conotoxin MI were altered by site-directed mutagenesis or by chemical modification. In physiological buffer, removal of charges at the N terminus, His-5, and Lys-10 had small (2-4-fold) effects on binding affinity to the mouse muscle AChR and the Torpedo AChR. It was also demonstrated that conotoxin had no effect on the conformational equilibrium of either receptor, as assessed by the effects of the noncompetitive antagonist proadifen on conotoxin binding and, conversely, the effect of conotoxin on the affinity of phencyclidine, proadifen, and ethidium. Conotoxin displayed higher binding affinity in low ionic strength buffer; neutralization of Lys-10 and the N terminus by acetylation blocked this affinity shift at the alphadelta site but not at the alphagamma site. It is concluded that Ctx residues Lys-10 and the N terminal interact with oppositely charged receptor residues only at the alphadelta site, and the two sites have distinct arrangements of charged residues. Ethidium fluorescence experiments demonstrated that conotoxin is formally competitive with a small cholinergic ligand, tetramethylammonium. Thus, alpha-conotoxin MI appears to interact with the portion of the binding site responsible for stabilizing agonist cations but does not do so with a cationic residue and is, consequently, incapable of inducing a conformational change.     

Site-directed mutagenesis to determine essential residues of ribulose-bisphosphate carboxylase ofRhodospirillum rubrum

Both Lys-166 and His-291 of ribulosebisphosphate carboxylase/oxygenase fromRhodospirillum rubrum have been implicated as the active-site residue that initiates catalysis. To decide between these two candidates, we resorted to site-directed mutagenesis to replace Lys-166 and His-291 with several amino acids. All 7 of the position-166 mutants tested are severely deficient in carboxylase activity, whereas the alanine and serine mutants at position 291 are ∼40% and ∼18% as active as the native carboxylase, essentially ruling out His-291 in theRhodospirillum rubrum carboxylase (and by inference His-298 in the spinach enzyme) as a catalytically essential residue. The ability of some of the mutant proteins to undergo carbamate formation or to bind either ribulosebisphosphate or a transition-state analogue remains largely unimpaired. This implies that Lys-166 is not required for substrate binding; rather, the results corroborate the earlier postulate that Lys-166 functions as an acid-base group in catalysis or in stabilizing a transition state in the reaction pathway.     

The second intracellular loop of the calcitonin gene-related peptide receptor provides molecular determinants for signal transduction and cell surface expression

The calcitonin gene-related peptide (CGRP) receptor is a heterodimer of a family B G-protein-coupled receptor, calcitonin receptor-like receptor (CLR), and the accessory protein receptor activity modifying protein 1. It couples to G(s), but it is not known which intracellular loops mediate this. We have identified the boundaries of this loop based on the relative position and length of the juxtamembrane transmembrane regions 3 and 4. The loop has been analyzed by systematic mutagenesis of all residues to alanine, measuring cAMP accumulation, CGRP affinity, and receptor expression. Unlike rhodopsin, ICL2 of the CGRP receptor plays a part in the conformational switch after agonist interaction. His-216 and Lys-227 were essential for a functional CGRP-induced cAMP response. The effect of (H216A)CLR is due to a disruption to the cell surface transport or surface stability of the mutant receptor. In contrast, (K227A)CLR had wild-type expression and agonist affinity, suggesting a direct disruption to the downstream signal transduction mechanism of the CGRP receptor. Modeling suggests that the loop undergoes a significant shift in position during receptor activation, exposing a potential G-protein binding pocket. Lys-227 changes position to point into the pocket, potentially allowing it to interact with bound G-proteins. His-216 occupies a position similar to that of Tyr-136 in bovine rhodopsin, part of the DRY motif of the latter receptor. This is the first comprehensive analysis of an entire intracellular loop within the calcitonin family of G-protein-coupled receptor. These data help to define the structural and functional characteristics of the CGRP-receptor and of family B G-protein-coupled receptors in general.     

