Modulating chitin synthesis in marine algae with iminosugars obtained by SmI2 and FeCl3-mediated diastereoselective carbonyl ene reaction

Strategies for synthesizing polyhydroxylated piperidines such as iminosugars have received broad attention. These substances are known to interact with carbohydrate related enzymes glycosidases and glycosyltransferases, to which also the large...

Mineralized scale patterns on the cell periphery of the chrysophyte Mallomonas determined by comparative 3D Cryo-FIB SEM data processing

Unicellular protists can biomineralize spatially complex and functional shells. A typical cell of the photosynthetic synurophyte Mallomonas is covered by about 60-100 silica scales. Their geometric arrangement, the so-called scale case, mainly depends on the species and on the cell cycle. In this study, the scale case of the synurophyte Mallomonas was preserved in aqueous suspension using high-pressure freezing (HPF). From this specimen, a three-dimensional (3D) data set spanning a volume of about 25.6 μm × 19.2 μm × 4.2 μm with a voxel size of 12.5 nm × 12.5 nm × 25.0 nm was collected by Cryo-FIB SEM in 3 h and 24 min. SEM imaging using In-lens SE detection allowed to clearly differentiate between mineralized, curved scales of less than 0.2 μm thickness and organic cellular ultrastructure or vitrified ice. The three-dimensional spatial orientations and shapes of a minimum set of scales (N = 13) were identified by visual inspection, and manually segmented. Manual and automated segmentation approaches were comparatively applied to one arbitrarily selected reference scale using the differences in grey level between scales and other constituents. Computational automated routines and principal component analysis of the experimentally extracted data created a realistic mathematical model based on the Fibonacci pattern theory. A complete in silico scale case of Mallomonas was reconstructed showing an optimized scale coverage on the cell surface, similarly as it was observed experimentally. The minimum time requirements from harvesting the living cells to the final scale case determination by Cryo-FIB SEM and computational image processing are discussed.

Thermal decomposition of the amino acids glycine, cysteine, aspartic acid, asparagine, glutamic acid, glutamine, arginine and histidine

Background

The pathways of thermal instability of amino acids have been unknown. New mass spectrometric data allow unequivocal quantitative identification of the decomposition products.

Results

Calorimetry, thermogravimetry and mass spectrometry were used to follow the thermal decomposition of the eight amino acids G, C, D, N, E, Q, R and H between 185 °C and 280 °C. Endothermic heats of decomposition between 72 and 151 kJ/mol are needed to form 12 to 70% volatile products. This process is neither melting nor sublimation. With exception of cysteine they emit mainly H₂O, some NH₃ and no CO₂. Cysteine produces CO₂ and little else. The reactions are described by polynomials, AA→a NH₃+b H₂O+c CO₂+d H₂S+e residue, with integer or half integer coefficients. The solid monomolecular residues are rich in peptide bonds.

Conclusions

Eight of the 20 standard amino acids decompose at well-defined, characteristic temperatures, in contrast to commonly accepted knowledge. Products of decomposition are simple. The novel quantitative results emphasize the impact of water and cyclic condensates with peptide bonds and put constraints on hypotheses of the origin, state and stability of amino acids in the range between 200 °C and 300 °C.

Aqueous ball milling of nacre constituents facilitates directional self-assembly of aragonite nanoparticles of the gastropod Haliotis glabra

A ball-milling approach was developed to investigate the constituents of isolated nacre tablets of the gastropod Haliotis glabra in aqueous suspension without additional chemical additives. The obtained particle mixtures were characterized using X-ray crystallography as well as scanning and transmission electron microscopy. Aragonite nanoparticles retained their crystal structure even after 14 h of ball milling. The long-term stability of the particle mixtures varied as a function of the ball-milling duration. An increased milling time led to rod-like stable assemblies of aragonite nanoparticles. Selected area electron diffraction investigations revealed that the longitudinal axes in about one-third of these nanoparticle rods were oriented along the crystallographic c-axis of aragonite, indicating oriented attachment of the aragonite nanoparticles. These in vitro observations support the idea that a two-stage process, separated into crystallization of nanoparticles and oriented assembly of nanocrystals, could also occur in vivo.

