Competition between Li+ and Mg2+ for red blood cell membrane phospholipids: A 31P, 7Li, and 6Li nuclear magnetic resonance study

The mode of action of the lithium ion (Li+) in the treatment of manic depression or bipolar illness is still under investigation, although this inorganic drug has been in clinical use for 50 yr. Several research reports have provided evidence for Li+/Mg2+ competition in biomolecules. We carried out this study to characterize the interactions of Li+ and Mg2+ with red blood cell (RBC) membrane components to see whether Li+/Mg2+ competition occurs. 31P nuclear magnetic resonance chemical shift measurements of the phospholipids extracted from the RBC membranes indicated that the anionic phospholipids, phosphatidylserine and phosphatidylinositol, bind Li+ and Mg2+ most strongly. From 6Li relaxation measurements, the Li+ binding constant to the phospholipid extract was found to be 45 +/- 5 M(-1). Thus, these studies showed that the phospholipids play a major role in metal ion binding. 7Li spin-lattice relaxation measurements conducted on unsealed and cytoskeleton-depleted RBC membrane in the presence of magnesium indicated that the removal of the cytoskeleton increases lithium binding to the more exposed anionic phospholipids (357 +/- 24 M(-1)) when compared to lithium binding in the unsealed RBC membrane (221 +/- 21 M(-1)). Therefore, it can be seen that the cytoskeleton does not play a major role in Li+ binding or in Li+/Mg2+ competition.

The yeast transcription factor Mac1 binds to DNA in a modular fashion

Mac1 is a metalloregulatory protein that regulates expression of the high affinity copper transport system in the yeast Saccharomyces cerevisiae. Under conditions of high copper concentration, Mac1 represses transcription of genes coding for copper transport proteins. Mac1 binds to DNA sequences called copper response elements (CuREs), which have the consensus sequence 5'-TTTGC(T/G)C(A/G)-3'. Mac1 contains two zinc binding sites, a copper binding site, and the sequence motif RGRP, which has been found in other proteins to mediate binding to the minor groove of A/T-rich sequences in DNA. We have used hydroxyl radical footprinting, missing nucleoside, and methylation interference experiments to investigate the structure of the complex of the DNA binding domain of Mac1 (called here Mac1(t)) with the two CuRE sites found in the yeast CTR1 promoter. We conclude from these experiments that Mac1(t) binds in a modular fashion to DNA, with its RGRP AT-hook motif interacting with the TTT sequence at the 5' end of the CTR1 CuRE site, and with another DNA-binding module(s) binding in the adjacent major groove in the GCTCA sequence.

Competition between Na(+) and Li(+) for unsealed and cytoskeleton-depleted human red blood cell membrane: a (23)Na multiple quantum filtered and (7)Li NMR relaxation study

Evidence for competition between Li(+) and Na(+) for binding sites of human unsealed and cytoskeleton-depleted human red blood cell (csdRBC) membranes was obtained from the effect of added Li(+) upon the (23)Na double quantum filtered (DQF) and triple quantum filtered (TQF) NMR signals of Na(+)-containing red blood cell (RBC) membrane suspensions. We found that, at low ionic strength, the observed quenching effect of Li(+) on the (23)Na TQF and DQF signal intensity probed Li(+)/Na(+) competition for isotropic binding sites only. Membrane cytoskeleton depletion significantly decreased the isotropic signal intensity, strongly affecting the binding of Na(+) to isotropic membrane sites, but had no effect on Li(+)/Na(+) competition for those sites. Through the observed (23)Na DQF NMR spectra, which allow probing of both isotropic and anisotropic Na(+) motion, we found anisotropic membrane binding sites for Na(+) when the total ionic strength was higher than 40 mM. This is a consequence of ionic strength effects on the conformation of the cytoskeleton, in particular on the dimer-tetramer equilibrium of spectrin. The determinant involvement of the cytoskeleton in the anisotropy of Na(+) motion at the membrane surface was demonstrated by the isotropy of the DQF spectra of csdRBC membranes even at high ionic strength. Li(+) addition initially quenched the isotropic signal the most, indicating preferential Li(+)/Na(+) competition for the isotropic membrane sites. High ionic strength also increased the intensity of the anisotropic signal, due to its effect on the restructuring of the membrane cytoskeleton. Further Li(+) addition competed with Na(+) for those sites, quenching the anisotropic signal. (7)Li T(1) relaxation data for Li(+)-containing suspensions of unsealed and csdRBC membranes, in the absence and presence of Na(+) at low ionic strength, showed that cytoskeleton depletion does not affect the affinity of Na(+) for the RBC membrane, but increases the affinity of Li(+) by 50%. This clearly indicates that cytoskeleton depletion favors Li(+) relative to Na(+) binding, and thus Li(+)/Na(+) competition for its isotropic sites. Thus, this relaxation technique proves to be very sensitive to alkali metal binding to the membrane, detecting a more pronounced steric hindrance effect of the cytoskeleton network to binding of the larger hydrated Li(+) ion to the membrane phosphate groups.