Synthetic peptides corresponding to sequences of snake venom neurotoxins and rabies virus glycoprotein bind to the nicotinic acetylcholine receptor

Peptides corresponding to portions of loop 2 of snake venom curare-mimetic neurotoxins and to a structurally similar region of rabies virus glycoprotein were synthesized. Interaction of these peptides with purified Torpedo electric organ acetylcholine receptor was tested by measuring their ability to block the binding of 125I-labeled alpha-bungarotoxin to the receptor. In addition, inhibition of alpha-bungarotoxin binding to a 32-residue synthetic peptide corresponding to positions 173-204 of the alpha-subunit was determined. Neurotoxin and glycoprotein peptides corresponding to toxin loop 2 inhibited labeled toxin binding to the receptor with IC50 values comparable to those of nicotine and the competitive antagonist d-tubocurarine and to the alpha-subunit peptides with apparent affinities between those of d-tubocurarine and alpha-cobratoxin. Substitution of neurotoxin residue Arg37, the proposed counterpart of the quaternary ammonium of acetylcholine, with a negatively charged Glu residue reduced the apparent affinity about 10-fold. Peptides containing the neurotoxin invariant residue Trp29 and 10- to 100-fold higher affinities than peptides lacking this residue. These results demonstrate that relatively short synthetic peptides retain some of the binding ability of the native protein from which they are derived, indicating that such peptides are useful in the study of protein-protein interactions. The ability of the peptides to compete alpha-bungarotoxin binding to the receptor with apparent affinities comparable to those of other cholinergic ligands indicates that loop 2 of the neurotoxins and the structurally similar segment of the rabies virus glycoprotein act as recognition sites for the acetylcholine receptor. Invariant toxin residues Arg37 and Trp29 and their viral homologs play important, although not essential, roles in binding, possibly by interaction with complementary anionic and hydrophobic subsites on the acetylcholine receptor. The alpha-subunit peptide most likely contains all of the determinants for binding of the toxin and glycoprotein peptides present on the alpha-subunit, because these peptides bind to the 32-residue alpha-subunit peptide with the same or greater affinity as to the intact subunit.     

Pyridoxal 5'-diphospho-5'-adenosine binds at a single site on isolated alpha-subunit from Escherichia coli F1-ATPase and specifically reacts with lysine 201

Pyridoxal 5'-diphospho-5'-adenosine (PLP-AMP), an adenine nucleotide affinity analog, was found to bind in a saturable fashion to isolated alpha-subunit from Escherichia coli F1-ATPase with a stoichiometry of one mol/mol and a Kd approximately 150 microM. The binding was shown to be specific by the following criteria: 1) ATP reduced the binding of PLP-AMP by 80%, and 2) PLP-AMP, like ATP, induced a conformational change which increased the mobility of alpha-subunit in nondenaturing polyacrylamide gel electrophoresis and rendered alpha-subunit resistant to mild trypsin proteolysis. A stable adduct was formed between isolated alpha-subunit and [3H] PLP-AMP after reduction with NaBH4. alpha-Subunit labeled to the extent of 0.4-0.7 mol/mol was digested with trypsin and subjected to high pressure liquid chromatography purification, yielding a single labeled peptide. Automated amino acid sequencing showed that residue alpha-Lys-201 was specifically labeled. The results suggest that Lys-201 occupies a position proximate to the phosphate groups of bound ATP in the alpha.ATP complex. PLP-AMP did not support repolymerization of isolated alpha-, beta-, and gamma-subunits, consistent with previous reports that subunit repolymerization in vitro is dependent upon the presence of nucleoside triphosphate. Further, PLP-AMP-labeled alpha-subunit could not be reconstituted with isolated beta- and gamma-subunits in the presence of ATP, showing that occupation of the alpha-subunit nucleotide site by PLP-AMP impairs normal subunit-subunit interaction.     

