A Cytokinin-binding Protein from Wheat Germ: Isolation by Affinity Chromatography and Properties

Cytokinin: From autoclaved DNA to two-component signaling

Since its first identification in the 1950s as a regulator of cell division, cytokinin has been linked to many physiological processes in plants, spanning growth and development and various responses to the environment. Studies from the last two and one-half decades have revealed the pathways underlying the biosynthesis and metabolism of cytokinin and have elucidated the mechanisms of its perception and signaling, which reflects an ancient signaling system evolved from two-component elements in bacteria. Mutants in the genes encoding elements involved in these processes have helped refine our understanding of cytokinin functions in plants. Further, recent advances have provided insight into the mechanisms of intracellular and long-distance cytokinin transport and the identification of several proteins that operate downstream of cytokinin signaling. Here, we review these processes through a historical lens, providing an overview of cytokinin metabolism, transport, signaling, and functions in higher plants.

Comparison of cytokinin-binding proteins from wheat and oat grains

Cytokinin-binding proteins (CBPs) isolated from mature grains of oat (Avena sativa L.) and wheat (Triticum aestivum L.) by acid precipitation, ion-exchange and affinity chromatography had similar characteristics, although they differed somewhat in apparent molecular weight of the native protein as determined by gel filtration (109 and 133 kDa, respectively) and subunit size as estimated by SDS-polyacrylamide gel electrophoresis (47 and 55 kDa, respectively). Highly purified oat CBP showed very weak but distinct immunochemical cross-reactivity with anti-wheat CBP IgG, indicating different immunogenic properties of the two CBPs. Nevertheless, both CBPs exhibited very similar binding of different cytokinins and were characterized by high affinity for N6-benzyladenine (BA)-type and by low affinity for zeatin-type cytokinins to both wheat and oat CBPs and by somewhat higher binding activities of oat CBP compared to wheat CBP (Kds for BA: 4.6 x 10-7 M and 6.8 x 10-7 M, respectively). The potential role of CBPs in regulating free BA-type cytokinin levels during cereal grain development and germination is discussed.

Cytokinins affect spore formation but not cell division in the yeast Saccharomyces cerevisiae

Cytokinins are N6-substituted adenine derivatives that function as essential growth hormones in higher plants. In experimental systems, cytokinins can influence cell growth and differentiation among both plant and non-plant tissues. The single-celled yeast, Saccharomyces cerevisiae, has served as an effective and useful model system for the study of a wide range of cellular phenomena generally associated with higher eukaryotes, including mammals. In an attempt to assess the efficacy of its use to dissect the molecular basis for plant hormone action, the effects of cytokinins on S. cerevisiae with respect to cell division rates and sporulation efficiencies were monitored. While none of the cytokinins tested influenced mitotic generation times, micromolar concentrations of kinetin enhanced the formation of yeast haploid ascospores and even lower concentrations of isopentenyladenine inhibited ascus formation.

Characterization of a benzyladenine binding-site peptide isolated from a wheat cytokinin-binding protein: sequence analysis and identification of a single affinity-labeled histidine residue by mass spectrometry

A wheat embryo cytokinin-binding protein was covalently modified with the radiolabeled photoaffinity ligand 2-azido-N6-[14C]benzyladenine. A single labeled peptide was obtained after proteolytic digestion and isolation by reversed-phase and anion-exchange HPLC. Sequencing by classical Edman degradation identified 11 of the 12 residues but failed to identify the labeled amino acid. Analysis by laser photodissociation Fourier-transform mass spectrometry of 10 pmol of the peptide independently confirmed the Edman data and also demonstrated that the histidine residue nearest the C terminus (underlined) was modified by the reagent in the sequence Ala-Phe-Leu-Gln-Pro-Ser-His-His-Asp-Ala-Asp-Glu.

