Enzyme immobilization on reactive polymer films

Immobilized enzymes are currently used in many bioanalytical and biomedical applications. This protocol describes the use of thin films of maleic anhydride copolymers to covalently attach enzymes directly to solid supports at defined concentrations. The concentration and activity of the surface-bound enzymes can be tuned over a wide range by adjusting the concentration of enzyme used for immobilization and the physicochemical properties of the polymer platform, as demonstrated here for the proteolytic enzyme Subtilisin A. The versatile method presented allows for the immobilization of biomolecules containing primary amino groups to a broad variety of solid carriers, ranging from silicon oxide surfaces to standard polystyrene well plates and metallic surfaces. The approach can be used to investigate the effects of immobilized enzymes on cell adhesion, and on the catalysis of specific reactions.

Immobilization of growth factors on solid supports for the modulation of stem cell fate

Surface- and matrix-bound signals modulate stem cell fate in vivo and in vitro. This protocol enables the immobilization of a wide range of biomolecules that contain primary amino groups to different types of solid carriers, including glass substrates and standard polystyrene well plates. We describe how thin polymer coatings of poly(octadecene-alt-maleic anhydride) can be used to covalently attach growth factors directly, or through poly(ethylene glycol) spacers, to solid supports at defined concentrations. Surface-immobilized growth factors can be presented over a wide range of concentrations (5-150 ng cm(-2)), as we have previously shown for leukemia inhibitory factor and stem cell factor. Cell activation can be achieved in the presence of adhesion-promoting extracellular matrix proteins. Depending on the methods used, the overall procedure takes 1.5-3 d. In general, the approach can be used to investigate the effect of defined amounts of immobilized growth factors on stem cells and on the maintenance, growth and differentiation of other cell types.

Functional immobilization of signaling proteins enables control of stem cell fate

The mode of ligand presentation has a fundamental role in organizing cell fate throughout development. We report a rapid and simple approach for immobilizing signaling ligands to maleic anhydride copolymer thin-film coatings, enabling stable signaling ligand presentation at interfaces at defined concentrations. We demonstrate the utility of this platform technology using leukemia inhibitory factor (LIF) and stem cell factor (SCF). Immobilized LIF supported mouse embryonic stem cell (mESC) pluripotency for at least 2 weeks in the absence of added diffusible LIF. Immobilized LIF activated signal transducer and activator of transcription 3 (STAT3) and mitogen-activated protein kinase (MAPK) signaling in a dose-dependent manner. The introduced method allows for the robust investigation of cell fate responses from interface-immobilized ligands.

Heparin intercalation into reconstituted collagen I fibrils: Impact on growth kinetics and morphology

Collagen type I fibrils, reconstituted in vitro in the presence of heparin, exhibit an unusually thick and straight shape. A detailed structural analysis by scanning force and scanning electron microscopy revealed a non-linear dependence in size distribution, width-to-length ratio, and morphology over a wide range of glycosaminoglycan (GAG) concentrations. By varying molecular weight, degree of sulphation, charge, and concentration of different GAGs we are able to correlate the morphological data with kinetic turbidimetric measurements, and quantitation of fibril-bound GAG. The experiments imply a pronounced impact of the prenucleation phase on the cofibril morphology as a result of the strong electrostatic interaction of heparin with tropocollagen. Heparin is assumed to stabilize the collagen microfibrils and to enhance their parallel accretion during cofibrillogenesis with preservation of the typical asymmetric collagen banding pattern. The heparin quantitation data show heparin to be intercalated as a linker molecule with one specific binding site inside the cofibrils. The reconstituted cofibrils with their unusual morphology and GAG intercalation-a phenomenon not reported in vivo-can be expected to exhibit interesting mechanical and biochemical behaviours as a biomaterial for extracellular matrix scaffolds.