Biogenic and Synthetic Peptides with Oppositely Charged Amino Acids as Binding Sites for Mineralization

Proteins regulate diverse biological processes by the specific interaction with, e.g., nucleic acids, proteins and inorganic molecules. The generation of inorganic hybrid materials, such as shell formation in mollusks, is a protein-controlled mineralization process. Moreover, inorganic-binding peptides are attractive for the bioinspired mineralization of non-natural inorganic functional materials for technical applications. However, it is still challenging to identify mineral-binding peptide motifs from biological systems as well as for technical systems. Here, three complementary approaches were combined to analyze protein motifs consisting of alternating positively and negatively charged amino acids: (i) the screening of natural biomineralization proteins; (ii) the selection of inorganic-binding peptides derived from phage display; and (iii) the mineralization of tobacco mosaic virus (TMV)-based templates. A respective peptide motif displayed on the TMV surface had a major impact on the SiO₂ mineralization. In addition, similar motifs were found in zinc oxide- and zirconia-binding peptides indicating a general binding feature. The comparative analysis presented here raises new questions regarding whether or not there is a common design principle based on acidic and basic amino acids for peptides interacting with minerals.

Datasets from a vapor diffusion mineral precipitation protocol for Dictyostelium stalks

Datasets from a slow carbonate vapor diffusion and mineral precipitation protocol for Dictyostelium ECM and cellulose stalks show examples for composite materials obtained by an in vitro approach, which differs substantially from the in vivo approach reported in The Journal of Structural Biology, doi: 10.1016/j.jsb.2016.03.015 [1]. Methods for obtaining the datasets include bright field transmitted light microscopy, fluorescence microscopy, LC-PolScope birefringence microscopy, variable pressure scanning electron microscopy (VP-SEM/ESEM), and Raman imaging spectroscopy.

In vivo modified organic matrix for testing biomineralization-related protein functions in differentiated Dictyostelium on calcite

This work reports an in vivo approach for identifying the function of biomineralization-related proteins. Synthetic sequences of n16N, OC-17 and perlucin with signal peptides are produced in a novel Gateway expression system for Dictyostelium under the control of the [ecmB] promoter. A fast and easy scanning electron microscopic screening method was used to differentiate on the colony level between interplay effects of the proteins expressed in the extracellular matrix (ECM). Transformed Dictyostelium, which migrated as multicellular colonies on calcite crystals and left their ECM remnants on the surface were investigated also by energy-dispersive X-ray spectroscopy (EDX). Calcium minerals with and without phosphorous accumulated very frequently within the matrix of the Dictyostelium colonies when grown on calcite. Magnesium containing phosphorous granules were observed when colonies were exposed on silica. The absence of calcium EDX signals in these cases suggests that the external calcite crystals but not living cells represent the major source of calcium in the ECM. Several features of the system provide first evidence that each protein influences the properties of the matrix in a characteristic mode. Colonies transformed with perlucin produced a matrix with cracks on the length scale of a few microns throughout the matrix patch. For colonies with OC-17, almost no cracks were observed, regardless of the length scale. The non-transformed Dictyostelium (Ax3-Orf+) produced larger cracks. The strategy presented here develops the first step toward an efficient eukaryotic screening system for the combinatorial functionalization of materials by bioengineering in close analogy to natural biomineralization concepts.

Real-time monitoring of calcium carbonate and cationic peptide deposition on carboxylate-SAM using a microfluidic SAW biosensor

A microfluidic biosensor with surface acoustic wave technology was used in this study to monitor the interaction of calcium carbonate with standard carboxylate self-assembled monolayer sensor chips. Different fluids, with and without biomolecular components, were investigated. The pH-dependent surface interactions of two bio-inspired cationic peptides, AS8 and ES9, which are similar to an extracellular domain of the chitin synthase involved in mollusc shell formation, were also investigated in a biological buffer system. A range of experimental conditions are described that are suitable to study non-covalent molecular interactions in the presence of ionic substances, such as, mineral precursors below the solubility equilibrium. The peptide ES9, equal to the mollusc chitin synthase epitope, is less sensitive to changes in pH than its counterpart AS8 with a penta-lysine core, which lacks the flanking acidic residues. This study demonstrates the extraordinary potential of microfluidic surface acoustic wave biosensors to significantly expand our experimental capabilities for studying the principles underlying biomineralization in vitro.