Fluorescent anticytokinins as a probe for binding. Isolation of cytokinin-binding proteins from the soluble fraction and identification of a cytokinin-binding site on ribosomes of tobacco callus cells

4-Substituted 2-methylthiopyrido[2,3-d]pyrimidines, a series of recently developed anticytokinins, have been found to fluoresce strongly in water and to be useful as probes for binding studies. The binding activity of the soluble proteins and particulate fraction of tobacco callus cells to the biologically most active member of the family, 4-n-butylamino-2-methylthiopyrido[2,3-d]pyrimidine (BAMPP), was studied fluorimetrically. We found that the binding activity is better monitored in terms of saturable binding rather than in terms of the amount of bound ligand, a conventional method used in isolation studies of hormone receptor proteins. Using this technique we isolated two kinds of high-affinity cytokinin-binding proteins from the soluble fraction and identified a high-affinity binding site on ribosomes.

Biosynthesis and Degradation of a Wheat Embryo Cytokinin-Binding Protein during Embryogenesis and Germination

The accumulation and degradation of a wheat (Triticum durum) embryo cytokinin-binding protein (CBF-1) was followed during embryo development and germination by its N(6)-benzyladenine (BA) binding activity and immunological reactivity (rocket immunoelectrophoresis and Western blotting). Both BA binding activity and CBF-1 appeared at 2 weeks post-anthesis and rose sharply between 2 to 4 weeks before leveling off to approximately 47 micrograms per embryo (9% of the soluble embryo protein at maturity). In vitro translation of polyadenylated RNA from 20-day-old embryos yielded a polypeptide which was immunoprecipitable with anti-CBF-1 IgG and migrated closely to the 54-kilodalton CBF-1 polypeptide on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Upon germination, both the amount of CBF-1 and BA binding activity dropped to low levels within 3 days. The data are discussed in relation to the possible role of CBF-1 as a regulator of cytokinin availability, and comparisons are drawn between the structural and biosynthetic similarities found between CBF-1 and the vicilin storage proteins of legumes. An improved method for isolating undegraded CBF-1 from whole seeds is also presented.

Hormone-binding studies and the misuse of precipitation assays

Many methods are available for determining binding of small-molecular ligands to macromolecules. In studies on plant hormone binding, precipitation of putative hormone-protein complexes with ammonium sulphate is often used as the sole method of assay. Several such reports on auxin binding have been re-examined, using additional assay procedures. The results show that apparent binding activity measured by ammonium sulphate precipitation may be undetectable by independent, less ambiguous methods. It is concluded that hormone-binding studies that rely solely on the precipitation assay can be grossly misleading, and that this assay, if used at all, should always be validated against a procedure less likely to generate artefacts. An example of such validation for a plant protein is shown.

Azido auxins : photolysis in solution and covalent binding to soybean

The potential of three auxin analogs, 4-, 5-, and 6-azidoindole-3-acetic (4-N(3)IAA, 5-N(3)IAA, and 6-N(3)IAA), as photoaffinity labeling agents for the detection and isolation of auxin receptors was assessed by irradiating these compounds at 365 nm on TLC plates, in solution, and in contact with soybean (Glycine max L. Merr. var. Wayne) hypocotyl. Photolysis on TLC plates produces immobile spots, indicating extremely polar or covalent binding of the photoproducts to the plates. On irradiation in buffer or in buffer containing sucrose, all three compounds decompose at rates that are first order in N(3)IAA to give fluorescent solutions. Photolysis through a Pyrex filter is slower than that through quartz, but the filter prevents tissue damage and allows a given dose of irradiation to photolyze all three N(3)IAAs to the same extent. The effects of photolysis of these compounds in vivo were evaluated with a straight growth assay using etiolated soybean hypocotyl segments. According to this assay, the photoproducts of the N(3)IAAs possess little auxin activity. Irradiation of soybean hypocotyl tissue after 1-hour exposure to 4-N(3)IAA in the dark causes the tissue to grow during 12 hours as much as tissue that is continuously exposed to 4-N(3)IAA in the dark for this period, suggesting that, on photolysis, this auxin analog binds irreversibly to an auxinsensitive site. Although the fluorescence intensity of the photolyzed N(3)IAAs is weak enough to require another method of detecting the bound analog under physiological conditions, the evidence for covalent binding of the N(3)IAAs on photolysis implies that these compounds will be satisfactory photoaffinity labeling agents.

