Photosensitized cleavage of dynein heavy chains. Cleavage at the "V1 site" by irradiation at 365 nm in the presence of ATP and vanadate

Irradiation of soluble dynein 1 from sea urchin sperm flagella at 365 nm in the presence of MgATP and 0.05-50 microM vanadate (Vi) cleaves the alpha and beta heavy chains (Mr 428,000) at their V1 sites to give peptides of Mr 228,000 and 200,000, without the nonspecific side effects produced by irradiation at 254 nm as described earlier (Lee-Eiford, A., Ow, R. A., and Gibbons, I. R. (1986) J. Biol. Chem. 261, 2337-2342). The decrease in intact heavy chain material is biphasic; in 10 microM Vi, approximately 80% occurs with a half-time of 7 min and the remainder with a half-time of about 90 min, and the yield of cleavage peptides is better than 90%. Loss of dynein ATPase activity appears to be a direct result of the cleavage process and is not significantly affected by the presence of up to 0.1 M cysteamine (CA, 60-23-1) or 2-aminoethyl carbamimidothioic acid dihydrobromide (CA, 56-10-0) as free radical trapping agents. The concentration of Vi required for 50% maximal initial cleavage rate is 4.5 microM, while that for 50% ATPase inhibition is 0.8 microM, both in a 0.6 M NaCl medium. In the presence of 20 microM Vi, CTP and UTP support cleavage at about half the rate of ATP, whereas GTP and ITP support cleavage only if the Vi concentration is raised to about 200 microM. Substitution of any of the transition metal cations Cr2+, Mn2+, Fe2+, or Co2+ for the usual Mg2+ suppresses the photocleavage, presumably by quenching the excited chromophore prior to scission of the heavy chain. The photocleaved dynein 1 binds to dynein-depleted flagella similarly to intact dynein 1, but upon reactivation of the flagella with 1 mM ATP their motility is partially inhibited, rather than being augmented as with intact dynein. These results indicate that Vi acts as a photosensitizing catalyst and suggest that the cleavage proceeds through excitation of Vi bound to dynein at the hydrolytic ATP binding site on each heavy chain, probably in a dynein X MgADP X Vi complex. The exquisite specificity of Vi-sensitized photocleavage will aid the peptide mapping of dynein heavy chains and may be of broader use in studies of protein structure.     

Photosensitized cleavage of dynein heavy chains. Cleavage at the V2 site by irradiation at 365 NM in the presence of oligovanadate

Irradiation of the outer-arm dynein ATPase from sea urchin sperm flagella at 365 nm in the presence of 50-200 microM vanadate (Vi) and 1 mM manganese acetate, in the absence of ATP, cleaves the alpha and beta heavy chains at a specific site, termed the V2 site, to form discrete peptides of Mr approximately 260,000 and 170,000 from the alpha chain and of Mr approximately 255,000 and 175,000 from the beta chain, with a yield of 80%. This cleavage at the V2 site is not correlated with any direct effect on the dynein ATPase activity. In the presence of 100 microM Vi, the half-times for cleavage of the alpha and beta chains are about 12 and 50 min, respectively. The rate of heavy chain cleavage shows a sigmoidal dependence upon Vi concentration, with half-maximal rate occurring at 58 +/- 7 microM, consistent with the chromophore responsible for cleavage being tri-vanadate. Addition of 10 microM ATP or ADP, or of 100 microM CTP or UTP, to the irradiation medium inhibits cleavage at the V2 site, and results in a slow cleavage occurring at the V1 site described previously. The peptides produced by sequential cleavage at the V2 and then the V1 sites indicate that the sites are separated by about 100,000 Da along the length of each heavy chain. Photoaffinity labeling with [alpha-32P] 8-azidoadenosine 5'-triphosphate (8-N3ATP) gives specific incorporation of 32P into both the Mr 255,000 and 175,000 peptides of the beta chain but into only the Mr 260,000 peptide of the alpha chain. These results suggest that V2 cleavage occurs on a loop of the heavy chain that forms part of the ATP-binding site, close to the locus of 8-N3ATP attachment.     

