Sequence and expression of the discoidin I gene family in Dictyostelium discoideum

Factor VIII moiety of recombinant Factor VIII Fc fusion protein impacts Fc effector function and CD16+ NK cell activation

Recombinant Factor VIII-Fc fusion protein (rFVIIIFc) is an enhanced half-life therapeutic protein product used for the management of hemophilia A. Recent studies have demonstrated that rFVIIIFc interacts with Fc gamma receptors (FcγR) resulting in the activation or inhibition of various FcγR-expressing immune cells. We previously demonstrated that rFVIIIFc, unlike recombinant Factor IX-Fc (rFIXFc), activates natural killer (NK) cells via Fc-mediated interactions with FcγRIIIA (CD16). Additionally, we showed that rFVIIIFc activated CD16+ NK cells to lyse a FVIII-specific B cell clone. Here, we used human NK cell lines and primary NK cells enriched from peripheral blood leukocytes to study the role of the FVIII moiety in rFVIIIFc-mediated NK cell activation. Following overnight incubation of NK cells with rFVIIIFc, cellular activation was assessed by measuring secretion of the inflammatory cytokine IFNγ by ELISA or by cellular degranulation. We show that anti-FVIII, anti-Fc, and anti-CD16 all inhibited indicating that these molecules were involved in rFVIIIFc-mediated NK cell activation. To define which domains of FVIII were involved, we used antibodies that are FVIII domain-specific and demonstrated that blocking FVIII C1 or C2 domain-mediated membrane binding potently inhibited rFVIIIFc-mediated CD16+ NK cell activation, while targeting the FVIII heavy chain domains did not. We also show that rFVIIIFc binds CD16 with about five-fold higher affinity than rFIXFc. Based on our results we propose that FVIII light chain-mediated membrane binding results in tethering of the fusion protein to the cell surface, and this, together with increased binding affinity for CD16, allows for Fc-CD16 interactions to proceed, resulting in NK cellular activation. Our working model may explain our previous results where we observed that rFVIIIFc activated NK cells via CD16, whereas rFIXFc did not despite having identical IgG1 Fc domains.

Discoidin domain receptors (DDRs): Potential implications in periodontitis

Periodontitis is a chronic inflammatory disease leading to the destruction of periodontal tissues associated with high prevalence and significant economic burden. As special collagen-binding tyrosine kinase receptors, the discoidin domain receptors (DDRs) can control cell migration, adhesion, proliferation, and extracellular matrix remodeling. DDRs are constitutively expressed and widely distributed in periodontal tissues which are rich in collagen. Ddr1/2 knockout mice showed significant periodontal defects including connective tissue destruction, alveolar bone loss, and even tooth loss. It has been demonstrated that bone homeostasis, inflammation, matrix metalloproteinases, and autophagy are crucial characteristics involved in the pathogenesis of periodontitis. Of note, DDRs have been reported to participate in the above pathophysiological processes, implicating the potential roles of DDRs in periodontitis. In this review article, we aim to illustrate the possible roles of DDRs in periodontitis in an attempt to explore their potential value as therapeutic targets for periodontitis.

Characterization of a phospholipid-regulated β-galactosidase from Akkermansia muciniphila involved in mucin degradation

The gut microbe Akkermansia muciniphila is important for the human health as the occurrence of the organism is inversely correlated with different metabolic disorders. The metabolism of the organism includes the degradation of intestinal mucins. Thus, the gut health‐promoting properties are not immediately obvious and mechanisms of bacteria‐host interactions are mostly unclear. In this study, we characterized a novel extracellular β‐galactosidase (Amuc_1686) with a preference for linkages from the type Galβ1–3GalNAc. Additionally, Amuc_1686 possesses a discoidin‐like domain, which enables the interaction with anionic phospholipids. We detected a strong inhibition by phosphatidylserine, phosphatidylglycerol, phosphatidic acid, and lysophosphatidic acid while phosphatidylcholine and phosphatidylethanolamine had no influence. Amuc_1686 is the first example of a prokaryotic hydrolase that is strongly inhibited by certain phospholipids. These inhibiting phospholipids have important signal functions in immune response and cell clearance processes. Hence, Amuc_1686 might be regulated based on the health status of the large intestine and could therefore contribute to the mutualistic relationship between the microbe and the host on a molecular level. In this sense, Amuc_1686 could act as an altruistic enzyme that does not attack the mucin layer of apoptotic epithelial cells to ensure tissue regeneration, for example, in areas with inflammatory damages.