On the function of chitin synthase extracellular domains in biomineralization

Molluscs with various shell architectures evolved around 542-525 million years ago, as part of a larger phenomenon related to the diversification of metazoan phyla. Molluscs deposit minerals in a chitin matrix. The mollusc chitin is synthesized by transmembrane enzymes that contain several unique extracellular domains. Here we investigate the assembly mechanism of the chitin synthase Ar-CS1 via its extracellular domain ArCS1_E22. The corresponding transmembrane protein ArCS1_E22TM accumulates in membrane fractions of the expression host Dictyostelium discoideum. Soluble recombinant ArCS1_E22 proteins can be purified as monomers only at basic pH. According to confocal fluorescence microscopy experiments, immunolabeled ArCS1_E22 proteins adsorb preferably to aragonitic nacre platelets at pH 7.75. At pH 8.2 or pH 9.0 the fluorescence signal is less intense, indicating that protein-mineral interaction is reduced with increasing pH. Furthermore, ArCS1_E22 forms regular nanostructures on cationic substrates as revealed by atomic force microscopy (AFM) experiments on modified mica cleavage planes. These experiments suggest that the extracellular domain ArCS1_E22 is involved in regulating the multiple enzyme activities of Ar-CS1 such as chitin synthesis and myosin movements by interaction with mineral surfaces and eventually by protein assembly. The protein complexes could locally probe the status of mineralization according to pH unless ions and pCO2 are balanced with suitable buffer substances. Taking into account that the intact enzyme could act as a force sensor, the results presented here provide further evidence that shell formation is coordinated physiologically with precise adjustment of cellular activities to the structure, topography and stiffness at the mineralizing interface.

GFP facilitates native purification of recombinant perlucin derivatives and delays the precipitation of calcium carbonate

Insolubility is one of the possible functions of proteins involved in biomineralization, which often limits their native purification. This becomes a major problem especially when recombinant expression systems are required to obtain larger amounts. For example, the mollusc shell provides a rich source of unconventional proteins, which can interfere in manifold ways with different mineral phases and interfaces. Therefore, the relevance of such proteins for biotechnological processes is still in its infancy. Here we report a simple and reproducible purification procedure for a GFP-tagged lectin involved in biomineralization, originally isolated from mother-of-pearl in abalone shells. An optimization of E. coli host cell culture conditions was the key to obtain reasonable yields and high degrees of purity by using simple one-step affinity chromatography. We identified a dual functional role for the GFP domain when it became part of a mineralizing system in vitro. First, the GFP domain improved the solubility of an otherwise insoluble protein, in this case recombinant perlucin derivatives. Second, GFP inhibited calcium carbonate precipitation in a concentration dependent manner. This was demonstrated here using a simple bulk assay over a time period of 400 seconds. At concentrations of 2 µg/ml and higher, the inhibitory effect was observed predominantly for HCO(3) (-) as the first ionic interaction partner, but not necessarily for Ca(2+). The interference of GFP-tagged perlucin derivatives with the precipitation of calcium carbonate generated different types of GFP-fluorescent composite calcite crystals. GFP-tagging offers therefore a genetically tunable tool to gently modify mechanical and optical properties of synthetic biocomposite minerals.

Hierarchical super-structure identified by polarized light microscopy, electron microscopy and nanoindentation: Implications for the limits of biological control over the growth mode of abalone sea shells

Background

Mollusc shells are commonly investigated using high-resolution imaging techniques based on cryo-fixation. Less detailed information is available regarding the light-optical properties. Sea shells of Haliotis pulcherina were embedded for polishing in defined orientations in order to investigate the interface between prismatic calcite and nacreous aragonite by standard materialographic methods. A polished thin section of the interface was prepared with a defined thickness of 60 μm for quantitative birefringence analysis using polarized light and LC-PolScope microscopy. Scanning electron microscopy images were obtained for comparison. In order to study structural-mechanical relationships, nanoindentation experiments were performed.

Results

Incident light microscopy revealed a super-structure in semi-transparent regions of the polished cross-section under a defined angle. This super-structure is not visible in transmitted birefringence analysis due to the blurred polarization of small nacre platelets and numerous organic interfaces. The relative orientation and homogeneity of calcite prisms was directly identified, some of them with their optical axes exactly normal to the imaging plane. Co-oriented "prism colonies" were identified by polarized light analyses. The nacreous super-structure was also visualized by secondary electron imaging under defined angles. The domains of the super-structure were interpreted to consist of crystallographically aligned platelet stacks. Nanoindentation experiments showed that mechanical properties changed with the same periodicity as the domain size.