Peroxidase and phosphatase activity of active-site mutants of vanadium chloroperoxidase from the fungus Curvularia inaequalis. Implications for the catalytic mechanisms

Mutation studies were performed on active-site residues of vanadium chloroperoxidase from the fungus Curvularia inaequalis, an enzyme which exhibits both haloperoxidase and phosphatase activity and is related to glucose-6-phosphatase. The effects of mutation to alanine on haloperoxidase activity were studied for the proposed catalytic residue His-404 and for residue Asp-292, which is located close to the vanadate cofactor. The mutants were strongly impaired in their ability to oxidize chloride but still oxidized bromide, although they inactivate during turnover. The effects on the optical absorption spectrum of vanadium chloroperoxidase indicate that mutant H404A has a reduced affinity for the cofactor, whereas this affinity is unchanged in mutant D292A. The effect on the phosphatase activity of the apoenzyme was investigated for six mutants of putative catalytic residues. Effects of mutation of His-496, Arg-490, Arg-360, Lys-353, and His-404 to alanine are in line with their proposed roles in nucleophilic attack, transition-state stabilization, and leaving-group protonation. Asp-292 is excluded as the group that protonates the leaving group. A model based on the mutagenesis studies is presented and may serve as a template for glucose-6-phosphatase and other related phosphatases. Hydrolysis of a phospho-histidine intermediate is the rate-determining step in the phosphatase activity of apochloroperoxidase, as shown by burst kinetics.     

Location of the polyamine binding site in the vestibule of the nicotinic acetylcholine receptor ion channel

To map the structure of a ligand-gated ion channel, we used the photolabile polyamine-containing toxin MR44 as photoaffinity label. MR44 binds with high affinity to the nicotinic acetylcholine receptor in its closed channel conformation. The binding stoichiometry was two molecules of MR44 per receptor monomer. Upon UV irradiation of the receptor-ligand complex, (125)I-MR44 was incorporated into the receptor alpha-subunit. From proteolytic mapping studies, we conclude that the site of (125)I-MR44 cross-linking is contained in the sequence alpha His-186 to alpha Leu-199, which is part of the extracellular domain of the receptor. This sequence partially overlaps in its C-terminal region with one of the three loops that form the agonist-binding site. The agonist carbachol and the competitive antagonist alpha-bungarotoxin had only minor influence on the photocross-linking of (125)I-MR44. The site where the hydrophobic head group of (125)I-MR44 binds must therefore be located outside the zone that is sterically influenced by agonist bound at the nicotinic acetylcholine receptor. In binding and photocross-linking experiments, the luminal noncompetitive inhibitors ethidium and triphenylmethylphosphonium were found to compete with (125)I-MR44. We conclude that the polyamine moiety of (125)I-MR44 interacts with the high affinity noncompetitive inhibitor site deep in the channel of the nicotinic acetylcholine receptor, while the aromatic ring of this compound binds in the upper part of the ion channel (i.e. in the vestibule) to a hydrophobic region on the alpha-subunit that is located in close proximity to the agonist binding site. The region of the alpha-subunit labeled by (125)I-MR44 should therefore be accessible from the luminal side of the vestibule.     

Site-directed mutagenesis of histidine-13 and histidine-114 of human angiogenin. Alanine derivatives inhibit angiogenin-induced angiogenesis