The collagen receptor DDR1 co-localizes with the non-muscle myosin IIA in mice inner ear and contributes to the cytoarchitecture and stability of motile cells

Discoidin domain receptor 1 (DDR1) is a tyrosine kinase receptor activated by native collagen. DDRs regulate cell adhesion, migration and various other cell functions. Deletion of the DDR1 gene in mice is associated with a severe decrease in auditory function and substantial structural alterations in a heterogeneous group of cells, including cells containing actin/myosin contractile elements, e.g., outer hair cells (OHCs) (Meyer zum Gottesberge et al. Lab Invest, 88: 27-37, 2008). The non-muscle myosin heavy chain isoform IIA (NM-IIA), encoded by MYH9, is implicated in the regulation of cell spreading, cellular reshaping and movement and cell migration and adhesion. In this study, we identify DDR1 and NM-IIA co-localization in the type III fibrocytes (tension fibrocytes) of the spiral ligament, the OHCs and the stereocilia of both OHCs and inner hair cells. We show for the first time that DDR1 malfunction causes OHC deformation and the separation of the lateral wall, the location of the cellular motor responsible for the electromotile property, explicitly in those regions showing DDR1 and NM-IIA co-localization. On the basis of our results, we propose that DDR1 acts in concert with proteins of the actin/myosin complex to maintain mechanical forces in the inner ear and to stabilize OHC cellular shape for proper auditory signal transduction.

Genome of Acanthamoeba castellanii highlights extensive lateral gene transfer and early evolution of tyrosine kinase signaling

BACKGROUND

The Amoebozoa constitute one of the primary divisions of eukaryotes, encompassing taxa of both biomedical and evolutionary importance, yet its genomic diversity remains largely unsampled. Here we present an analysis of a whole genome assembly of Acanthamoeba castellanii (Ac) the first representative from a solitary free-living amoebozoan.

RESULTS

Ac encodes 15,455 compact intron-rich genes, a significant number of which are predicted to have arisen through inter-kingdom lateral gene transfer (LGT). A majority of the LGT candidates have undergone a substantial degree of intronization and Ac appears to have incorporated them into established transcriptional programs. Ac manifests a complex signaling and cell communication repertoire, including a complete tyrosine kinase signaling toolkit and a comparable diversity of predicted extracellular receptors to that found in the facultatively multicellular dictyostelids. An important environmental host of a diverse range of bacteria and viruses, Ac utilizes a diverse repertoire of predicted pattern recognition receptors, many with predicted orthologous functions in the innate immune systems of higher organisms.

CONCLUSIONS

Our analysis highlights the important role of LGT in the biology of Ac and in the diversification of microbial eukaryotes. The early evolution of a key signaling facility implicated in the evolution of metazoan multicellularity strongly argues for its emergence early in the Unikont lineage. Overall, the availability of an Ac genome should aid in deciphering the biology of the Amoebozoa and facilitate functional genomic studies in this important model organism and environmental host.

Collagen recognition and transmembrane signalling by discoidin domain receptors

The discoidin domain receptors, DDR1 and DDR2, are two closely related receptor tyrosine kinases that are activated by triple-helical collagen in a slow and sustained manner. The DDRs have important roles in embryo development and their dysregulation is associated with human diseases, such as fibrosis, arthritis and cancer. The extracellular region of DDRs consists of a collagen-binding discoidin (DS) domain and a DS-like domain. The transmembrane region mediates the ligand-independent dimerisation of DDRs and is connected to the tyrosine kinase domain by an unusually long juxtamembrane domain. The major DDR binding site in fibrillar collagens is a GVMGFO motif (O is hydroxyproline), which is recognised by an amphiphilic trench at the top of the DS domain. How collagen binding leads to DDR activation is not understood. GVMGFO-containing triple-helical peptides activate DDRs with the characteristic slow kinetics, suggesting that the supramolecular structure of collagen is not required. Activation can be blocked allosterically by monoclonal antibodies that bind to the DS-like domain. Thus, collagen most likely causes a conformational change within the DDR dimer, which may lead to the formation of larger DDR clusters. This article is part of a Special Issue entitled: Emerging recognition and activation mechanisms of receptor tyrosine kinases.

Putative chitin synthases from Branchiostoma floridae show extracellular matrix-related domains and mosaic structures

The transition from unicellular to multicellular life forms requires the development of a specialized structural component, the extracellular matrix (ECM). In Metazoans, there are two main supportive systems, which are based on chitin and collagen/hyaluronan, respectively. Chitin is the major constituent of fungal cell walls and arthropod exoskeleton. However, presence of chitin/chitooligosaccharides has been reported in lower chordates and during specific stages of vertebrate development. In this study, the occurrence of chitin synthases (CHSs) was investigated with a bioinformatics approach in the cephalochordate Branchiostoma floridae, in which the presence of chitin was initially reported in the skeletal rods of the pharyngeal gill basket. Twelve genes coding for proteins containing conserved amino acid residues of processive glycosyltransferases from GT2 family were found and 10 of them display mosaic structures with novel domains never reported previously in a chitin synthase. In particular, the presence of a discoidin (DS) and a sterile alpha motif (SAM) domain was found in nine identified proteins. Sequence analyses and homology modelling suggest that these domains might interact with the extracellular matrix and mediate protein–protein interaction. The multi-domain putative chitin synthases from B. floridae constitute an emblematic example of the explosion of domain innovation and shuffling which predate Metazoans.