Conclusions

In this study, we have demonstrated that insights into the growth mechanisms of nacre can be obtained by conventional light-optical methods. For example, we observed super-structures formed by co-oriented nacre platelets as previously identified using X-ray Photo-electron Emission Microscopy (X-PEEM) [Gilbert et al., Journal of the American Chemical Society 2008, 130:17519–17527]. Polarized optical microscopy revealed unprecedented super-structures in the calcitic shell part. This bears, in principle, the potential for in vivo studies, which might be useful for investigating the growth modes of nacre and other shell types.

Functional expression of Ca²⁺ dependent mammalian transmembrane gap junction protein Cx43 in slime mold Dictyostelium discoideum

In this paper, we expressed murine gap junction protein Cx43 in Dictyostelium discoideum by introducing the specific vector pDXA. In the first step, the successful expression of Cx43 and Cx43-eGFP was verified by (a) Western blot (anti-Cx43, anti-GFP), (b) fluorescence microscopy (eGFP-Cx43 co-expression, Cx43 immunostaining), and (c) flow cytometry analysis (eGFP-Cx43 co-expression). Although the fluorescence signals from cells expressing Cx43-eGFP detected by fluorescence microscopy seem relatively low, analysis by flow cytometry demonstrated that more than 60% of cells expressed Cx43-eGFP. In order to evaluate the function of expressed Cx43 in D. discoideum, we examined the hemi-channel function of Cx43. In this series of experiments, the passive uptake of carboxyfluorescein was monitored using flow cytometric analysis. A significant number of the transfected cells showed a prominent dye uptake in the absence of Ca(2+). The dye uptake by transfected cells in the presence of Ca(2+) was even lower than the non-specific dye uptake by non-transformed Ax3 orf+ cells, confirming that Cx43 expressed in D. discoideum retains its Ca(2+)-dependent, specific gating function. The expression of gap junction proteins expressed in slime molds opens a possibility to the biological significance of intercellular communications in development and maintenance of multicellular organisms.

Calcium and silicon mineralization in land plants: transport, structure and function

Plant biomineralization involves calcium and silicon transport and mineralization. Respective analytical methods and case studies are listed. Calcium carbonate is deposited in cystoliths, calcium oxalate in idioblasts. Silicon is deposited in phytoliths. Biomineralization is a coordinated process.

Non-invasive LC-PolScope imaging of biominerals and cell wall anisotropy changes

The formation of defined shapes by cells is one of the challenging questions in biology. Due to the anisotropy of cell walls and of certain biominerals, the LC-PolScope represents a promising tool for tracking dynamic structural changes in vivo non-invasively and, to some extent, quantitatively. A complex three-dimensional biogenic system, the in vitro precipitation of calcium oxalate induced by cellulose stalks produced by Dictyostelium discoideum, was analyzed. Although the retardance values and orientation of the crystals with respect to the stalk were quickly and easily detected, this study raised a number of issues that were addressed in this work. The effect of the refractive index of the embedding medium was examined by taking advantage of the homogeneous size and shape distribution of kiwifruit raphides, a biologically controlled calcium oxalate biomineral and of cotton (Gossypium) seed fibers. The retardance remained consistent when embedding these samples in media with increasing refractive indices from 1.33 to 1.42 or 1.47 for sucrose or glycerol gradients, respectively. The general applicability of LC-PolScope image processing for biominerals and cell wall formation during development in vivo was demonstrated in a particular group of green algae, the Desmidiaceae. Various organization levels of the cell wall were identified, thus confirming earlier findings based on electron microscopy and immunostaining investigations. It can be concluded that LC-PolScope microscopy is an attractive tool for studying dynamic ordering of biomolecules, such as plant cell walls, when additional parameters regarding the structure, composition, and refractive indices of the specimen are available.

The peacock's train (Pavo cristatus and Pavo cristatus mut. alba) I. structure, mechanics, and chemistry of the tail feather coverts

The feathers in the train of the peacock serve not for flying but for sexual display. They are long, slender beams loaded in bending by their own weight. An outer circular conical shell, the cortex, is filled by a closed foam of 7.6% relative density, the medulla, both of feather keratin. Outer diameter and thickness of the cortex decrease linearly from the body toward the tip. This self-similar geometry leads to a division of labor. The cortex (longitudinal Young's modulus 3.3 GPa, transverse modulus 1 GPa) provides 96% of the longitudinal strength and bending rigidity of the feather. The medulla (Young's modulus 10 MPa) provides 96% of the transverse compressive rigidity. Fracture stress of the cortex, both longitudinal and transverse, is 120 MPa.