Electrostatic interactions modulate the conformation of collagen I

The pH- and electrolyte-dependent charging of collagen I fibrils was analyzed by streaming potential/streaming current experiments using the Microslit Electrokinetic Setup. Differential scanning calorimetry and circular dichroism spectroscopy were applied in similar electrolyte solutions to characterize the influence of electrostatic interactions on the conformational stability of the protein. The acid base behavior of collagen I was found to be strongly influenced by the ionic strength in KCl as well as in CaCl(2) solutions. An increase of the ionic strength with KCl from 10(-4) M to 10(-2) M shifts the isoelectric point (IEP) of the protein from pH 7.5 to 5.3. However, a similar increase of the ionic strength in CaCl(2) solutions shifts the IEP from 7.5 to above pH 9. Enhanced thermal stability with increasing ionic strength was observed by differential scanning calorimetry in both electrolyte systems. In line with this, circular dichroism spectroscopy results show an increase of the helicity with increasing ionic strength. Better screening of charged residues and the formation of salt bridges are assumed to cause the stabilization of collagen I with increasing ionic strength in both electrolyte systems. Preferential adsorption of hydroxide ions onto intrinsically uncharged sites in KCl solutions and calcium binding to negatively charged carboxylic acid moieties in CaCl(2) solutions are concluded to shift the IEP and influence the conformational stability of the protein.

Gene-expression profiling of CD34+ hematopoietic cells expanded in a collagen I matrix

CD34+ hematopoietic stem/progenitor cells (HSCs) reside in the bone marrow in close proximity to the endosteal bone surface, surrounded by osteoblasts, stromal cells, and various extracellular matrix molecules. We used a bioartificial matrix of fibrillar collagen I, the major matrix component of bone, as a scaffold for ex vivo expansion of HSCs. CD34+ HSCs were isolated from umbilical cord blood and cultivated within reconstituted collagen I fibrils in the presence of fms-like tyrosine kinase-3 ligand, stem cell factor, and interleukin (IL)-3. After 7 days of culture, the cell number, number of colony-forming units (CFU-C), and gene-expression profile of the cultured cells were assessed. Although the total expansion factor of CD34+ cells was slightly lower when cells were cultivated in the collagen I gel, the frequency of CFU-C was greater than in control suspension cultures. Gene-expression analysis with microarray chip technology revealed the upregulation of more than 50 genes in the presence of collagen I. Among these, genes for several growth factors, cytokines, and chemokines (e.g., IL-8 and macrophage inhibitory protein 1alpha) could be confirmed using quantitative polymerase chain reaction. Furthermore, greater expression levels of the negative cell-cycle regulator BTG2/TIS21 and an inhibitor of the mitogen-activated protein kinase pathway, DUSP2, underline the regulatory role of the extracellular matrix. Together, these data show that the expansion of CD34+ cord blood cells in a culture system containing a three-dimensional collagen I matrix induces a qualitative change in the gene-expression profile of cultivated HSCs.

Immobilization of an anticoagulant benzamidine derivative: effect of spacer arms and carrier hydrophobicity on thrombin binding

Prevention of blood coagulation is very often a prerequisite for successful medical devices. For that purpose, passivation of the key coagulation enzyme thrombin through the derivatization of the material's surface with an amidine-based molecule has been found to be promising. To further enhance the efficiency of this approach, thin layers of maleic anhydride copolymers offering different physico-chemical characteristics were tethered with carboxyl terminated polyethylene glycol to covalently immobilize a benzamidine-type derivative. The free carboxyl surface groups produced by the attachment of polyethylene glycol (PEG) were quantified by Ag(+) labeling and subsequent XPS detection. The film thickness as well as the carboxyl group content were found to be clearly dependent on the copolymer hydrophobicity and the nature of the PEG molecule. For the assessment of the anchorage of the thrombin to the benzamidine-derivative functionalized surfaces, the substrates were immersed in a buffered thrombin solution and the enzyme adsorption was studied using immunostaining/confocal laser scanning microscopy. Higher degrees of thrombin binding were observed for substrates configured with the hydrophilic compared to the more hydrophobic copolymer. Moreover, surface-bound spacers based on alpha,omega-heterobifunctional PEG amino acids (alphaAm,omegaAc-PEG) also enhanced the benzamidine surface density in comparison to homofunctional PEG diacids (alphaAc,omegaAc-PEG) because of a lower degree of carboxyl inactivation due to PEG 'bridging'. Altogether, the choice of copolymer coatings and the type of PEG spacers were demonstrated to enhance the efficiency of the thrombin scavenging by the covalently immobilized coagulation inhibitor.