X-linked juvenile retinoschisis: clinical diagnosis, genetic analysis, and molecular mechanisms

X-linked juvenile retinoschisis (XLRS, MIM 312700) is a common early onset macular degeneration in males characterized by mild to severe loss in visual acuity, splitting of retinal layers, and a reduction in the b-wave of the electroretinogram (ERG). The RS1 gene (MIM 300839) associated with the disease encodes retinoschisin, a 224 amino acid protein containing a discoidin domain as the major structural unit, an N-terminal cleavable signal sequence, and regions responsible for subunit oligomerization. Retinoschisin is secreted from retinal cells as a disulphide-linked homo-octameric complex which binds to the surface of photoreceptors and bipolar cells to help maintain the integrity of the retina. Over 190 disease-causing mutations in the RS1 gene are known with most mutations occurring as non-synonymous changes in the discoidin domain. Cell expression studies have shown that disease-associated missense mutations in the discoidin domain cause severe protein misfolding and retention in the endoplasmic reticulum, mutations in the signal sequence result in aberrant protein synthesis, and mutations in regions flanking the discoidin domain cause defective disulphide-linked subunit assembly, all of which produce a non-functional protein. Knockout mice deficient in retinoschisin have been generated and shown to display most of the characteristic features found in XLRS patients. Recombinant adeno-associated virus (rAAV) mediated delivery of the normal RS1 gene to the retina of young knockout mice result in long-term retinoschisin expression and rescue of retinal structure and function providing a 'proof of concept' that gene therapy may be an effective treatment for XLRS.

Specific cell-cell contact serves as the developmental signal to deactivate discoidin I gene expression in Dictyostelium discoideum

Specific cell-cell contact is a major regulatory signal controlling cell differentiation in Dictyostelium discoideum, causing dramatic changes in the developmental program of gene expression. In this report, we focus on the relationships between specific cell-cell contact and the activity of the genes for discoidin I, an endogenous lectin that has been implicated in the cell-cell cohesion process. By performing quantitative RNA dot-hybridization assays and RNA gel blot-hybridization analyses, using as a probe a recombinant plasmid containing a discoidin I cDNA insert, we have measured changes in discoiding I mRNA levels during normal development and in response to specific manipulations of the state of cellular aggregation. Our major findings are as follows. (i) During normal development on filters, there is a close temporal correspondence between the establishment of specific cell-cell contacts and the decline in discoidin I mRNA levels. By the tight-aggregate stage, discoidin I mRNA is barely detectable. (ii) When tight aggregates are disaggregated and the cells are maintained in the disaggregated state, there is a dramatic rise in discoidin I mRNA content. (iii) When cells are developed in suspension (conditions that interfere with the establishment of tight cell-cell contacts), discoidin I mRNA accumulates to abnormally high levels, and these persist well after the levels in filter-developed cells have declined. Taken together, these results strongly suggest that cell-cell contact is the normal developmental signal to deactivate discoidin I gene expression; thus, a contact-deactivated gene for which a recombinant DNA probe is available has now been identified. Furthermore, we demonstrate that exogenous cAMP almost completely blocks the disaggregation-induced reactivation of discoidin I gene expression. Possible mechanistic relationships between specific cell-cell contact, intracellular cAMP levels, and developmental gene expression are discussed.

Discoidin I from Dictyostelium discoideum and Interactions with oligosaccharides: specificity, affinity, crystal structures, and comparison with discoidin II

Discoidin I (DiscI) and discoidin II (DiscII) are N-acetylgalactosamine (GalNAc)-binding proteins from Dictyostelium discoideum. They consist of two domains: an N-terminal discoidin domain and a C-terminal H-type lectin domain. They were cloned and expressed in high yield in recombinant form in Escherichia coli. Although both lectins bind galactose (Gal) and GalNAc, glycan array experiments performed on the recombinant proteins displayed strong differences in their specificity for oligosaccharides. DiscI and DiscII bind preferentially to Gal/GalNAcbeta1-3Gal/GalNAc-containing and Gal/GalNAcbeta1-4GlcNAcbeta1-6Gal/GalNAc-containing glycans, respectively. The affinity of the interaction of DiscI with monosaccharides and disaccharides was evaluated using isothermal titration calorimetry experiments. The three-dimensional structures of native DiscI and its complexes with GalNAc, GalNAcbeta1-3Gal, and Galbeta1-3GalNAc were solved by X-ray crystallography. DiscI forms trimers with involvement of calcium at the monomer interface. The N-terminal discoidin domain presents a structural similarity to F-type lectins such as the eel agglutinin, where an amphiphilic binding pocket suggests possible carbohydrate-binding activity. In the C-terminal H-type lectin domain, the GalNAc residue establishes specific hydrogen bonds that explain the observed affinity (K(d)=3x10(-4) M). The different specificities of DiscI and DiscII for oligosaccharides were rationalized from the different structures obtained by either X-ray crystallography or molecular modeling.

Functional analysis of conserved aromatic amino acids in the discoidin domain of Paenibacillus beta-1,3-glucanase

The 190-kDa Paenibacillus beta-1,3-glucanase (LamA) contains a catalytic module of the glycoside hydrolase family 16 (GH16) and several auxiliary domains. Of these, a discoidin domain (DS domain), present in both eukaryotic and prokaryotic proteins with a wide variety of functions, exists at the carboxyl-terminus. To better understand the bacterial DS domain in terms of its structure and function, this domain alone was expressed in Escherichia coli and characterized. The results indicate that the DS domain binds various polysaccharides and enhances the biological activity of the GH16 module on composite substrates. We also investigated the importance of several conserved aromatic residues in the domain's stability and substrate-binding affinity. Both were affected by mutations of these residues; however, the effect on protein stability was more notable. In particular, the forces contributed by a sandwiched triad (W1688, R1756, and W1729) were critical for the presumable beta-sandwich fold.