Keratin homogeneity in the tail feathers of Pavo cristatus and Pavo cristatus mut. alba

The keratin structure in the cortex of peacocks’ feathers is studied by X-ray diffraction along the feather, from the calamus to the tip. It changes considerably over the first 5 cm close to the calamus and remains constant for about 1 m along the length of the feather. Close to the tip, the structure loses its high degree of order. We attribute the X-ray patterns to a shrinkage of a cylindrical arrangement of β-sheets, which is not fully formed initially. In the final structure, the crystalline beta-cores are fixed by the rest of the keratin molecule. The hydrophobic residues of the beta-core are locked into a zip-like arrangement. Structurally there is no difference between the blue and the white bird.

The structure of mollusc larval shells formed in the presence of the chitin synthase inhibitor Nikkomycin Z

BACKGROUND

Chitin self-assembly provides a dynamic extracellular biomineralization interface. The insoluble matrix of larval shells of the marine bivalve mollusc Mytilus galloprovincialis consists of chitinous material that is distributed and structured in relation to characteristic shell features. Mollusc shell chitin is synthesized via a complex transmembrane chitin synthase with an intracellular myosin motor domain.

RESULTS

Enzymatic mollusc chitin synthesis was investigated in vivo by using the small-molecule drug NikkomycinZ, a structural analogue to the sugar donor substrate UDP-N-acetyl-D-glucosamine (UDP-GlcNAc). The impact on mollusc shell formation was analyzed by binocular microscopy, polarized light video microscopy in vivo, and scanning electron microscopy data obtained from shell material formed in the presence of NikkomycinZ. The partial inhibition of chitin synthesis in vivo during larval development by NikkomycinZ (5 microM - 10 microM) dramatically alters the structure and thus the functionality of the larval shell at various growth fronts, such as the bivalve hinge and the shell's edges.

CONCLUSION

Provided that NikkomycinZ mainly affects chitin synthesis in molluscs, the presented data suggest that the mollusc chitin synthase fulfils an important enzymatic role in the coordinated formation of larval bivalve shells. It can be speculated that chitin synthesis bears the potential to contribute via signal transduction pathways to the implementation of hierarchical patterns into chitin mineral-composites such as prismatic, nacre, and crossed-lamellar shell types.

Quantitative in Vitro Biopolymerization to Chitin in Native Chitosomal Membranes Supported by Silica Microparticles

To investigate the unknown physical mechanisms of chitin biosynthesis quantitatively, we designed a quantitative in vitro biopolymerization assay by deposition of native chitosomal membranes from Saccharomyces cerevisiae onto solid silica microparticles of a defined size (ø = 3 microm). The homogeneous coating of particle surfaces with native chitosomal membranes observed by confocal microscopy agrees well with the surface coverage calculated by the phosphate analysis. The amount of the synthesized chitin polymers is determined by radioactive assays, which demonstrate that chitin synthase in particle-supported membranes retains its specific enzymatic activity. In comparison to planar substrates, particle supports of defined size (and thus surface area) enable us to amplify the signals from immobilized proteins owing to the much larger surface area and to the capability of concentrating the sample to any given sample volume. Moreover, the large density of particle supports offers unique advantages over purified chitosomes in the quick separation of particle-supported membranes and materials in bulk within 1 min. This allows for the termination of the polymerization reaction without the disruption of the whole membranes, and thus the chitin polymers released in bulk can quantitatively be extracted. The obtained results demonstrate that the native biological membranes on particle supports can be utilized as a new in vitro biopolymerization assay to study the function of transmembrane enzyme complexes.

The chitin synthase involved in marine bivalve mollusk shell formation contains a myosin domain

Chitin is a key component in mollusk nacre formation. However, the enzyme complex responsible for chitin deposition in the mollusk shell remained unknown. We cloned and characterized the chitin synthase of the marine bivalve mollusk Atrina rigida. We present here the first chitin synthase sequence from invertebrates containing an unconventional myosin motor head domain. We further show that a homologous gene for chitin synthase is expressed in the shell forming tissue of larval Mytilus galloprovincialis even in early embryonic stages. The new data presented here are the first clear-cut indication for a functional role of cytoskeletal forces in the precisely controlled mineral deposition process of mollusk shell biogenesis.