Fibrillar collagen assembled in the presence of glycosaminoglycans to constitute bioartificial stem cell niches in vitro

Fibrillar collagen was reconstituted from mixtures of monomeric tropocollagen and heparin or hyaluronic acid, respectively. Turbidity measurements were utilized to follow the fibrillar assembly and demonstrated the influence of the concentration of the glycosaminoglycan on the maximum optical densities. Thin film coatings of maleic anhydride copolymers were utilized for the covalent immobilization of the fibrillar assemblies to solid supports. Quantification of surface-bound collagen was accomplished by ellipsometry and HPLC-based amino acid analysis indicating that less collagen was immobilized in the presence of the glycosaminoglycans. SEM and AFM revealed various sizes and shapes of the immobilized fibrillar assemblies if collagen fibrils were prepared in the presence of heparin or hyaluronic acid. Human hematopoietic stem cells (HSCs) were cultivated on the surface-bound collagen fibrils and the migration of adherent cells was studied by time-lapse microscopy. Migration rates on fibrillar structures were significantly lower then on tropocollagen indicating a more intimate contact of HSCs to the fibrillar substrates.

Fibronectin displacement at polymer surfaces

The interactions of fibronectin with thin polymer films are studied in displacement experiments using human serum albumin. Fibronectin adsorption and exchange on two different maleic anhydride copolymer surfaces differing in hydrophobicity and surface charge density have been analyzed by quartz crystal microbalance and laser scanning microscopy with respect to adsorbed amounts, viscoelastic properties, and conformation. Fibronectin is concluded to become attached onto hydrophilic surfaces as a "softer", less rigid protein layer, in contrast to the more rigid, densely packed layer on hydrophobic surfaces. As a result, the fibronectin conformation is more distorted on the hydrophobic substrates together with remarkably different displacement characteristics in dependence on the adsorbed fibronectin surface concentration and the displacing albumin solution concentration. While the displacement kinetic remains constant for the strongly interacting surface, an acceleration in fibronectin exchange is observed for the weakly interacting surface with increasing fibronectin coverage. For displaced amounts, no change is determined for the hydrophobic substrate, in contrast to the hydrophilic substrate with a decrease of fibronectin exchange with decreasing coverage leading finally to a constant nondisplaceable amount of adsorbed proteins. Furthermore, the variation of the albumin exchange concentration reveals a stronger dependence of the kinetic for the weakly interacting substrate with higher rates at higher albumin concentrations.

Covalently immobilized thrombomodulin inhibits coagulation and complement activation of artificial surfaces in vitro

Thrombomodulin (TM) serves as the endothelial cell receptor for thrombin and alters its characteristics from pro- to anticoagulant. Additionally, it promotes the formation of activated protein C. We evaluated the conservation of the overall outcome of these functions in recombinant TM linked to artificial surfaces by incubation with human whole blood in vitro. TM was covalently immobilized through poly(ethylene glycol) (PEG) spacers onto thin films of poly(octadecene alt maleic anhydride) covering planar glass substrates. TM binding to the polymer films was achieved after active ester formation at the carboxylic acid terminus of the PEG spacers and thoroughly characterized by HPLC-based amino acid analysis, immunofluorescence and ellipsometry. TM-coated samples were incubated for 3h with freshly drawn whole human blood anticoagulated with heparin (5IU/ml) using in-house developed incubation systems. The substantially reduced activation of blood coagulation (TAT) for TM-coated samples correlates well with the degree of contact activation (bradykinin and FXIIa formation) while no significant effects were observed for the platelet activation (PF4). Further, complement activation (C5a levels), was strongly diminished at the TM-containing surfaces. We conclude that the suggested method for preparation of TM immobilization may serve to prepare model substrates for studies on TM interactions but similarly provides a promising coating strategy for blood contacting medical devices.