Nucleocytoplasmic plant lectins

During the last decade it was unambiguously shown that plants synthesize minute amounts of carbohydrate-binding proteins upon exposure to stress situations like drought, high salt, hormone treatment, pathogen attack or insect herbivory. In contrast to the 'classical' plant lectins, which are typically found in storage vacuoles or in the extracellular compartment this new class of lectins is located in the cytoplasm and the nucleus. Based on these observations the concept was developed that lectin-mediated protein-carbohydrate interactions in the cytoplasm and the nucleus play an important role in the stress physiology of the plant cell. Hitherto, six families of nucleocytoplasmic lectins have been identified. This review gives an overview of our current knowledge on the occurrence of nucleocytoplasmic plant lectins. The carbohydrate-binding properties of these lectins and potential ligands in the nucleocytoplasmic compartment are discussed in view of the physiological role of the lectins in the plant cell.

Characterization and purification of the discoidin domain-containing protein retinoschisin and its interaction with galactose

RS1, also known as retinoschisin, is an extracellular discoidin domain-containing protein that has been implicated in maintaining the cellular organization and synaptic structure of the vertebrate retina. Mutations in the gene encoding RS1 are responsible for X-linked retinoschisis, a retinal degenerative disease characterized by the splitting of the retinal cell layers and visual impairment. To better understand the role of RS1 in retinal cell biology and X-linked retinoschisis, we have studied the interaction of wild-type and mutant RS1 with various carbohydrates coupled to agarose supports. RS1 bound efficiently to galactose-agarose and to a lesser extent lactose-agarose, but not agarose, N-acetylgalactosamine-agarose, N-acetylglucosamine-agarose, mannose-agarose, or heparin-agarose. RS1 cysteine mutants (C59S/C223S and C59S/C223S/C40S) which prevent disulfide-linked octamer formation exhibited little if any binding to galactose-agarose. The disease-causing R141H mutant bound galactose-agarose at levels similar to that of wild-type RS1, whereas the R141S mutant resulted in a marked reduction in the level of galactose-agarose binding. RS1 bound to galactose-agarose could be effectively displaced by incubation with isopropyl beta- d-1-thiogalactopyranoside (IPTG). This property was used as a basis to develop an efficient purification procedure. Anion exchange and galactose affinity chromatography was used to purify RS1 from the culture media of stably transformed Sf21 insect cells that express and secrete RS1. This cell expression and protein purification method should prove useful in the isolation of RS1 for detailed structure-function studies.

Evolution of polydom-like molecules: identification and characterization of cnidarian polydom (Cnpolydom) in the basal metazoan Hydractinia

End sequencing of random BAC clones from a Hydractinia symbiolongicarpus (Cnidaria: Hydrozoa) genomic library revealed a gene across a approximately 37.5kb region of the H. symbiolongicarpus genome sharing highest sequence identity and domain architecture to mammalian polydom that we in turn named cnidarian polydom (CnPolydom). Sharing all eight domain types characteristic of polydom and organized in a similar 5'-3' manner, CnPolydom was predicted to contain three additional domain types: PAN, FA58C, and CUB that are characteristic of CnPolydom. Expression analysis of CnPolydom from H. symbiolongicarpus (Hysy-CnPolydom) showed upregulation in response to bacterial and primarily fungal challenges, with transcripts produced specifically by a subset of interstitial stem cells (i-cells) and/or neural cells throughout the ectodermal tissue layer of feeding polyps (gastrozooids). This is the first description of a polydom-like molecule outside of Mammalia and provides evolutionary perspective on the ancestral structure and role of this pentraxin family clade.

Retinoschisin (RS1), the Protein Encoded by the X-linked Retinoschisis Gene, Is Anchored to the Surface of Retinal Photoreceptor and Bipolar Cells through Its Interactions with a Na/K ATPase-SARM1 Complex

Retinoschisin or RS1 is a discoidin domain-containing protein encoded by the gene responsible for X-linked retinoschisis (XLRS), an early onset macular degeneration characterized by a splitting of the retina. Retinoschisin, expressed and secreted from photoreceptors and bipolar cells as a homo-octameric complex, associates with the surface of these cells where it serves to maintain the cellular organization of the retina and the photoreceptor-bipolar synaptic structure. To gain insight into the role of retinoschisin in retinal cell adhesion and the pathogenesis of XLRS, we have investigated membrane components in retinal extracts that interact with retinoschisin. Unlike the discoidin domain-containing blood coagulation proteins Factor V and Factor VIII, retinoschisin did not bind to phospholipids or retinal lipids reconstituted into unilamellar vesicles or immobilized on microtiter plates. Instead, co-immunoprecipitation studies together with mass spectrometric-based proteomics and Western blotting showed that retinoschisin is associated with a complex consisting of Na/K ATPase (alpha3, beta2 isoforms) and the sterile alpha and TIR motif-containing protein SARM1. Double labeling studies for immunofluorescence microscopy confirmed the co-localization of retinoschisin with Na/K ATPase and SARM1 in photoreceptors and bipolar cells of retina tissue. We conclude that retinoschisin binds to Na/K ATPase on photoreceptor and bipolar cells. This interaction may be part of a novel SARM1-mediated cell signaling pathway required for the maintenance of retinal cell organization and photoreceptor-bipolar synaptic structure.