The distribution of chitin in larval shells of the bivalve mollusk Mytilus galloprovincialis

The insoluble matrix of larval shells of the marine bivalve mollusk Mytilus galloprovincialis is investigated by confocal laser scanning microscopy using a GFP fusion protein with a chitin-binding domain for labeling of chitinous structures. We show that chitinous material is present in the larval shell, presumably as a chitin-protein complex. We further show that the structure of the chitinous material changes with the development of the larvae. We conclude from the presence of characteristic chitinous structures in certain shell regions that chitin fulfills an important function in the formation and functionality of larval bivalve shells.

Perlustrin, a Haliotis laevigata (abalone) nacre protein, is homologous to the insulin-like growth factor binding protein N-terminal module of vertebrates

The 84-amino-acid-long sequence of perlustrin showed homology of the abalone nacre protein to the N-terminal domain of mammalian insulin-like growth factor binding proteins (IGFBPs). Despite the evolutionary distance between mollusks and mammals, the sequence identity was 40% including 12 conserved cysteines. However, the residues which were suggested recently to bind IGF-II in a complex with IGFBP-5 were conserved only partially. Nevertheless, perlustrin bound human IGFs with K(D) approximately 10(-7) M. This was the same affinity range as measured before for the interaction of isolated IGFBP-5 N-terminal domains with IGFs. Moreover, perlustrin bound bovine insulin with only approximately two- to sevenfold lower affinity than IGFs. Sequence similarity and growth factor binding identified perlustrin unequivocally as a member of the IGFBP family, the first found in an invertebrate biomineral. Nacre is known to contain proteinaceous factors which promote bone formation in vitro and in vivo. Bone contains IGFBPs which influence bone metabolism in many ways by modulating either IGF effects or IGF independently. Thus, perlustrin may provide a first clue at the molecular level to what these two phylogenetically rather distant biomineralization systems have in common.

The amino-acid sequence of the abalone (Haliotis laevigata) nacre protein perlucin. Detection of a functional C-type lectin domain with galactose/mannose specificity

Perlucin isolated from abalone nacre consists of 155 amino acids including a glycosylated asparagine. The sequence of the first 130 amino acids shows a high similarity to the C-type carbohydrate-recognition domains of asialoglycoprotein receptors and other members of the group of C-type lectins but also a weaker similarity to related proteins without carbohydrate-binding activity. This C-type module is followed by a short C-terminal domain containing two almost identical sequence repeats with a length of 10 amino acids. Solid phase assays show a divalent metal ion-dependent binding of perlucin to (neo)glycoproteins containing D-galactose or D-mannose/D-glucose indicating that perlucin is a functional C-type lectin with broad carbohydrate-binding specificity. Our results also indicate that it may be difficult to predict carbohydrate-binding specificity and the occurrence of alternative binding configurations by amino-acid sequence comparisons and homology modeling.

Imaging microtubules and kinesin decorated microtubules using tapping mode atomic force microscopy in fluids

The atomic force microscope has been used to investigate microtubules and kinesin decorated microtubules in aqueous solution adsorbed onto a solid substrate. The netto negatively charged microtubules did not adsorb to negatively charged solid surfaces but to glass covalently coated with the highly positively charged silane trimethoxysilylpropyldiethylenetriamine (DETA) or a lipid bilayer of 1,2-dipalmitoyl-3-dimethylammoniumpropane. Using electron beam deposited tips for microtubules adsorbed on DETA, single protofilaments could be observed showing that the resolution is up to 5 nm. Under conditions where the silane coated surfaces are hydrophobic, microtubules opened, presumably at the seam, whose stability is lower than that of the bonds between the other protofilaments. This led to a "sheet" with a width of about 100 nm firmly attached to the surface. Microtubules decorated with a stoichiometric low amount of kinesin molecules in the presence of the non-hydrolyzable ATP-analog 5'-adenylylimidodiphosphate could also be adsorbed onto silane-coated glass. Imaging was very stable and the molecules did not show any scan-induced deformation even after hundreds of scans with a scan frequency of 100 Hz.