Mapping of the DNA binding domain of the copper-responsive transcription factor Mac1 from Saccharomyces cerevisiae

Mac1 from Saccharomyces cerevisiae activates transcription of genes, including CTR1 in copper-deficient cells. N-terminal fusions of Mac1 with the herpes simplex VP16 activation domain were used to show that residues 1-159 in Mac1 constitute the minimal DNA binding domain. Mac1-(1-159) purified from Escherichia coli contains two bound Zn(II) ions. Electrophoretic mobility shift assays showed direct and specific binding by Mac1-(1-159) to a DNA duplex containing the copper-responsive element TTTGCTCA. The DNA binding affinity of Mac1-(1-159) for a duplex containing a single promoter element or an inverted repeat was 5 nM for the 1:1 complex. The N-terminal 40-residue segment of Mac1 is homologous to the DNA binding zinc module found in the copper-activated transcription factors Ace1 and Amt1. A MAC1 mutation yielding a Cys11 --> Tyr substitution at the first candidate zinc ligand position relative to Ace1 resulted in a loss of in vivo function. Two TTTGCTCA promoter elements are necessary for efficient Mac1-mediated transcriptional activation. The elements appear to function synergistically. Increasing the number of elements yields more than additive enhancements in CTR1 expression.

Characterization of the copper chaperone Cox17 of Saccharomyces cerevisiae

Assembly of functional cytochrome oxidase in yeast requires Cox17, which has been postulated to deliver copper ions to the mitochondrion for insertion into the enzyme. This role for Cox17 is supported by the observation that it binds copper as a binuclear cuprous-thiolate cluster. X-ray absorption spectroscopy, together with UV-visible absorption and emission spectroscopy, indicates the presence of bound cuprous ions, trigonally coordinated by thiolate ligands. Analysis of the EXAFS shows three Cu-S bonds at 2.26 A, plus a short Cu-Cu distance of 2.7 A, indicating a binuclear cluster in Cox17. The cuprous-thiolate cluster in Cox17 is substantially more labile than structurally related clusters in metallothioneins.

Metal-ion regulation of gene expression in yeast

Metal-responsive transcription factors exist in yeast to modulate expression of genes that encode proteins involved in cellular uptake of copper, iron and zinc ions. These signal transduction pathways function in the cellular regulation of the intracellular concentration of free metal ions. A second component of metal homeostasis is the regulation of metal-ion binding through protein-mediated metallation. Copper-specific chaperones exist in yeast that route copper ions to the site of biosynthesis of copper-metalloenzymes.

Competition between Li+ and Mg2+ for the phosphate groups in the human erythrocyte membrane and ATP: an NMR and fluorescence study

We investigated the mechanism of competition between Li+ and Mg2+ in Li(+)-loaded human red blood cells (RBCs) by making 7Li and 31P NMR and fluorescence measurements. We used 7Li NMR relaxation times to probe Li+ binding to the human RBC membrane and ATP; an increase in Mg2+ concentration caused an increase in both 7Li T1 and T2 values in packed Li(+)-loaded RBCs, in suspensions of Li(+)-loaded RBC ghosts, in suspensions of Li(+)-containing RBC membrane, and in aqueous solutions of ATP, indicating competition between Li+ and Mg2+ for binding sites in the membrane and ATP. We found that increasing concentrations of either Li+ or Mg2+ in the presence of human RBC membrane caused an increase in the 31P NMR chemical shift anisotropy parameter, which describes the observed axially symmetric powder pattern, indicating metal ion binding to the phosphate groups in the membrane. Competition between Li+ and Mg2+ for phosphate groups in ATP and in the RBC membrane was also observed by both fluorescence measurements and 31P NMR spectroscopy at low temperature. The ratio of the stoichiometric binding constants of Mg2+ to Li+ to the RBC membrane was approximately 20; the ratio of the conditional binding constants in the presence of a free intracellular ATP concentration of 0.2 mM was approximately 4, indicating that Li+ competes for approximately 20% of the Mg(2+)-binding sites in the RBC membrane. Our results indicate that, regardless of the spectroscopic method used, Li+ competes with Mg2+ for phosphate groups in both ATP and the RBC membrane; the extent of metal ion competition for the phosphate head groups of the phospholipids in the RBC membrane is enhanced by the presence of ATP. Competition between Li+ and Mg2+ for anionic phospholipids or Mg(2+)-activated proteins present in cell membranes may constitute the basis of a general molecular mechanism for Li+ action in human tissues.

Transgenic plants of turfgrass (Agrostis palustris Huds.) from microprojectile bombardment of embryogenic callus

Transgenic creeping bentgrass (Agrostis palustris Huds., cv. Pencross; Poaceae) plants have been obtained by microprojectile bombardment of and regeneration from embryogenic calli with a vector designed to deliver the β-glucuronidase (GUS) gene under the control of rice actin 1 5' regulatory sequences. Southern analysis of polymerase chain reaction (PCR)-amplified and restriction-digested genomic DNA of four transgenic plants regenerated from these cultures showed the unscrambled integration of the gus fragment. Northern blot analysis confirmed the expression of gus mRNA in one of the transgenic plants. Western blot analysis revealed a high level of accumulation of gus protein. Histochemical assays showed enzymatic activity of β-glucuronidase in all parts of the transgenic turfgrass plant. The order of gus expression level in different tissues of the transgenic plant is as follows: stem node > first young leaf > root tip > second / third / fourth young leaf > stem internode > root hair-zone.