In vitro blood compatibility of polymeric biomaterials through covalent immobilization of an amidine derivative

We present a surface coating with anticoagulant characteristics showing significantly reduced coagulation activation. The synthesis of a monomeric conjugate containing a benzamidine moiety was carried out and its inhibitory activity against human thrombin, the key enzyme of the blood coagulation cascade, was determined using a chromogenic assay. Based on that, low-thrombogenic interfaces were prepared by covalent attachment of this low-molecular weight thrombin inhibitor on poly(octadecene-alt-maleic anhydride) copolymer thin films and characterized using ellipsometry, XPS and dynamic contact angle measurements. The in vitro hemocompatibility tests using freshly drawn human whole blood showed, in agreement with the SEM images, that a PO-MA film modified with a benzamidine moiety using a PEG spacer decreased the activation of coagulation, platelets and the complement system. The decreased protein adsorption, in addition to the specific inhibition of thrombin, effectively enhanced the short-term hemocompatibility characteristics.

In Vitro Reconstitution of Fibrillar Collagen Type I Assemblies at Reactive Polymer Surfaces

The reconstitution of fibrillar collagen and its assemblies with heparin and hyaluronic acid was studied in vitro. Fibril formation kinetics were analyzed by turbidity and depletion measurements in solutions containing varied concentrations of collagen and glycosaminoglycans. Fibril-forming collagen solutions were further applied for the coating of planar substrates which had been modified with alternating maleic anhydride copolymer films before. The immobilized collagen assemblies were characterized with respect to the deposited amount of protein using ellipsometry and acidic hydrolysis/HPLC-based amino acid analysis, respectively. AFM, SEM, and cLSM were utilized to gain information on structural features and patterns formed by surface-attached fibrils depending on the initial solution concentrations of collagen. The results revealed that the addition of heparin and hyaluronic acid affected both the fibril dimensions and the meshwork characteristics of the surface-bound fibrils.

Fibronectin anchorage to polymer substrates controls the initial phase of endothelial cell adhesion

Early stages of the adhesion of human endothelial cells onto a set of smooth polymer films were analyzed to reveal the modulation of cell-matrix interactions by the physicochemical constraints of predeposited fibronectin (FN). Hydrophobic and hydrophilic polymer substrates, consisting of poly(octadecene-alt-maleic anhydride) and poly(propene-alt-maleic anhydride) films, were coated with similar amounts of FN at conditions of either covalent or noncovalent immobilization. The well-defined substrates permit variation of the anchorage of FN at invariant topography, pliability, and molecular composition. Although all of the compared FN coatings were effective in stimulating attachment of endothelial cells, the initial formation of cell-matrix adhesions was found to be controlled by the type of interaction between predeposited FN and the underlying substrate. Covalent linkage and hydrophobic interactions of the predeposited FN with the polymer films interfered with the rapid generation of focal and fibrillar adhesions. It was demonstrated that this was caused by the fact that only weakly bound FN could become readily reorganized by the adherent cells. Upon prolonged culture periods at standard cell culture conditions, secretion and deposition of organized extracellular matrix by the attached cells was found to balance out the differences of the substrates.

Control of fibronectin displacement on polymer substrates to influence endothelial cell behaviour

Thin layered biomaterial surfaces of maleic anhydride copolymers are provided as a versatile platform for biomaterial applications. The provided comonomers define the character of the surface and its behaviour towards biomolecules and biosystems, such as proteins and cells. The kinetics of adsorption, desorption, and exchange of fibronectin and human serum albumin were investigated on different copolymer surfaces. Two different species of adsorbed proteins were found, a fast and a slow desorbing one. Furthermore, the exchange process depends on the kind of pre-adsorbed protein and the kind of exchange protein, as well as of the hydrophobicity of the copolymer surface. In this context adhesion, proliferation, and differentiation of endothelial cells from the umbilical cord vein onto fibronectin pre-coated surfaces were studied. Strong correlation between fibronectin exchange characteristics and the formation of focal adhesions, reorganisation of fibronectin, and generation of vascular-like structures by the cells was observed.