Purification and characterization of perlucin and perlustrin, two new proteins from the shell of the mollusc Haliotis laevigata

Two new proteins, named perlucin and perlustrin, with M(r) 17,000 and 13,000, respectively, were isolated from the shell of the mollusc Halotis laevigata (abalone) by ion-exchange chromatography and reversed-phase HPLC after demineralization of the shell in 10% acetic acid. The sequence of the first 32 amino acids of perlucin indicated that this protein belonged to a heterogeneous group of proteins consisting of a single C-type lectin domain. Perlucin increased the precipitation of CaCO(3) from a saturated solution, indicating that it may promote the nucleation and/or the growth of CaCO(3) crystals. With pancreatic stone protein (lithostathine) and the eggshell protein ovocleidin 17, this is the third C-type lectin domain protein isolated from CaCO(3) biominerals. This indicates that this type of protein performs an important but at present unrecognized function in biomineralization. Perlustrin was a minor component of the protein mixture and the sequence of the first 33 amino acids indicated a certain similarity to part of the much larger nacre protein lustrin A.

Oestradiol regulation of the components of the plasminogen-plasmin system in MDA-MB-231 human breast cancer cells stably expressing the oestrogen receptor

To understand the hormonal regulation of the components of the plasminogen-plasmin system in human breast cancer, we examined the oestradiol (E2) regulation of plasminogen activators (PAs), namely urokinase-type plasminogen activator (uPA) and tissue-type plasminogen activator (tPA), plasminogen activator inhibitor type 1 (PAI-1) and uPA receptor (uPAR), in our model system. We used stable transfectants of the MDA-MB-231 human breast cancer cells that express either the wild-type (S30 cells) or the mutant 351asp-->tyr oestrogen receptor (ER) (BC-2 cells). Northern blot analysis showed that there was a concentration-dependent down-regulation of uPA, tPA and PAI-1 mRNAs by E2. In contrast, uPAR mRNA was not modulated by E2. The pure anti-oestrogen ICI 182,780 was able to block E2 action, indicating that the regulation of these genes is ER mediated. The E2 also inhibited the expression and secretion of uPA, tPA and PAI-1 proteins as determined by enzyme-linked immunosorbent assay (ELISA) in cell extracts (CEs) and conditioned media (CM). Zymography of the CM confirmed the inhibitory effect of E2 on uPA activity. Thus, we now report the regulation of uPA, PAI-1 and tPA by E2 in both mRNA and protein levels in ER transfectants. The association between down-regulation of the uPA by E2 and known E2-mediated growth inhibition of these cells was also explored. Our findings indicate that down-regulation of uPA by E2 is an upstream event of inhibitory effects of E2 on growth of these cells as the addition of exogenous uPA did not block the growth inhibition by E2.

Erythroid cell-specific mRNA stability elements in the alpha 2-globin 3' nontranslated region

Very little is known about the mechanisms mediating longevities of mRNAs. As a means of identifying potential cis- and trans-acting elements which stabilize an individual mRNA, naturally occurring mutations that decreased stability of the normally long-lived globin mRNA were analyzed. Our previous studies demonstrated that a subset of mutations which allowed the translating ribosome to read through into the alpha 2-globin 3' nontranslated region (NTR) targeted the mutant mRNAs for accelerated turnover in erythroid cells but not in several nonerythroid cell lines (I. M. Weiss and S. A. Liebhaber, Mol. Cell. Biol. 14:8123-8132, 1994). These results suggested that translational readthrough interfered with some feature of the alpha 2-globin 3' NTR required for message stability in erythroid cells. To define the cis-acting sequences which comprise this erythroid cell-specific stability determinant, scanning mutagenesis was performed on the alpha 2-globin 3' NTR, and the stability of each mutant mRNA was examined during transient expression. Three cytidine-rich regions which are required for longevity of the alpha 2-globin mRNA were identified. However, in contrast to the readthrough mutations, base substitutions in these elements destabilize the message through a translation-independent mechanism. To account for these results, we propose that the cis-acting elements form a complex or determinant in the normal alpha 2-globin mRNA which protects the message from degradation in erythroid cells. Disruption of this determinant, by translational readthrough or because mutations in an element prevent or inhibit its formation, targets the message for accelerated turnover in these cells.