The roles of His-13 and His-114 in the ribonucleolytic and angiogenic activities of human angiogenin have been investigated by site-directed mutagenesis. Replacement of either residue by alanine (H13A and H114A) decreases enzymatic activity toward tRNA by at least 10,000-fold and virtually abolishes 10,000-fold and virtually abolishes angiogenic activity in the chick embryo chorioallantoic membrane assay. Both the H13A and H114A mutant proteins compete effectively with angiogenin in the latter assay; only a 5-fold molar excess of H13A over unmodified protein is required for complete inhibition. The His----Ala substitutions, however, do not have any significant effect on the interaction of angiogenin with human placental ribonuclease inhibitor, an extremely potent inhibitor of angiogenin (Ki approximately 7 x 10(-16 M) previously shown to interact with another active-site residue, Lys-40. The effects of more conservative replacements-glutamine at position 13 and asparagine at position 114--were also examined. While the enzymatic activity of the H114N mutant was at least 3300-fold less than for the unmodified protein, the H13Q derivative had only 300-fold reduced activity toward tRNA and cytidylyl(3'----5') adenosine. Both substitutions substantially decreased angiogenic activity. The parallel effects on ribonucleolytic and biological activities observed with all four mutant proteins provide strong evidence that the latter activity of angiogenin is dependent on a functional enzymatic active site. The capacity of the H13A and H114A derivatives to compete with angiogenin in the chorioallantoic membrane assay suggests several additional features of the biological mode of action of this protein.     

Orientation of cobra alpha-toxin on the nicotinic acetylcholine receptor. Fluorescence studies

Four flourescein isothiocyanate (FITC) derivatives of Naja naja siamemsis 3 neurotoxin (alpha-toxin), labeled at the epsilon-amino groups of Lys-23, Lys-35, Lys-49, or Lys-69, and a tetramethylrhodamine isothiocyanate (TRITC) derivative, labeled at epsilon-amino group of Lys-23, were prepared and used to analyze the orientation of cobra alpha-toxin on the nicotinic acetylcholine receptor (AcChR) relative to both the plane of the membrane and the central ion channel. Fluorescence-quenching studies of the AcChR-bound FITC derivatives indicated significant solute accessibility to each site of labeling and suggested that none of the sites of FITC labeling is included in the binding surface of the alpha-toxin. Labeling of Lys-23 with TRITC did not affect the affinity of the alpha-toxin toward the AcChR and confirmed, contrary to some previous reports, a minimal role of Lys-23 in the binding surface of the alpha-toxin. Measurements of energy transfer between the lipid-membrane surface and the sites of labeling on receptor-bound alpha-toxin derivatives show that the relative distances of closest approach between the surface of the lipid membrane domain and the sites of labeling are in the order Lys-23 less than or equal to Lys-49 less than Lys-35 less than or equal to Lys-69. Energy transfer between AcChR tryptophans and the sites of labeling of bound derivatives was about 50% greater to Lys-49 than to Lys-23, Lys-35, or Lys-69, suggesting that Lys-49 is closer to receptor tryptophans and to the center of the extracellular domain of the receptor than Lys-23, Lys-35, or Lys-69. Combined with previous observations that the tip of the central loop of the alpha-toxin directly interacts with the AcChR, the above results suggest a model of the approximate orientation of the snake neurotoxins on the receptor. This model shows the tip of the central loop of the toxin directly interacting with the receptor surface and the major axis of the neurotoxin tilting from a perpendicular projection from the membrane. The surface of the alpha-toxin that includes Lys-23 projects away from the central ion channel and the surface that includes Lys-35 and Lys-69 faces the ion channel.     

Different amino acid substitutions at the same position in the nucleotide-binding site of aspartate transcarbamoylase have diverse effects on the allosteric properties of the enzyme