Outer arm dynein from trout spermatozoa. Purification, polypeptide composition, and enzymatic properties

Extraction of isolated axonemes from trout (Salmo gairdneri) sperm with 0.6 M NaCl removed 97% of the outer arms, approximately 12% of the protein, and approximately 50% of the MgATPase activity. Fractionation of this high salt extract by sucrose density gradient centrifugation yielded a single peak of ATPase activity with an apparent sedimentation coefficient of 19 S. Electrophoretic analysis showed that this 19 S particle was composed of two heavy chains (termed alpha and beta; Mr 430,000 and 415,000, respectively), five intermediate molecular weight chains (IC1-IC5; Mr 85,000, 73,000, 65,000, 63,000, and 57,000), and six light chains (LC1-LC6; Mr 22,000-6,000). A similar complex was obtained following further purification by DEAE-Sephacel column chromatography. Quantitative densitometry of Coomassie Blue-stained gels indicated that the heavy and intermediate chains were present in equimolar amounts. Electron microscopic examination of the 19 S particles revealed that it consisted of two globular heads joined together by a Y-shaped stem. The 19 S particle had a specific MgATPase activity of 1.1 +/- 0.3 mumol of phosphate released/min/mg and exhibited an apparent Km for MgATP2- of 40 +/- 16 microM. MnATP2- and CaATP2- were hydrolyzed at rates 100 and 80% that of MgATP2-, respectively. The Mg-ATPase activity was inhibited by vanadate, but not by ouabain or oligomycin, and exhibited a high activity between pH 7.0 and 10.0 with a maximum at pH 9.0-9.5. ATP was the preferred nucleotide, although GTP and CTP (but not ITP) did interact with the dynein to a minor extent. Based on its origin, sedimentation coefficient, polypeptide composition, and enzymatic properties, we conclude that this two-headed 19 S particle represents the entire trout sperm axonemal outer arm dynein. This dynein is probably exemplary of the outer arm dyneins of other vertebrates.     

Isolated beta-heavy chain subunit of dynein translocates microtubules in vitro

Our goal was to assess the microtubule translocating ability of individual ATPase subunits of outer arm dynein. Solubilized outer arm dynein from sea urchin sperm (Stronglocentrotus purpuratus) was dissociated into subunits by low ionic strength buffer and fractionated by zonal centrifugation. Fractions were assessed by an in vitro functional assay wherein microtubules move across a glass surface to which isolated dynein fractions had been absorbed. Microtubule gliding activity was coincident with the 12-S beta-heavy chain-intermediate chain 1 ATPase fractions (beta/IC1). Neither the alpha-heavy chain nor the intermediate chains 2 and 3 fractions coincided with microtubule gliding activity. The beta/IC1 ATPase induced very rapid gliding velocities (9.7 +/- 0.88 micron/s, range 7-11.5 micron/s) in 1 mM ATP-containing motility buffers. In direct comparison, isolated intact 21-S outer arm dynein, from which the beta/IC1 fraction was derived, induced slower microtubule gliding rates (21-S dynein, 5.6 +/- 0.7 micron/s; beta/IC1, 8.7 +/- 1.2 micron/s). These results demonstrate that a single subdomain in dynein, the beta/IC1 ATPase, is sufficient for microtubule sliding activity.     

Outer-arm dynein from trout spermatozoa: substructural organization.