Detection and characterization of a 3' untranslated region ribonucleoprotein complex associated with human alpha-globin mRNA stability

The highly stable nature of globin mRNA is of central importance to erythroid cell differentiation. We have previously identified cytidine-rich (C-rich) segments in the human alpha-globin mRNA 3' untranslated region (alpha-3'UTR) which are critical in the maintenance of mRNA stability in transfected erythroid cells. In the present studies, we have detected trans-acting factors which interact with these cis elements to mediate this stabilizing function. A sequence-specific ribonucleoprotein (RNP) complex is assembled after incubation of the alpha-3'UTR with a variety of cytosolic extracts. This so-called alpha-complex is sequence specific and is not formed on the 3'UTR of either beta-globin or growth hormone mRNAs. Furthermore, base substitutions within the C-rich stretches which destabilize alpha-globin mRNA in vivo result in a parallel disruption of the alpha-complex in vitro. Competition studies with a series of homoribopolymers reveals a striking sensitivity of alpha-complex formation to poly(C), suggesting the presence of a poly(C)-binding activity within the alpha-complex. Three predominant proteins are isolated by alpha-3'UTR affinity chromatography. One of these binds directly to poly(C). This cytosolic poly(C)-binding protein is distinct from previously described nuclear poly(C)-binding heterogeneous nuclear RNPs and is necessary but not sufficient for alpha-complex formation. These data suggest that a messenger RNP complex formed by interaction of defined segments within the alpha-3'UTR with a limited number of cytosolic proteins, including a potentially novel poly(C)-binding protein, is of functional importance in establishing high-level stability of alpha-globin mRNA.

Erythroid cell-specific determinants of alpha-globin mRNA stability

Although globin mRNAs are considered prototypes of highly stable messages, the mechanisms responsible for their longevity remain largely undefined. As an initial step in identifying potential cis-acting elements or structures which contribute to their stability, we analyzed the defect in expression of a naturally occurring alpha 2-globin mutant, alpha Constant Spring (CS). The CS mutation is a single-base change in the translation termination codon (UAA-->CAA) that allows the ribosome to read through into the 3' nontranslated region (NTR). The presence of CS mRNA in transcriptionally active erythroid precursors and its absence (relative to normal alpha-globin mRNA) in the more differentiated transcriptionally silent erythrocytes suggest that this mutation disrupts some feature of the alpha-globin mRNA required for its stability. Using a transient transfection system, we demonstrate that in murine erythroleukemia cells the CS mRNA is unstable compared with the normal alpha 2-globin mRNA. The analyses of several other naturally occurring and site-directed mutant alpha-globin genes in murine erythroleukemia cells indicate that entry of a translating ribosome into the 3' NTR targets the message for accelerated degradation in erythroid cells. In contrast, both the CS and alpha 2-globin mRNAs are stable in several nonerythroid cell lines. These results suggest that translational readthrough disrupts a determinant associated with the alpha 2-globin 3' NTR which is required for mRNA stability in erythroid cells.

Cartilage-specific 5' end of chick alpha 2(I) collagen mRNAs

Chondrocytes grown in suspension contain both type I and type II collagen mRNAs, yet synthesize only type II collagen. The inability of chondrocytes to synthesize the alpha 2 subunit of type I collagen, alpha 2(I), results from a severely reduced translation elongation rate (Bennett, V.D., and Adams, S.L. (1987) J. Biol. Chem. 262, 14806-14814). Furthermore, the alpha 2(I) collagen mRNAs from chondrocytes are translated inefficiently in vitro and appear slightly smaller than those from other cells (Focht, R.J., and Adams, S.L. (1984) Mol. Cell. Biol. 4, 1843-1852). These observations suggest that the reduced translation elongation rate may be due to an intrinsic property of the mRNAs. In this report we demonstrate that the alpha 2(I) collagen mRNAs from suspended chondrocytes are 120 bases shorter than those from other cells, and that the first 94 bases of the chondrocyte mRNAs differ from the corresponding region of the calvaria mRNAs. The unique 5' end of the chondrocyte alpha 2(I) collagen mRNAs accounts for their smaller size and may be responsible for the translation elongation defect. Interestingly, the alpha 2(I) collagen mRNAs from chondrocytes grown in monolayer, rather than in suspension, no longer display the cartilage-specific 5' end, suggesting that cell shape and/or adhesion may modulate the structure of the 5' end of the chondrocyte alpha 2(I) collagen mRNAs.