Dynamic alterations of fibronectin layers on copolymer substrates with graded physicochemical characteristics

Desorption and exchange of preadsorbed fibronectin layers in pure buffer solution and solutions of human serum albumin or fibronectin, respectively, were studied in dependence on the physicochemical characteristics of maleic acid copolymer films used as substrates. Although the preadsorbed amount of fibronectin differed only slightly, the protein was found to exhibit a significantly enhanced anchorage at the more hydrophobic polymer surface as compared to the more hydrophilic and more negatively charged polymer surface. The preadsorbed fibronectin layer was most efficiently exchanged by fibronectin (i.e., in the homodisplacement process) while pure buffer solution and human serum albumin solutions induced desorption or exchange of fibronectin to lower and similar degrees. An increase of the total adsorbed amount of protein due to additional adsorption of fibronectin or human serum albumin accompanied the partial exchange of the preadsorbed fibronectin in the displacement experiments. Evaluation of the kinetics of desorption and exchange of fibronectin at any of the substrates revealed two kinds of surface-attached protein populations--a fast desorbing species and a species with a slow desorption and exchange rate. By a multivariate regression analysis the surface characteristics of the polymer substrate were confirmed to determine the degree of protein desorption and exchange while the dynamics of the layer alteration was found to solely depend on the diffusion behavior of the proteins.

Comparison of flow cytometry and laser scanning cytometry for the analysis of CD34+hematopoietic stem cells

BACKGROUND

Characterization of hematopoietic stem cells (HSCs) by laser scanning cytometry (LSC) was compared with conventional flow cytometry (FCM). The method was evaluated for application in the development of advanced cell culture substrates that were supposed to support the ex vivo expansion of HSC. For this purpose, adherent HSCs were grown in culture on thin polymer films coated with reconstituted collagen I fibrils and subsequently analyzed by LSC.

METHODS

CD34+ HSCs were isolated from cord blood by immunomagnetic separation and cultivated on polymer films coated with reconstituted collagen I fibrils. Cell surface antigens (CD34, CD29) were stained with antibodies, and nuclei were labeled with a DNA stain (TO-PRO-3 iodide) that does not interfere with the fluorochromes of the antibodies. Fluorescence intensity of the adherent cells was measured by means of LSC. Before and after in vitro expansion for time periods of up to 7 days, suspension cells were analyzed with LSC and FCM.

RESULTS

LSC-based analysis enabled reliable quantification of CD34+ cells with bright antigen expression before cell culture. At this stage, LSC and FCM data for CD34 expression at given HSC samples largely coincided. After in vitro expansion, LSC data deviated from FCM data for cells with dim CD34 antigen expression, whereas the fluorescence intensity of the CD29 antigen remained comparable. The deviation between LSC and FCM data for CD34dim was attributed to the better resolution of weak fluorescence by FCM. Based on the preceding evaluation of the method, LSC analysis could be applied to characterize HSCs cultivated on collagen I-coated polymer films without detachment of the cells from the substrate.

CONCLUSIONS

LSC-based analysis allows for the automated evaluation of adherent HSCs. Although resolution of weakly expressed antigens can be achieved more precisely with FCM, the method provides a valuable tool to study interactions of HSCs with bioartificial substrates.

Quantitative analysis of immobilized proteins and protein mixtures by amino acid analysis

Biomolecular surface engineering of materials often requires precise, versatile and efficient quantification of immobilized proteins at solid surfaces. Acidic hydrolysis of surface-bound proteins and subsequent HPLC analysis of fluorescence-derivatized amino acids were adapted and critically evaluated for that purpose. Contaminations and concentration-dependent amino acid retrieval during HPLC were found to influence the accuracy of the method. In addition to the choice of adequate conditions for hydrolysis, derivatization and chromatographic separation extensions of the data evaluation were suggested to improve the accuracy of the approach when applied to single protein systems: comparing the experimentally obtained amino acid ratio to the protein constitution enabled to identify the properly separated and detected amino acids. Those amino acids were selected for a more precise calculation of the amount of immobilized protein. To further increase the accuracy of the method, the retrieval of amino acids corresponding to protein amounts in the range between 0.5 and 4.0 microg was analyzed for a variety of proteins of interest to derive protein-specific correction factors. The evaluation of amino acid data was furthermore applied to quantify binary protein mixtures at similar settings. This method was proven useful to detect the composition of protein mixtures throughout a wide range of absolute and relative concentrations.