Outer-arm dynein purified from trout spermatozoa was disrupted by low-ionic-strength dialysis, and the resulting subunits were separated by sucrose density-gradient centrifugation. The intact 19 S dynein, containing the alpha- an beta-heavy chains, intermediate chains (ICs) 1-5 and light chains (LCs) 1-6, yielded several discrete particles: a 17.5 S adenosine triphosphatase (ATPase) composed of the alpha- and beta-chains ICs 3-5 and LC 1; a 9.5 S complex containing ICs 1 and 2 together with LCs 2, 3, 4, and 6; and a single light chain (LC 5), which sedimented at approximately 4 S. In some experiments, ICs 3-5 also separated from the heavy chain complex and were obtained as a distinct subunit. Further dissociation of the 17.5 S particle yielded a 13.1 S ATPase that contained the beta-heavy chain and ICs 3-5. The polypeptide compositions of the complexes provide new information on the intermolecular associations that occur within dynein. Substructural features of the trout dynein polypeptides also were examined. The heavy chains were subjected to vanadate-mediated photolysis at the V1 sites by irradiation at 365 nm in the presence of Mg2+, ATP, and vanadate. Fragment pairs of relative molecular mass (Mr) 245,000/185,000 and 245,000/170,000 were obtained from the alpha- and beta-heavy chains, respectively. Photolysis of these molecules at their V2 sites, by irradiation in the presence of vanadate and Mn2+, yielded fragments of Mr 160,000/270,000 and 165,000/250,000, respectively. These values confirm that the alpha- and beta-heavy chains have masses of 430,000 and 415,000 daltons, respectively. Immunological analysis using monoclonal antibodies revealed that one intermediate chain from trout dynein (IC 2) contains epitopes present in two different intermediate chains from Chlamydomonas dynein. This indicates that specific sequences within the dynein intermediate chains have been highly conserved throughout evolution.     

Conformational changes of the beta chain of the outer-arm dynein from sea urchin sperm flagella coupled with ATP hydrolysis

Conformational changes of the beta chain of the outer-arm dynein from sea urchin sperm flagella in relation to ATP hydrolysis was examined by tryptic digestion. Tryptic digestion of the beta chain in the presence of 2 mM ATP (ADP) and 100 microM vanadate (Vi) or in the presence of 4 mM ATP gamma S produced different polypeptides from in the case of no addition. The difference was similar to the result previously reported for 21S outer-arm dynein heavy chains [Inaba, K. & Mohri, H. (1989) J. Biol. Chem. 264, 8384-8388]. Unlike the tryptic digestion pattern of 21S dynein heavy chains, however, the 135-kDa polypeptide was consistently produced from the beta chain, even in the presence of ATP (ADP) and Vi. The tryptic digestion pattern of the 21S particle reconstituted from the separated a chain, the beta/IC1 complex and the IC2/IC3 complex [Tang, W.-J.Y., Bell, C.W., Sale, W.S., & Gibbons, I.R. (1982) J. Biol. Chem. 257, 508-515] was similar to that of intact 21S dynein; the 135-kDa polypeptide was only slightly produced in the presence of ATP and Vi. The digestion rate constant of the 135-kDa polypeptide from the beta chain in the presence of ATP and Vi was significantly decreased as compared with in the case of 21S dynein or that of the reconstituted 21S particle. These results suggest that the trypsin sensitivity of the 135-kDa region of the beta chain changes with the association of the beta/ICI complex with the alpha chain and the IC2/IC3 complex in the presence of ATP and Vi.     

Characterization of a protein serine kinase from yeast plasma membrane

A casein kinase activity, which copurifies with the H+-ATPase activity during isolation of plasma membranes Saccharomyces cerevisiae and during centrifugation of the solubilized membrane extract through a sucrose gradient, is separated from the Mr = 100,000 ATPase catalytic polypeptide by subsequent DEAE-cellulose chromatography. The purified casein kinase activity exhibits a low Km of 12 microM MgATP, is maximally stimulated by 6 mM free Mg2+, and is 50% inhibited by 300 microM Zn2+, by 7.5 micrograms of heparin/ml, and by 300 microM orthovanadate. It phosphorylates only seryl residues. The purified casein kinase contains two polypeptides of Mr = 45,000 and 39,000 which yield antibodies which do not cross-react to each other. The two polypeptides seem to originate from a precursor of Mr = 85,000 which is detected by both antibodies in partly purified fractions. In the absence of casein, a zinc and heparin-sensitive phosphorylation of the ATPase polypeptide is observed in partly purified ATPase fractions, and a peptide of similar mobility is phosphorylated, among others, in isolated plasma membranes. The purified ATPase activity is markedly inhibited by incubation in the presence of acid phosphatase. In agreement with a recent report that the purified active ATPase molecule is largely phosphorylated (Yanagita, Y., Abdel-Ghany, M., Raden, D., Nelson, N., and Racker, E. (1987) Proc. Natl. Acad. Sci. U. S. A. 894, 925-929) this data suggests that dephosphorylation leads to deactivation of ATPase activity.     