Structural similarities and functional diversity of eukaryotic discoidin-like domains

The discoidin domain is a approximately 150 amino acid motif common in both eukaryotic and prokaryotic proteins. It is found in a variety of extracellular, intracellular and transmembrane multidomain proteins characterized by a considerable functional diversity, mostly involved in developmental processes. The biological role of the domain depends on its interactions with different molecules, including growth factors, phospholipids and lipids, galactose or its derivatives, and collagen. The conservation of the motif, as well as the serious physiological consequences of discoidin domain disorders underscore the importance of the fold, while the ability to accommodate such an extraordinarily broad range of ligand molecules makes it a fascinating research target. In present review we characterize the distinctive features of discoidin domains and briefly outline the biological role of this module in various eukaryotic proteins.

Transcriptional pulsing of a developmental gene

It has not been possible to view the transcriptional activity of a single gene within a living eukaryotic cell. It is therefore unclear how long and how frequently a gene is actively transcribed, how this is modulated during differentiation, and how transcriptional events are dynamically coordinated in cell populations. By means of an in vivo RNA detection technique , we have directly visualized transcription of an endogenous developmental gene. We found discrete "pulses" of gene activity that turn on and off at irregular intervals. Surprisingly, the length and height of these pulses were consistent throughout development. However, there was strong developmental variation in the proportion of cells recruited to the expressing pool. Cells were more likely to re-express than to initiate new expression, indicating that we directly observe a transcriptional memory. In addition, we used a clustering algorithm to reveal synchronous transcription initiation in neighboring cells. This study represents the first direct visualization of transcriptional pulsing in eukaryotes. Discontinuity of transcription may allow greater flexibility in the gene-expression decisions of a cell.

Identification and isolation of Dictyostelium microtubule-associated protein interactors by tandem affinity purification

Tandem affinity purification (TAP) is a method originally established in yeast to isolate highly purified protein complexes in a very gentle and efficient way. In this work, we have modified TAP for Dictyostelium applications and have proved it as a useful method to specifically isolate and identify microtubule-associated protein (MAP) complexes. MAPs are known to interact with other proteins to fulfill their complex functions in balancing the dynamic instability of microtubules as well as anchoring microtubules at the cell cortex, controlling mitosis at the centrosome and guiding transport along them. DdEB1 and the Dictyostelium member of the XMAP215 protein family, DdCP224, are known to be part of complexes at the microtubule tips as well as at the centrosome. Employing TAP and mass spectrometry we were able to prove an interaction between EB1 and the DdCP224. Additionally, among other interactions that remain to be confirmed by other methods, an interaction between DdCP224 and a TACC-family protein could be shown for the first time in Dictyostelium and was confirmed by colocalization and co-immunoprecipitation analyses.

Structural and functional diversity of lectin repertoires in invertebrates, protochordates and ectothermic vertebrates

During the past few years, substantial progress has been accomplished in the elucidation of the structural diversity of the lectin repertoires of invertebrates, protochordates and ectothermic vertebrates, providing particularly valuable information on those groups that constitute the invertebrate/vertebrate 'boundary'. Although representatives of lectin families typical of mammals, such as C-type lectins, galectins and pentraxins, have been described in these taxa, the detailed study of selected model species has yielded either novel variants of the structures described for the mammalian lectin representatives or novel lectin families with unique sequence motifs, multidomain arrangements and a new structural fold. Along with the high structural diversity of the lectin repertoires in these taxa, a wide spectrum of biological roles is starting to emerge, underscoring the value of invertebrate and lower vertebrate models for gaining insight into structural, functional and evolutionary aspects of lectins.

Small FVIII gene rearrangements in 18 hemophilia A patients: five novel mutations

Hemophilia A (HA) is a disorder caused by mutations of the FVIII gene, which is located on the tip of the long arm of the X chromosome. In a cohort of 18 unrelated Italian patients affected with HA of varying severity, we performed mutational screening of the gene by denaturing high-performance liquid chromatography (DHPLC) and direct sequencing of abnormal peaks. We identified five novel mutations and 9 previously reported DNA alterations. Two of the 9 previously reported alterations were each common to 3 unrelated patients. Six different mutations were characterized as missense alterations, while 8 were non-missense mutations. Among the new gene alterations, one created a stop codon, one consisted of an out-of frame deletion, and one was a splice-site mutation. The last two were missense alterations. In an attempt to better understand the causative effect of the mutations and the clinical variability of the patients, we investigated the consequences of each missense mutation and visualized the effect of the amino acid change on structural FVIII models.