Resolution and properties of a protein kinase catalyzing the phosphorylation of a wheat germ cytokinin-binding protein

The major cytokinin binding protein of wheat germ (CBP) was extensively purified employing chromatography on Cibacron F3GA-Sepharose CL6B and concanavalin A-agarose as key purification steps. The major polypeptides present in the purified CBP preparations have molecular weights of 60,000 +/- 4,000, 42,000 +/- 3,000, and 37,000 +/- 3,000, respectively. A protein kinase that catalyzes the phosphorylation of CBP (CBP kinase) was extensively purified from wheat germ by affinity chromatography on casein-Sepharose 4B and CBP-Sepharose 4B. The purification procedure resolves CBP kinase from an abundant casein kinase that does not phosphorylate CBP. CBP kinase catalyzes the phosphorylation of casein, phosvitin, CBP, and the wheat germ cyclic AMP-binding protein cABPII. CBP kinase phosphorylates the major 60,000 dalton subunit of CBP as well as 16,000 to 18,000 dalton polypeptides present in CBP preparations. CBP fractions with differing activities as substrates for CBP kinase were partly resolved by gel filtration and by chromatography on DEAE-Sephacel.

A comparative study of the localization of wheat-germ agglutinin and its potential receptors in wheat grains

Wheat-germ agglutinin is located only in the embryo of a dry wheat seed and not in the endosperm tissue. This distribution remains unaltered for up to 96 h of germination and growth. The lectin is found not only in a freely soluble form but also in reversible association with particulate subcellular components. There appear to be no poly-peptides that can be solubilized with sonication and aqueous buffers from the embryo tissue that can interact with the agglutinin. This suggests that in vivo the lectin remains uncomplexed to endogenous glycoconjugates or is only able to bind to glycosylated integral membrane polypeptides. Alternatively the potential endogenous receptor(s) to wheat-germ agglutinin may not contain a polypeptide. Although the lectin is not present in the endosperm, seven polypeptides able to interact in a reversible way with wheat-germ agglutinin could be purified from that tissue.

Uptake of Benzyladenine by Tuber Slices of Jerusalem Artichoke (Helianthus tuberosus L.) over a Wide Concentration Range

Uptake of the cytokinin benzyladenine by tuber slices of Jerusalem artichoke (Helianthus tuberosus L.) increased with the concentration of benzyladenine in the medium in the range 10(-8) to 10(-4) molar. The kinetics of uptake could be precisely represented by four phases of a single, multiphasic isotherm. K(m) and V(max) increased in a regular manner upon transition to higher phases. The fit to multiphasic kinetics was significantly better than the fit to cooperative kinetics or to the sum of four independent Michaells-Menten terms.

Metabolism of cytokinin: deribosylation of cytokinin ribonucleoside by adenosine nucleosidase from wheat germ cells

Adenosine nucleosidase (adenosine ribohydrolase, EC 3.2.2.7) which catalyzes the deribosylation of N(6)-(Delta(2)-isopentenyl)adenosine and adenosine to form the corresponding bases was partially purified from wheat germ. This enzyme (molecular weight 59,000 +/- 3,000) deribosylates the ribonucleosides at an optimum pH of 4.7 K(m) values for the cytokinin nucleoside and adenosine are 2.38 and 1.43 micromolar, respectively, in 50 millimolar Tris-citrate buffer (pH 4.7) at 30 C. The presence of adenosine and other cytokinin nucleosides inhibited the hydrolysis of N(6)-(Delta(2)-isopentenyl)adenosine but this reaction was insensitive to guanosine, uridine, or 3'-deoxyadenosine. It is hypothesized that an adequate level of "active cytokinin" in plant cells may be provided through the deribosylation of cytokinin riboside in concert with other cytokinin metabolic enzymes.