Modification of high molecular weight plasmic degradation products of human crosslinked fibrin.

The predominant high molecular weight products of plasmic digestion of human crosslinked fibrin Fragments DD, E and (DD)E complex were purified by column gel filtration in a non-dissociating buffer or by ion-exchange chromatography on DEAE-cellulose. The structure of the degradation products was studied by proteolytic degradation, polyacrylamide gel electrophoresis immunodiffusion and sucrose density gradient centrifugation. Unaltered derivatives were very resistant to proteolytic degradation by plasmin. In the the presence of 10 mM EDTA the (DD)E complex did not dissociate, but similar to Fragment DD, became susceptible to plasmic degradation forming Fragment D derivatives. The (DD)E complex dissociated in 3 M urea at pH 5.5, had an altered conformation as evidenced by its aggregability and by its increased susceptibility to degradation by plasmin resulting in the formation of Fragment d. The gammagamma chain remnants of Fragment DD were attacked first, followed by cleavage of the beta chain remnants. It is concluded that plasmin resistance is a function of the intact structure and it is not directly dependent on the presence of the crosslink bonds or calcium ions.     

A map of photolytic and tryptic cleavage sites on the beta heavy chain of dynein ATPase from sea urchin sperm flagella

NH2-terminal analysis of the alpha and beta heavy chain polypeptides (Mr greater than 400,000) from the outer arm dynein of sea urchin sperm flagella, compared with that of the 230,000- and 200,000-Mr peptides formed upon photocleavage of dynein by irradiation at 365 nm in the presence of vanadate and ATP, shows that the NH2 termini of the intact chains are acetylated and that the 230,000- and 200,000 Mr peptides constitute the amino- and carboxy-terminal portions of the heavy chains, respectively. Tryptic digestion of the beta heavy chain is known to separate it into two particles, termed fragments A and B, that sediment at 12S and 6S (Ow, R. A., W.-J. Y. Tang, G. Mocz, and I. R. Gibbons, 1987. J. Biol. Chem. 262:3409-3414). Immunoblots against monoclonal antibodies specific for epitopes on the beta heavy chain, used in conjunction with photoaffinity labeling, show that the ATPase-containing fragment A is derived from the amino-terminal region of the beta chain, with the two photolytic sites thought to be associated with the purine-binding and the gamma-phosphate-binding areas of the ATP-binding site spanning an approximately 100,000 Mr region near the middle of the intact beta chain. Fragment B is derived from the complementary carboxy-terminal region of the beta chain.     

A latent adenosine triphosphatase form of dynein 1 from sea urchin sperm flagella.