Aggregation of maternal pigment granules is induced by the cytosolic discoidin domain of theXenopus Del1 protein

Xenopus oocytes generate pigment granules (melanosomes) that predominantly localize to the animal hemisphere cortex. During embryonic development, these granules are located near the membranes of outer layer ectoderm cells. We report a novel phenotype found during an expression cloning screen in Xenopus laevis embryos. The phenotype is characterized by dissociation of pigment granules from the cell membrane to form large central aggregates. This phenomenon was induced by a truncated form of the Xenopus Del1 (XDel1) protein that contains only the C-terminal discoidin (D2) domain. This truncated form of XDel1 localized to membranes as shown by a chimeric enhanced green fluorescent protein construct. Although a similar localization occurred in immature oocytes, dissociation of pigment granules was limited to the oocyte vegetal hemisphere. The full-length XDel1 cDNA was cloned, and XDel1 mRNA expression was found to be ubiquitous and continuous from early oocyte to tail bud stages, with a transient enrichment in the cement gland. Ectopic expression of various deletion or full-length constructs or antisense morpholino oligonucleotides did not induce any significant developmental phenotypes.

Molecular properties of mycelial aggregate-specific lectin of Pleurotus cornucopiae

By cloning and sequencing cDNA, the primary structure of a mycelial aggregate-specific lectin of Pleurotus cornucopiae was determined. The amino acid sequence was novel and elucidated unique properties of this lectin: It was composed of 373 amino acids, 33 of which constitute a signal sequence. The sequence of the mature lectin consisted of two homologous regions having five glycosylation recognition signals and six cysteine residues. However, the distribution of these elements in the two regions was biased. Expression of cDNA in Escherichia coli and Pichia pastoris revealed the requirement of glycosylation to produce the functional lectin. Gel filtration followed by gel electrophoretic analyses of the purified lectin showed that the active component moved faster than the bulk of the protein, suggesting that the most active lectin formed an oligomer of subunits through disulfide bonds. From these observations, a model for the structure of the active form of this lectin is proposed. Southern hybridization using the cDNA as a probe revealed the presence of several genes. The lectin gene was composed of five exons and five introns.

Analysis of 18 novel mutations in the factor VIII gene

We describe 18 novel mutations, unreported in the Haemophilia A mutation Databases, that have been identified in a cohort of unrelated, Italian patients affected with haemophilia A (HA). Screening of the factor VIII gene (FVIII) was performed using denaturing high-performance liquid chromatography (DHPLC) and direct sequencing. Eight mutations were characterized as non-missense alterations, and the remaining 10 were missense mutations. Heterozygosity for the identified mutations was observed in the female relatives of patients belonging to eight families with sporadic cases. In an attempt to understand better the causative effect of the mutations and the clinical variability of the patients, missense mutation consequences were investigated for: (1) the nature of the new amino acid; (2) the location of the substituted amino acid within crystallographic and theoretical models; and (3) the degree of conservation of the native residue in factor VIII (FVIII) protein and FVIII-related protein family aligned sequences. These research tools have provided evidence that the mutations we describe involve residues that were conserved, at least in FVIII proteins, in all the species we compared.

The nonenzymatic subunit of pseutarin C, a prothrombin activator from eastern brown snake (Pseudonaja textilis) venom, shows structural similarity to mammalian coagulation factor V

Pseutarin C is a group C prothrombin activator from the venom of the eastern brown snake Pseudonaja textilis. It is a multi-subunit protein complex consisting of catalytic and nonenzymatic subunits similar to coagulation factor Xa and factor Va, respectively. Here we describe the complete sequence of the nonenzymatic subunit. Based on the partial amino acid sequence of the nonenzymatic subunit, degenerate primers were designed. Using a "walking" strategy based on sequentially designed primers, we determined the complete cDNA sequence of the nonenzymatic subunit. The cDNA encodes a protein of 1461 amino acid residues, which includes a 30-residue signal peptide, a mature protein of 1430 amino acid residues, and a stop codon. cDNA blot analysis showed a single transcript of approximately 4.6 kb. The deduced amino acid sequence shows approximately 50% identity to mammalian factor V and by homology has a similar domain structure consisting of domains A1-A2-B-A3-C1-C2. Interestingly, the B domain of pseutarin C is shorter than that of mammalian factor V (FV). Although most of the proteolytic activation sites are conserved, 2 of 3 proteolytic sites cleaved by activated protein C are mutated, and thus activated protein C is not able to inactivate this procoagulant toxin. The predicted posttranslational modifications, including disulfide bonds, N-glycosylation, phosphorylation, and sulfation, in pseutarin C are significantly different compared with bovine factor V. Thus, our data demonstrate that the nonenzymatic subunit of group C prothrombin activators is structurally similar to mammalian FV.