Maleic anhydride copolymers--a versatile platform for molecular biosurface engineering

A platform of thin polymer coatings was introduced for the functional modulation of immobilized bioactive molecules at solid/liquid interfaces. The approach is based on covalently attached alternating maleic acid anhydride copolymers with a variety of comonomers and extended through conversion of the anhydride moieties by hydrolysis, reaction with functional amines, and other conversions of the anhydride moieties. We demonstrate that these options permit control of the physicochemical constraints for bioactive molecules immobilized at interfaces to influence important performance characteristics of biofunctionalized materials for medical devices and molecular diagnostics. Examples concern the impact of the substrate-anchorage of fibronectin on the formation of cell-matrix adhesions, the orientation of endothelial cells according to lateral anti-adhesive micropatterns using grafted poly(ethylene oxide), and the spacer-dependent activity of immobilized synthetic thrombin inhibitors.

A TERMINAL FLOWER1-like gene from perennial ryegrass involved in floral transition and axillary meristem identity

Control of flowering and the regulation of plant architecture have been thoroughly investigated in a number of well-studied dicot plants such as Arabidopsis, Antirrhinum, and tobacco. However, in many important monocot seed crops, molecular information on plant reproduction is still limited. To investigate the regulation of meristem identity and the control of floral transition in perennial ryegrass (Lolium perenne) we isolated a ryegrass TERMINAL FLOWER1-like gene, LpTFL1, and characterized it for its function in ryegrass flower development. Perennial ryegrass requires a cold treatment of at least 12 weeks to induce flowering. During this period a decrease in LpTFL1 message was detected in the ryegrass apex. However, upon subsequent induction with elevated temperatures and long-day photoperiods, LpTFL1 message levels increased and reached a maximum when the ryegrass apex has formed visible spikelets. Arabidopsis plants overexpressing LpTFL1 were significantly delayed in flowering and exhibited dramatic changes in architecture such as extensive lateral branching, increased growth of all vegetative organs, and a highly increased trichome production. Furthermore, overexpression of LpTFL1 was able to complement the phenotype of the severe tfl1-14 mutant of Arabidopsis. Analysis of the LpTFL1 promoter fused to the UidA gene in Arabidopsis revealed that the promoter is active in axillary meristems, but not the apical meristem. Therefore, we suggest that LpTFL1 is a repressor of flowering and a controller of axillary meristem identity in ryegrass.

Functional identification of an Arabidopsis snf4 ortholog by screening for heterologous multicopy suppressors of snf4 deficiency in yeast

Yeast Snf4 is a prototype of activating gamma-subunits of conserved Snf1/AMPK-related protein kinases (SnRKs) controlling glucose and stress signaling in eukaryotes. The catalytic subunits of Arabidopsis SnRKs, AKIN10 and AKIN11, interact with Snf4 and suppress the snf1 and snf4 mutations in yeast. By expression of an Arabidopsis cDNA library in yeast, heterologous multicopy snf4 suppressors were isolated. In addition to AKIN10 and AKIN11, the deficiency of yeast snf4 mutant to grown on non-fermentable carbon source was suppressed by Arabidopsis Myb30, CAAT-binding factor Hap3b, casein kinase I, zinc-finger factors AZF2 and ZAT10, as well as orthologs of hexose/UDP-hexose transporters, calmodulin, SMC1-cohesin and Snf4. Here we describe the characterization of AtSNF4, a functional Arabidopsis Snf4 ortholog, that interacts with yeast Snf1 and specifically binds to the C-terminal regulatory domain of Arabidopsis SnRKs AKIN10 and AKIN11.