Treatment of demembranated sea urchin sperm axonemes with an extraction solution containing 0.6 M NaCl, pH 7.0 for 10 min at 4 degrees C yields a solution of dynein 1 having a low, latent specific ATPase activity of about 0.25 mumol of Pi mg(-1) min(-1). Exposure of this dynein solution to 0.1% Triton-X-100 for 10 min at 25 degrees C causes an increase in its ATPase activity to about 3 mumol of Pi mg(-1) min(-1). A similar activation can be obtained by treating at 42 degrees C or by reacting with 60 mol of p-chloromercuribenzene sulfonate/10(6) g of protein. The effects of these activating procedures are not additive, suggesting that they lead to a common activated state. Purification of the latent activity dynein 1 by sucrose density gradient centrifugation yields a monodisperse preparation sedimenting at 21 S, and having a molecular weight of 1,250,000 as determined by sedimentation diffusion and sedimentation equilibrium. Activation of the latent dynein 1 with Triton X-100 converts it to a form sedimenting at 10 to 14 S. The 21 S dynein is also converted to a 10 S form by dialysis against 5 mM imidazole/NaOH buffer, 0.1 mM EDTA, 5 mM 2-mercaptoethanol, pH 7, although in this case, the ATPase activity is increased only about 3-fold, with another 3-fold activation being obtainable upon subsequent treatment with Triton X-100. The 21 S latent form of dynein 1 may represent the intact dynein arms that form moving cross-bridges and generate active sliding between adjacent doublet tubules of the flagellar axoneme. Electrophoretic analysis on polyacrylamide gels in the presence of sodium dodecyl sulfate suggests a model in which the 21 S dynein 1 particle is composed of three subunits of about 330,000 daltons and one of each of three medium weight subunits of 126,000, 95,000, and 77,000 daltons. When latent dynein 1 is added back to NaCl-extracted axonemes in the presence of 0.15 M NaCl, it recombines stoichiometrically and restores the arms on the doublet tubules with a 6-fold activation of its ATPase activity measured in the absence of KCl.     

Structure of the dynein-1 outer arm in sea urchin sperm flagella. II. Analysis by proteolytic cleavage

The structure of the 21 S latent activity dynein-1 (LAD-1) particle has been investigated by limited proteolytic cleavage with trypsin and with chymotrypsin. The A alpha and A beta heavy polypeptide chains show different characteristic digestion patterns which remain essentially unchanged whether the chains are components of the 21 S LAD-1 particle or are in the form of separated fractions, although changes in their relative digestion rates upon separation suggest that the A beta chain in the 21 S particle is partially protected from digestion by the presence of the A alpha chain and intermediate chains 2 and 3. The progressive digestion of the A chains and intermediate chains causes an eventual dissociation of the 21 S particle to smaller particles sedimenting in the range 10 to 14 S. Within this broad peak, the fragments from the A alpha chain peak in the 10 to 12 S region, while those from the A beta chain peak in the 12 to 14 S region. Digestion of whole axonemes to a stage at which the A alpha chain is substantially digested but the A beta chain remains mostly intact, enables a large amount of 21 S dynein-1 to be solubilized by 3 mM MgATP2(-) in the presence of 0.1 M NaCl, pH 7.0. This indicates that the affinity of the 0.6 M NaCl-sensitive bond of the outer arm to the A-tubule is diminished substantially by the early stages of digestion of the A alpha chain.     

The Mr 78,000 intermediate chain of Chlamydomonas outer arm dynein interacts with alpha-tubulin in situ

We have used the zero-length cross-linker 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) to examine protein-protein associations within purified outer arm dynein and axonemes from Chlamydomonas flagella. When axonemes were treated with 0.5-1 mM EDC in either the presence or absence of ATP/vanadate, a polypeptide band of Mr 127,000 recognized by monoclonal antibody 1878A (specific for the Mr 78,000 intermediate chain (IC78) of outer arm dynein) was generated. This conjugate was not obtained when purified dynein was treated with EDC. Further immunological analysis demonstrated that this complex also contained alpha- (but not beta-) tubulin. These results indicate that IC78 interacts with alpha-tubulin in situ in an ATP-insensitive manner. Identification of this interface between dynein and tubulin suggests that IC78, which probably is located at the base of the dynein particle (King, S. M., and Witman, G. B. (1990) J. Biol. Chem. 265, 19807-19811), contributes to the structural attachment of the dynein arms to the A-tubules of the outer doublet microtubules. Analysis of the cross-linked products from the purified dynein revealed several additional interactions involving the intermediate chains; these adducts provide further evidence for an intermediate chain/light chain complex within dynein and confirm that IC78 and IC69 associate directly.     