Defective discoidin domain structure, subunit assembly, and endoplasmic reticulum processing of retinoschisin are primary mechanisms responsible for X-linked retinoschisis

Retinoschisin is a 24-kDa discoidin domain-containing protein that is secreted from photoreceptor and bipolar cells as a large disulfide-linked multisubunit complex. It functions as a cell adhesion protein to maintain the cellular organization and synaptic structure of the retina. Over 125 different mutations in the RS1 gene are associated with X-linked juvenile retinoschisis, the most common form of early onset macular degeneration in males. To identify molecular determinants important for retinoschisin structure and function and elucidate molecular and cellular mechanisms responsible for X-linked juvenile retinoschisis, we have analyzed the expression, protein folding, disulfide-linked subunit assembly, intracellular localization, and secretion of wild-type retinoschisin, 15 Cys-to-Ser variants and 12 disease-linked mutants. Our studies, together with molecular modeling of the discoidin domain, identify Cys residues involved in intramolecular and intermolecular disulfide bonds essential for protein folding and subunit assembly. We show that misfolding of the discoidin domain, defective disulfide-linked subunit assembly, and inability of retinoschisin to insert into the endoplasmic reticulum membrane as part of the protein secretion process are three primary mechanisms responsible for the loss in the function of retinoschisin as a cell adhesion protein and the pathogenesis of X-linked juvenile retinoschisis.

Expression of factor VIII in recombinant and transgenic systems

Deficiency in a coagulation factor VIII (FVIII) causes a genetic disorder hemophilia A, which is treated by repeated infusions of expensive FVIII products. Recombinant FVIII (rFVIII), the culmination of years of extensive international research, is an important alternative to plasma-derived FVIII (pdFVIII) and is considered to have a higher margin of safety. Advances in biotechnology allowed production of rFVIII at industrial scale, which significantly improved treatment of hemophilia A patients. We review the contemporary methods used for FVIII expression in mammalian cell culture systems and discuss the factors responsible for insufficient recoveries of rFVIII, such as inefficient accumulation of FVIII mRNA in the cell, complexity of the mechanisms of FVIII secretion, and instability of secreted FVIII. The approaches to improve the yield of rFVIII in cell culture systems include genetic engineering of B-domain-deleted FVIII, introduction of introns into FVIII cDNA constructs for more efficient processing and accumulation of FVIII mRNA, and introduction of mutations into chaperone-binding sites of FVIII to improve its secretion. Design of FVIII with prolonged half-life in vivo is considered as another promising direction in improving rFVIII protein and efficiency of hemophilia A therapy. As an alternative to expression of rFVIII in cell culture systems, we discuss production of rFVIII in transgenic animals, where high levels of rFVIII have been successfully secreted into milk. We also pay attention to the major limitations of this approach, such as safety issues associated with potential transmission of animal pathogens. Finally, we present a brief characterization of commercial recombinant FVIII products currently available on the market for hemophilia A treatment.

Functional analysis of discoidin domain receptor 1: effect of adhesion on DDR1 phosphorylation

Discoidin domain receptor 1 (DDR1), a receptor tyrosine kinase (RTK), has been shown to be activated mainly by soluble fibrillar collagen. Unusually, the kinetics of phosphorylation of the receptor is slow, with maximal phosphorylation observed after 90 min. To understand the reasons for slow phosphorylation of the receptor, we examined several cell lines under different conditions. We confirm that endogenous DDR1 is phosphorylated slowly by collagen in adherent T47D and HCT116 cells. In detached and resuspended cells, collagen induced rapid phosphorylation of DDR1. This was further confirmed with a semiadherent cell line (COLO201) and one that grows as a suspension (K562), both of which express endogenous DDR1. Replating K562 on fibronectin to mimic adherent conditions altered the kinetics of phosphorylation from rapid to slow, similar to those of adherent cells. The slow kinetics of phosphorylation in the adherent state was probably not due to cell-cell contacts because EDTA had no major effect. However, pervanadate in the absence of collagen was able to induce strong DDR1 phosphorylation, indicating that a phosphatase may inhibit or delay the phosphorylation of DDR1. Further, downstream signals after phosphorylation of DDR1 by collagen were not transmitted through the classical mitogen-activated protein kinase pathway. In addition, a chimeric TrkA-DDR1 receptor failed to become phosphorylated on stimulation with nerve growth factor (NGF), although it dimerized normally. This is the first RTK whose kinetics of phosphorylation is dependent on cellular context. The interaction of the cells with the matrix, rather than cell-cell contact, is probably responsible for the inhibition of phosphorylation.

ESDN, a novel neuropilin-like membrane protein cloned from vascular cells with the longest secretory signal sequence among eukaryotes, is up-regulated after vascular injury

A novel cDNA has been isolated from primary culture of human coronary arterial cells by a signal sequence trap method, and designated ESDN (endothelial and smooth muscle cell-derived neuropilin-like molecule). ESDN is a type-I transmembrane protein with the longest cleavable secretory signal sequence among eukaryotes. ESDN contains a CUB domain and a coagulation factor V/VIII homology domain, which reminds us of the structure of neuropilins. ESDN also harbors an LCCL domain, which is shared by Limulus factor C and Coch. Mouse and rat counterparts were also identified revealing >84% amino acid identity with human ESDN. The human ESDN gene was mapped between D3S1552 and D3S1271. Northern blot analysis showed that ESDN mRNA was expressed in various tissues; particularly highly expressed in cultured vascular smooth muscle cells. The ESDN expression was up-regulated in platelet-derived growth factor-BB-stimulated vascular smooth muscle cells in vitro and neointima of the balloon-injured carotid artery in vivo. Overexpression of ESDN in 293T cells suppressed their bromodeoxyuridine uptake. In addition, ESDN protein was strongly expressed in nerve bundles in rodents. Thus, ESDN is considered to play a role in regulation of vascular cell growth and may have a wide variety of functions in other tissues including the nervous system, like neuropilins.