A conserved domain of the arabidopsis GNOM protein mediates subunit interaction and cyclophilin 5 binding

The Arabidopsis GNOM protein, a guanine nucleotide exchange factor (GEF) that acts on ADP ribosylation factor (ARF)-type G proteins, is required for coordination of cell polarity along the apical-basal embryo axis. Interallelic complementation of gnom mutants suggested that dimerization is involved in GNOM function. Here, direct interaction between GNOM molecules is demonstrated in vitro and by using a yeast two-hybrid system. Interaction was confined to an N-terminal domain conserved within a subgroup of large ARF GEFs. The same domain mediated in vitro binding to cyclophilin 5 (Cyp5), which was identified as a GNOM interactor in two-hybrid screening. Cyp5 displayed peptidylprolyl cis/trans-isomerase and protein refolding activities that were sensitive to cyclosporin A. Cyp5 protein accumulated in several plant organs and, like GNOM, was partitioned between cytosolic and membrane fractions. Cyp5 protein was also expressed in the developing embryo. Our results suggest that Cyp5 may regulate the ARF GEF function of the GNOM protein during embryogenesis.

Regulatory interaction of PRL1 WD protein with Arabidopsis SNF1-like protein kinases

Mutation of the PRL1 gene, encoding a regulatory WD protein, results in glucose hypersensitivity and derepression of glucose-regulated genes in Arabidopsis. The yeast SNF1 protein kinase, a key regulator of glucose signaling, and Arabidopsis SNF1 homologs AKIN10 and AKIN11, which can complement the Deltasnf1 mutation, were found to interact with an N-terminal domain of the PRL1 protein in the two-hybrid system and in vitro. AKIN10 and AKIN11 suppress the yeast Deltasnf4 mutation and interact with the SNF4p-activating subunit of SNF1. PRL1 and SNF4 bind independently to adjacent C-terminal domains of AKIN10 and AKIN11, and these protein interactions are negatively regulated by glucose in yeast. AKIN10 and AKIN11, purified in fusion with glutathione S-transferase, undergo autophosphorylation and phosphorylate a peptide of sucrose phosphate synthase in vitro. The sucrose phosphate synthase-peptide kinase activity of AKIN complexes detected by immunoprecipitation is stimulated by sucrose in light-grown Arabidopsis plants. In comparison with wild type, the activation level of AKIN immunocomplexes is higher in the prl1 mutant, suggesting that PRL1 is a negative regulator of Arabidopsis SNF1 homologs. This conclusion is supported by the observation that PRL1 is an inhibitor of AKIN10 and AKIN11 in vitro.

Pleiotropic control of glucose and hormone responses by PRL1, a nuclear WD protein, in Arabidopsis

The prl1 mutation localized by T-DNA tagging on Arabidopsis chromosome 4-44 confers hypersensitivity to glucose and sucrose. The prl1 mutation results in transcriptional derepression of glucose responsive genes defining a novel suppressor function in glucose signaling. The prl1 mutation also augments the sensitivity of plants to growth hormones including cytokinin, ethylene, abscisic acid, and auxin; stimulates the accumulation of sugars and starch in leaves; and inhibits root elongation. PRL1 encodes a regulatory WD protein that interacts with ATHKAP2, an alpha-importin nuclear import receptor, and is imported into the nucleus in Arabidopsis. Potential functional conservation of PRL1 homologs found in other eukaryotes is indicated by nuclear localization of PRL1 in monkey COS-1 cells and selective interaction of PRL1 with a nuclear protein kinase C-betaII isoenzyme involved in human insulin signaling.

Control of cell elongation and stress responses by steroid hormones and carbon catabolic repression in plants

Molecular analysis of Arabidopsis mutants displaying hypocotyl elongation defects in both the dark and light revealed recently that steroids play an essential role as hormones in plants. Deficiencies in brassinosteroid biosynthesis and signalling permit photomorphogenic development and light-regulated gene expression in the dark, and result in severe dwarfism, male sterility and de-repression of stress-induced genes in the light. A cytochrome P450 steroid hydroxylase (CYP90) controls a rate limiting step in brassinosteroid biosynthesis and appears to function as a signalling factor in stress responses. Another key step in steroid biosynthesis is controlled by the Arabidopsis SNF1 kinases that phosphorylate the 3-hydroxy-3methylglutaryl-CoA reductase. The activity of SNF1 kinases is regulated by PRL1, an evolutionarily conserved alpha-importin-binding nuclear WD-protein. The prl1 mutation results in cell elongation defects, de-repression of numerous stress-induced genes, and augments the sensitivity of plants to glucose, cold stress and several hormones, including cytokinin, ethylene, auxin, and abscisic acid.