Polypeptide subunits of dynein 1 from sea urchin sperm flagella

A high-resolution sodium dodecyl sulfate polyacrylamide gel electrophoresis system has been used to show the presence, in both whole sperm and isolated flagellar axonemes, of eight polypeptides migrating in the 300,000–350,000 molecular weight range characteristic of the heavy chains of dynein ATPase. Previously, only five such chains have been discernible. Extraction of isolated axonemes for 10 min at 4°C with a solution containing 0.6 M NaCl, pH 7, releases a mixture of particles that separate, in sucrose density gradient centrifugation, into a major peak, dynein 1 ATPase, sedimenting at 21 S and a minor peak at 12–14S. The polypeptide compositions of these two peaks are different. The dynein 1 peak, which contains most of the protein on the gradient, contains approximately equal quantities of two closely migrating heavy chains, with a small amount of a third, more slowly migrating chain; no other heavy chains appear in this peak. Two groups of smaller polypeptides (three intermediate chains, within the apparent molecular weight range 76,000–122,000 and four newly discovered light chains, within the apparent molecular weight range 14,000–24,000) cosediment with the 21 S peak. The heavy chain composition of the 12–14S peak is more complex, all eight heavy chains occurring in approximately the same ratios as occur in intact axonemes.     

Heparin-copper biaffinity chromatography of fibroblast growth factors

A novel method is described to separate and identify the various forms of fibroblast growth factor (FGF) based on their differential affinities for both heparin and copper. FGFs were extracted from bovine hypothalamus and purified by batchwise adsorption to heparin-Sepharose. The partially purified FGFs were then applied to an affinity column prepared by mixing equal portions of heparin-Sepharose and copper-Sepharose. The column was rinsed consecutively with the following four reagents: (i) 2 M NaCl, (ii) 0.6 M NaCl, (iii) 0.6 M NaCl plus 10 mM imidazole, and (iv) 0.6 m NaCl. FGFs were then eluted with a linear NaCl/imidazole gradient (from 0.6 m NaCl without imidazole to 2 M NaCl plus 10 mM imidazole). Fractions eluted from the column were analyzed by sodium dodecyl sulfate-gel electrophoresis with silver staining and electrophoretic immunoblot using site-specific antibodies against basic and acidic FGF. The results demonstrate that it is possible to resolve from hypothalamus at least two basic FGF species (with Mr values of 19,000 and 18,000) and three acidic FGF species (with Mr values of 18,000, 16,400, and 15,600). These findings indicate that heparin-copper biaffinity chromatography may have wide applicability in the study of the structure and activity of FGFs.     

Isolation and molecular kinetic properties of the angiotensin-converting enzyme from the human heart

An angiotensin-converting enzyme was isolated from human heart using N[-1(S)-carboxy-5-aminopentyl]glycyl-glycine as an affinity adsorbent. The isolation procedure resulted in an enzyme purified 1650-fold. The enzyme specific activity was 38.0 u./mg protein, Mr = 150 kD. The pH optimum for the angiotensin-converting enzyme towards Hip-His-Leu lies at 7.8, Km = 1.2 mM. The enzyme was inhibited by the substrate (Ks' = 14 mM). The enzyme effectively catalyzed the hydrolysis of angiotensin I (Km = 10 microM; kcat = 250 s-1). NaCl, CaCl2 as well as Na2SO4 in the absence of Cl- activated the enzyme, whereas CH3COONa and NaNO3 did not influence the enzyme activity. It was found that the bradykinin-potentiating factor inhibited the cardiac angiotensin-converting enzyme with IC50 = 4.0 X 10(-8) M.     