In-vitro morphogenesis of corn (Zea mays L.) : I. Differentiation of multiple shoot clumps and somatic embryos from shoot tips

In-vitro methods have been developed to regenerate clumps of multiple shoots and somatic embryos at high frequency from shoot tips of aseptically-grown seedlings as well as from shoot apices of precociously-germinated immature zygotic embryos of corn (Zea mays L.). About 500 shoots were produced from a shoot tip after eight weeks of culture (primary culture and one subculture of four weeks) in darkness on Murashige and Skoog basal medium (MS) supplemented with 500 mg/L casein hydrolysate (CH) and 9 μM N(6)-benzyladenine (BA). In this medium, shoots formed in shoot tips as tightly packed "multiple shoot clumps" (MSC), which were composed of some axillary shoots and many adventitious shoots. When the shoot tips were cultured on MS medium containing 500 mg/L CH, 9 μM BA and 2.25 μM 2,4-dichlorophenoxyacetic acid (2,4-D), most of the shoots in the clumps were adventitious in origin. Similar shoot tips cultured on MS medium containing 500 mg/L CH, 4.5 μM BA and 2.25 μM 2,4-D regenerated many somatic embryos within eight weeks of culture. Somatic embryos were produced either directly from the shoot apical meristems or from calli derived from the shoots apices. Both the MSC and the embryos produced normal shoots on MS medium containing 2.25 μM BA and 1.8 μM indole-3-butyric acid (IBA). These shoots were rooted on MS medium containing 3.6 μM IBA, and fertile corn plants were grown in the greenhouse. The sweet-corn genotype, Honey N Pearl, was used for the experiments described above, but shoot-tip cultures from all of 19 other corn genotypes tested also formed MSC on MS medium containing 500 mg/L CH and 9 μM BA.

In-vitro morphogenesis of corn (Zea mays L.) : II. Differentiation of ear and tassel clusters from cultured shoot apices and immature inflorescences

The objective of this research was to study the in-vitro morphogenetic pattern of corn (Zea mays L.) shoot tips excised from aseptically-grown seedlings, and of expiants of axillary shoot buds, immature tassels and ears (staminate and pistillate inflorescences) obtained from greenhouse-grown corn plants. The seedling shoot tips and immature ears first regenerated clumps of multiple shoots within four weeks of culture on Murashige and Skoog (MS) basal medium supplemented with 500 mg/L casein hydrolysate (CH) and 9.0 μM N(6)-benzyladenine (BA). Multiple shoot clumps were also differentiated from spikelets of immature tassels cultured on MS medium containing 500 mg/L CH, 4.5 μM BA and 0.45 μM 2,4-dichlorophenoxy acetic acid (2,4-D). All these multiple shoot clumps in turn differentiated clusters of ears after further four subcultures at four-week intervals under light on MS medium supplemented with 500 mg/L CH and 2.25, 4.5, 9.0 or 18 μM BA. Axillary shoot buds readily differentiated clusters of ears within four weeks of the initial culture on these media. Secondary and tertiary ear clusters were initiated following subculture of primary ears on MS medium containing 500 mg/L CH and 4.5 or 9.0 μM BA. Most of the ear primordia developed into ears with well-developed ovaries and styles on subculture on MS medium containing 500 mg/L CH and 1.0 μM BA. Corn kernels were obtained after pollination of in-vitro-formed ears with pollens collected from greenhouse-grown corn. These kernels germinated in vitro and developed into mature corn plants in the greenhouse. Clusters of tassels were also differentiated in darkness from the multiple shoot clumps after six months successive subcultures but the spikelet primordia of tassels failed to develop fully under the in-vitro conditions tested. Somatic embryos arose directly from spikelet primordia of young tassels or ears on MS medium containing 500 mg/L CH and 4.5 μM 2,4-D, or indirectly from calli derived from spikelets of young tassels and ears on MS medium containing 500 mg/L CH and 9.0 μM 2.4-D.

Plant regeneration via somatic embryogenesis in creeping bentgrass (Agrostis palustris Huds.)

We have established a high-frequency plant regeneration system via somatic embryogenesis from mature seeds of creeping bentgrass (Agrostis palustris Huds). The effects of 2,4-dichlorophenoxyacetic acid (2,4-D), 3.6-dichloroo-anisic acid (dicamba) and 6-benzyladenine (BA) on callus formation and embryogenesis were evaluated. Callus produced on the Murashige and Skoog (MS) (1962) medium containing 2,4-D had low embryogenic potency. In the presence of 30 μM dicamba, addition of 2.25 to 9 μM BA significantly enhanced embryogenic callus formation over dicamba alone. Optimum frequency of somatic embryogenesis was achieved on MS basal medium containing 30 μM dicamba and 2.25 μM BA. Over 80% of somatic embryos germinated and formed plantlets on half-strength MS basal medium. These plantlets grew normally in the greenhouse.