Site-directed mutagenesis was used to determine how the allosteric properties of aspartate transcarbamoylase (ATCase) are affected by amino acid replacements in the nucleotide binding region of the regulatory polypeptide chains. Amino acid substitutions were made for both Lys-60 and Lys-94 in the regulatory chain since those residues have been implicated by x-ray diffraction studies, chemical modification experiments, and site-directed mutagenesis as playing a role in binding CTP and ATP. Lys-60 was replaced by His, Arg, Gln, and Ala, and Lys-94 was changed to His. These mutant forms of ATCase exhibit bewildering changes in the allosteric properties compared to the wild-type enzyme as well as altered affinities for the nucleotide effectors. The enzyme containing His-60 lacks both homotropic and heterotropic effects and exhibits no detectable binding of nucleotides. In contrast, the holoenzymes containing either Gln-60 or Arg-60 retain both homotropic and heterotropic effects. Replacement of Lys-60 by Ala yields a derivative exhibiting altered heterotropic effects involving insensitivity to CTP and activation by ATP. The mutant enzyme containing His-94 in place of Lys exhibits cooperativity with reduced affinity for nucleotides. The multiple substitutions at Lys-60 in the nucleotide binding region of the regulatory chains of ATCase demonstrate that different amino acids in the same location can alter indirectly the delicate balance of interactions responsible for the allosteric properties of ATCase. The studies show that it is hazardous and frequently unwarranted from single amino acid replacements of a specific residue to attribute to that residue the properties observed for the wild-type enzyme.     

Tryptic and chymotryptic cleavage sites in sequence of alpha-subunit of (Na+ + K+)-ATPase from outer medulla of mammalian kidney

Localization of selective proteolytic splits in alpha-subunit of (Na+ + K+)-ATPase is important for understanding the mechanism of active Na+,K+-transport. Proteolytic fragments of alpha-subunit from pig kidney were purified by chromatography in NaDodSO4 on TSK 3000 SW columns. NH2-terminal amino acid sequences of fragments as determined in a gas phase sequenator were unambiguously located within the total sequence of alpha-subunit from sheep kidney (Shull, C.E., et al. (1985) Nature 316, 691-695) and pig kidney (Ovchinnikov, Y.A., et al. (1985) Proc. Acad. Sci. USSR 285, 1490-1495). The primary chymotryptic split in the E1-form is located between Leu-266 and Ala-267 while the tryptic cleavage site appears to be between Arg-262 and Ile-263 (Bond 3). Tryptic cleavage in the initial fast phase of inactivation of the E1-form is located between Lys-30 and Glu-31 (Bond 2). In the E2-form, primary tryptic cleavage is between Arg-438 and Ala-439 (Bond 1). Chymotryptic cleavage between Leu-266 and Ala-267 stabilizes the E1-form of the protein without affecting the sites for binding of cations or nucleotides. Titration of fluorescence responses demonstrates the importance of the NH2-terminal for E1-E2 transition. Protonation of His-13 facilitates transition from E1- to E2-forms of the protein. Removal of His-13 after cleavage of bond 2 can explain the increase in apparent affinity of the cleaved enzyme for Na+ and the shift in poise of E1-E2 equilibrium in direction of E1-forms. The NH2-terminal sequence in renal alpha-subunit is not conserved in alpha + from rat neurolemma or in alpha-subunit from Torpedo or brine shrimp. A regulatory function of the NH2-terminal part of the alpha-subunit may thus be a unique feature of the alpha-subunit in (Na+ + K+)-ATPase from mammalian kidney.     

Site-specific mutagenesis of annexin V: role of residues from Arg-200 to Lys-207 in phospholipid binding.

Annexin V (placental anticoagulant protein I) binds tightly to anionic phospholipid vesicles in the presence of calcium. Four mutant proteins were expressed in Escherichia coli in which Ala replaced one of the following residues in the third repeat of annexin V: Arg-200, His-204, Arg-206, or Lys-207. In a competitive fluorescence quenching assay, the wild-type recombinant protein had the same affinity for phosphatidylserine-containing vesicles as the placentally derived protein. The affinity of the four mutant proteins for phosphatidylserine-containing vesicles was unchanged relative to wild-type protein. We conclude that His-204 and adjacent basic residues, including the highly conserved Arg-200 residue, are not required for high-affinity phospholipid binding.     

Proton transfer roles of lysine 64 and glutamic acid 64 replacing histidine 64 in the active site of human carbonic anhydrase II.