The kinetics of assembly of normal and variant human oxyhemoglobins

The kinetics of assembly have been monitored spectrophotometrically for normal and variant human oxyhemoglobins in 0.1 M Tris, 0.1 M NaCl, 1 mM Na2EDTA, pH 7.4, at 21.5 degrees C. Oxyhemoglobin versus oxy chain static difference spectra were performed and revealed subtle but significant absorption changes in both the visible and Soret regions. Kinetic experiments were performed by rapidly mixing equivalent (in heme) concentrations of alpha and beta A chains and following the change in absorbance at 583 nm with time. Over a protein concentration range of 10-100 microM in heme prior to mixing, these time courses were homogeneous and followed first-order kinetics, yielding a value of 0.069 s-1 for the apparent rate constant of dissociation of oxygenated beta A chain tetramers. Under these conditions, the overall assembly of oxyhemoglobins S (beta 6Glu----Val) and N-Baltimore (beta 95Lys----Glu) were also governed by the rates of dissociation of their respective oxygenated beta S and beta N-Baltimore chain tetramers with the apparent first-order rate constants of 0.044 and 0.15 s-1, respectively. In the Soret region, the alpha, beta monomer combination reaction could be observed if the protein concentration (heme basis) was lowered and if protein nonequivalency (beta chain exceeded alpha chain concentration) mixing experiments were performed. A kinetic oxyhemoglobin A, oxy-alpha, oxy-beta A monomer difference spectrum could be generated, and simple second-order kinetics were observed (415 nm) yielding rate constants of 2.3, 3.3, and 4.8 X 10(5) M-1 s-1 for the assembly of oxyhemoglobins S, A, and N-Baltimore, respectively. To our knowledge, this is the first kinetic study to reveal significant differences between the rate of association of alpha and beta monomers of hemoglobin A and those of two distinctly charged hemoglobin variants.     

Chromium(III)ATP inactivating (Na+ + K+)-ATPase supports Na+-Na+ and Rb+-Rb+ exchanges in everted red blood cells but not Na+,K+ transport

The chromium(III) complex of ATP, an MgATP complex analogue, inactivates (Na+ + K+)-ATPase by forming a stable chromo-phosphointermediate. The rate constant k2 of inactivation at 37 degrees C of the beta, gamma-bidentate of CrATP is enhanced by Na+ (K0.5 = 1.08 mM), imidazole (K0.5 = 15 mM) and Mg2+ (K0.5 = 0.7 mM). These cations did not affect the dissociation constant of the enzyme-chromium-ATP complex. The inactive chromophosphoenzyme is reactivated slowly by high concentrations of Na+ at 37 degrees C. The half-maximal effect on the reactivation was reached at 40 mM NaCl, when the maximally observable reactivation was studied. However, 126 mM NaCl was necessary to see the half-maximal effect on the apparent reactivation velocity constant. K+ ions hindered the reactivation with a Ki of 70 microM. Formation of the chromophosphoenzyme led to a reduction of the Rb+ binding sites and of the capacity to occlude Rb+. The beta, gamma-bidentate of chromium(III)ATP (Kd = 8 microM) had a higher than the alpha, beta, gamma-tridentate of chromium(III)ATP (Kd = 44 microM) or the cobalt tetramine complex of ATP (Kd = 500 microM). The beta, gamma-bidentate of the chromium(III) complex of adenosine 5'-[beta, gamma-methylene]triphosphate also inactivated (Na+ + K+)ATPase. Although CrATP could not support Na+, K+ exchange in everted vesicles prepared from human red blood cells, it supported the Na+-Na+ and Rb+-Rb+ exchange. It is concluded that CrATP opens up Na+ and K+ channels by forming a relatively stable modified enzyme-CrATP complex. This stable complex is also formed in the presence of the chromium complex of adenosine 5'-[beta, gamma-methylene]triphosphate. Because the beta, gamma-bidentate of chromium ATP is recognized better than the alpha, beta, gamma-tridentate, it is concluded that the triphosphate site recognizes MgATP with a straight polyphosphate chain and that the Mg2+ resides between the beta- and the gamma-phosphorus. The enhancement of inactivation by Mg2+ and Na+ may be caused by conformational changes at the triphosphate site.     