Azido auxins: synthesis and biological activity of fluorescent photoaffinity labeling agents

Three auxin analogs, 4-, 5-, and 6-azido-3-indoleacetic acid (4-N(3)-IAA, 5-N(3)-IAA, and 6-N(3)-IAA) have been synthesized for use as fluorescent photoaffinity labeling agents. The pK(a) values of these compounds (4-N(3)-IAA, 4.67; 5-N(3)-IAA, 4.65; 6-N(3)-IAA, 4.66; all +/- 0.04) are experimentally indistinguishable from the pK(a) of 3-indoleacetic acid (IAA, 4.69 +/- 0.04). The auxin activity of these IAA derivatives has been determined in several systems. In soybean, pea, and corn straight growth assays, all three analogs induce growth comparable to that caused by IAA. In the tobacco pith assay, all three analogs elicit a maximum increase in fresh weight at least 40 to 50% of that caused by IAA. Optimal growth is attained in the tobacco pith assay at slightly higher concentrations of 4-N(3)-IAA and 6-N(3)-IAA (30 micromolar) than required for IAA (10 micromolar); however, maximal growth is achieved at a slightly lower concentration of 5-N(3)-IAA (3 micromolar). The N(3)-IAAs, like IAA, are transported basipetally through tobacco pith tissue.

Purification and Properties of a Protein Which Binds Cytokinin-active 6-Substituted Purines

A protein which binds 6-substituted purines of the cytokinin type with relatively high affinity has been extensively purified from wheat germ. Conventional chromatographic techniques, as well as an affinity matrix to which a cytokinin was covalently coupled, were used in the purification. The wheat germ cytokinin-binding protein (CBF-1) has four unlike subunits and an apparent molecular weight of 183,000 as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis.CBF-1 is saturated at one cytokinin molecule per tetramer with a K(d) for 6-benzylaminopurine of 5 x 10(-7) molar. The protein exists both on the native wheat germ ribosome (1 molecule CBF-1 per 80S ribosome) and free in the cytosol with approximately three copies of the latter for each of the former. Data from affinity chromatography studies and cross-linking experiments strongly suggest that a specific binding site for CBF-1 occurs on the wheat germ ribosome.

Active cytokinins: photoaffinity labeling agents to detect binding

FOUR SERIES OF AZIDOPURINES HAVE BEEN SYNTHESIZED AND TESTED FOR CYTOKININ ACTIVITY IN THE TOBACCO CALLUS BIOASSAY: 2- and 8-azido-N(6)-benzyladenines, -N(6)-(Delta(2)-isopentenyl)adenines, and -zeatins, and N(6)-(2- and 4-azidobenzyl)adenines. The compounds having 2-azido substitution on the adenine ring are as active as the corresponding parent compounds, while those with 8-azido substitution are about 10 or more times as active. The 8-azidozeatin, which is the most active cytokinin observed, exhibited higher than minimal detectable activity at 1.2 x 10(-5) micromolar, the lowest concentration tested. The shape of the growth curve indicates that even a concentration as low as 5 x 10(-6) micromolar would probably be effective. By comparison, the lowest active concentration ever reported for zeatin has been 5 x 10(-5) micromolar, representing a sensitivity rarely attained.All of the azido compounds have been submitted to photolysis in aqueous ethanol, and the photoproducts have been detected and identified by low and high resolution mass spectrometry. They are rationalized as products of abstraction and insertion reactions of the intermediate nitrenes. The potential of the major released products as cytokinins was also assessed by bioassay. 2-Azido-N(6)-(Delta(2)-isopentenyl)adenine competed with [(14)C]kinetin for the cytokinin-binding protein isolated from wheat germ. When the azido compound was photolysed in the presence of this protein, its attachment effectively blocked the binding of [(14)C]kinetin.