The CO2 hydration activities of cloned human carbonic anhydrase II (carbonate hydro-lyase, EC 4.2.1.1) and variants with Lys, Glu, Gln or Ala replacing His at sequence position 64 have been measured in a variety of different buffers in the pH range 6-9. The variants with Lys-64, Gln-64 and Ala-64 showed non-Michaelis-Menten behavior under some conditions, apparent substrate inhibition being prominent near pH 9. However, asymptotic Michaelis-Menten parameters could be estimated for the limit of low substrate concentrations. All variants show distinct buffer specificities, and imidazole derivatives, Ches and phosphate buffers yield higher kcat values that Bicine, Taps and Mops buffers under otherwise similar conditions. These results are interpreted in terms of different pathways for a rate-limiting proton transfer. In unmodified enzyme, the very high catalytic activity depends on His-64 functioning as an efficient proton transfer group, but this pathway is not available in the variants with Gln-64 and Ala-64. Imidazoles, Ches and phosphate are thought to participate in a metal center-to-buffer proton transfer pathway, whereas Bicine, Taps, Mops and Mes appear to lack this capacity, so that the rate-limiting proton transfer occurs in a metal center-to-bulk water pathway for these variants. The Lys-64 and Glu-64 variants give significantly higher kcat values in Taps, Mops and Mes buffers than the Ala-64 and Gln-64 variants. The pH dependencies of these kcat values are compatible with the hypothesis that Lys-64 and Glu-64 can function as proton transfer groups. Thus, at pH near 9, Lys-64 appears to be only 5-times less efficient than His-64, while Glu-64 is inefficient. At pH 6, Lys-64 is an inefficient proton transfer group, but Glu-64 is only 2-3-times less efficient than His-64. The data indicate that Lys-64 and Glu-64 have pKa values near 8 and below 6, respectively.     

Biotinylated human beta-endorphins as probes for the opioid receptor

The reaction of human beta-endorphin and biotinyl N-hydroxysuccinimide with or without spacer arm, afforded a series of products that were separated by high performance liquid chromatography (HPLC). Liquid secondary ion mass spectrometry of the biotinylated products and their tryptic digests produced abundant protonated molecular ions (MH+), which specified the number and location of biotinylation. Between 1 and 4 biotinyl residues were incorporated per human beta-endorphin molecule, at Lys-9, -19, -24, -28, and -29, but not at the amino-terminal Tyr-1. Three HPLC fractions were isolated for receptor binding studies with monobiotinylation of Lys-9 (B1 beta and B1X beta; X = C6 spacer arm), Lys-19 (B1 gamma), and a mixture of Lys-24, Lys-28, and Lys-29 derivatives (B1 alpha, BX1 alpha). All derivatives displayed tight binding to avidin, and no dissociation from avidin was detectable over several hours at 0 degrees C for the derivatives (BX1 alpha) tested. IC50 values for binding to mu and delta opioid receptor sites were 3-8 times higher for monobiotinylated derivatives than for the parent human beta-endorphin (IC50,mu = 1.5 nM, IC50,delta = 1.3 nM). Association with avidin decreased opioid receptor affinities for the C6 spacer derivative biotinylated at position Lys-9, which is close to the (1-5) enkephalin receptor region. In contrast, avidin did not affect or even increased apparent affinities to mu and delta sites for derivatives biotinylated at the alpha-helical part of the molecule (Lys-19, -24, -28, and -29). Thus, when bound to avidin, the biotinylated human beta-endorphin derivatives with spacer arm (BX1 alpha), substituted near the carboxyl terminal (Lys-24, -28, and -29), displayed mu binding affinities equal to and delta binding affinities only four times lower than underivatized human beta-endorphin. Biotinylated human beta-endorphins also bound to low affinity nonopioid binding sites on NG-108-15 cells; however, affinities to these sites were considerably reduced when derivatives were bound to avidin. The ability of biotinylated human beta-endorphin to cross-link the mu and delta opioid receptors to avidin allows application of the biotin-avidin system as a molecular probe of the opioid receptor.