Characterization of Saccharomyces cerevisiae Atm1p: functional studies of an ABC7 type transporter

Saccharomyces cerevisiae Atm1p has been cloned, over-expressed and purified from a yeast expression system. The sequence includes both the soluble ATPase and transmembrane-spanning domains. With the introduction of an N-terminal Kozak sequence and a C-terminal (His)(6)-tag, a yield of 1 mg of Atm1p was obtained from 3 g wet yeast cells, which is comparable to other membrane-associated proteins isolated from eukaryotic expression systems. The ATPase activity of Atm1p is sensitive to sodium vanadate, a P-type ATPase inhibitor, with an IC(50) of 4 microM. MgADP is a product inhibitor for Atm1p with an IC(50) of 0.9 mM. The Michaelis-Menten constants V(max), K(M) and k(cat) of Atm1p were measured as 8.7+/-0.3 microM/min, 107+/-16 microM and 1.24+/-0.06 min(-1), respectively. A plot of ATPase activity versus concentration of Atm1p exhibits a nonlinear relationship, suggesting an allosteric response and an important role for the transmembrane domain in mediating both ATP hydrolysis and MgADP release. The metal dependence of Atm1p ATPase activity demonstrated a reactivity order of Mg(2+)>Mn(2+)>Co(2+), while each divalent ion was found to be inhibitory at higher concentrations. The activation and inhibitory effect of phospholipids suggest that formation of a lipid-micelle complex is important for enzymatic activity and stability. Structural analysis of Atm1p by CD spectroscopy suggested a similarity of secondary structure to that found for other members of this ABC protein family.     

Stereoselectivity for the (R)-Enantiomer of HA-966 (l-Hydroxy-3-Aminopyrrolidone-2) at the Glycine Site of the N-Methyl-d-Aspartate Receptor Complex

HA-966 (1-hydroxy-3-aminopyrrolidone-2) is an antagonist at the glycine allosteric site of the N-methyl-D-aspartate receptor ionophore complex. Unlike presently known glycine antagonists, HA-966 is chiral. We report stereoselectivity for the (R)-enantiomer at the glycine antagonist site. In [3H]glycine binding, the (R)-enantiomer has an IC50 of 4.1 +/- 0.6 microM. The racemic mixture has an IC50 of 11.2 +/- 0.5 microM, whereas (S)-HA-966 has an IC50 greater than 900 microM. In glycine-stimulated [3H]1-[1-(2- thienyl)cyclohexyl]piperidine binding, the (R)-enantiomer inhibits with an IC50 of 121 +/- 61 microM, whereas the racemic mixture has an IC50 of 216 +/- 113 microM and (S)-HA-966 is inactive. The inhibition by (R)-HA-966 can be prevented by the addition of glycine. (R)-HA-966 and racemic HA-966, but not (S)-HA-966, also prevent N-methyl-D-aspartate cytotoxicity in cortical cultures. The (R)-enantiomer and, less potently, the (S)-enantiomer inhibit N-methyl-D-aspartate-evoked [3H]norepinephrine release from rat hippocampal slices (IC50 values of about 0.3 mM and 1.6 mM, respectively), but only the inhibition by (R)-HA-966 is reversed by added glycine. In glutamate-evoked contractions of the guinea pig ileum, (R)-HA-966 causes a glycine-reversible inhibition (IC50 of about 150 microM), whereas (S)-HA-966 is much less potent (IC50 of greater than 1 mM). These results demonstrate stereoselectivity of the glycine antagonist site of the N-methyl-D-aspartate receptor complex in a variety of tissues and assays. The stereoselectivity also confirms the specificity of N-methyl-D-aspartate receptors in glutamate-evoked contractions of the guinea pig ileum, and supports their similarity to central N-methyl-D-aspartate receptors.