Expression and one-step purification of Plasmodium proteins in dictyostelium

Nearly full-length Circumsporozoite protein (CSP) from Plasmodium falciparum, the C-terminal fragments from both P. falciparm and P. yoelii CSP and a fragment comprising 351 amino acids of P.vivax MSPI were expressed in the slime mold Dictyostelium discoideum. Discoidin-tag expression vectors allowed both high yields of these proteins and their purification by a nearly single-step procedure. We exploited the galactose binding activity of Discoidin Ia to separate the fusion proteins by affinity chromatography on Sepharose-4B columns. Inclusion of a thrombin recognition site allowed cleavage of the Discoidin-tag from the fusion protein. Partial secretion of the protein was obtained via an ER independent pathway, whereas routing the recombinant proteins to the ER resulted in glycosylation and retention. Yields of proteins ranged from 0.08 to 3 mg l(-1) depending on the protein sequence and the purification conditions. The recognition of purified MSPI by sera from P. vivax malaria patients was used to confirm the native conformation of the protein expressed in Dictyostelium. The simple purification procedure described here, based on Sepharose-4B, should facilitate the expression and the large-scale purification of various Plasmodium polypeptides.

Dynacortin, a genetic link between equatorial contractility and global shape control discovered by library complementation of a Dictyostelium discoideum cytokinesis mutant

We have developed a system for performing interaction genetics in Dictyostelium discoideum that uses a cDNA library complementation/multicopy suppression strategy. Chemically mutagenized cells were screened for cytokinesis-deficient mutants and one mutant was subjected to library complementation. Isolates of four different genes were recovered as modifiers of this strain's cytokinesis defect. These include the cleavage furrow protein cortexillin I, a novel protein we named dynacortin, an ezrin-radixin-moesin-family protein, and coronin. The cortexillin I locus and transcript were found to be disrupted in the strain, identifying it as the affected gene. Dynacortin is localized partly to the cell cortex and becomes enriched in protrusive regions, a localization pattern that is similar to coronin and partly dependent on RacE. During cytokinesis, dynacortin is found in the cortex and is somewhat enriched at the poles. Furthermore, it appears to be reduced in the cleavage furrow. The genetic interactions and the cellular distributions of the proteins suggest a hypothesis for cytokinesis in which the contraction of the medial ring is a function of spatially restricted cortexillin I and myosin II and globally distributed dynacortin, coronin, and RacE.

The mouse X-linked juvenile retinoschisis cDNA: expression in photoreceptors

Retinal photoreceptor cells are particularly vulnerable to degenerations that can eventually lead to blindness. Our purpose is to identify and characterize genes expressed specifically in photoreceptors in order to increase our understanding of the biochemistry and function of these cells, and then to use these genes as candidates for the sites of mutations responsible for degenerative retinal diseases. We have characterized a cDNA, a fragment of which (SR3.1) was originally isolated by subtractive hybridization of adult, photoreceptorless rd mouse retinal cDNAs from the cDNAs of normal mouse retina. The full-length sequence of this cDNA was determined from clones obtained by screening mouse retinal and eye cDNA libraries and by using the 5'- and 3'-RACE methods. Both Northern blot analysis and in situ hybridization showed that the corresponding mRNA is expressed in rod and cone photoreceptors. The gene encoding this cDNA was mapped to the X chromosome using an interspecific cross. Based on the nucleotide and amino acid sequences, as well as chromosome mapping, we determined that this gene is the mouse ortholog (Xlrs1) of the human X-linked juvenile retinoschisis gene (XLRS1). Analysis of the predicted amino acid sequence indicates that the Xlrs1 mRNA may encode a secretable, adhesion protein. Therefore, our data suggest that X-linked juvenile retinoschisis originates from abnormalities in a photoreceptor-derived adhesion protein.

Identification of mouse CPX-1, a novel member of the metallocarboxypeptidase gene family with highest similarity to CPX-2

The recent finding that Cpe(fat)/Cpe(fat) mice, which lack carboxypeptidase E (CPE) activity because of a point mutation, are still capable of a reduced amount of neuroendocrine peptide processing suggested that additional carboxypeptidases (CPs) participate in this processing reaction. Searches for novel members of the CPE gene family led to the discovery of CPD, CPZ, AEBP1, and CPX-2. In the present report, we describe mouse CPX-1, another novel member of this gene family. Like AEBP1 and CPX-2, CPX-1 contains an N-terminal region of 160 amino acids with sequence similarity to the discoidin domain of a variety of proteins. The 410-residue CP-like domain of CPX-1 has 54% to 62% amino acid sequence identity with AEBP1 and CPX-2 and 33% to 49% amino acid identity with other members of the CPE subfamily. However, several active-site residues that are important for catalytic activity of other CPs are not conserved in CPX-1. Furthermore, CPX-1 expressed in either the baculovirus system or the mouse AtT-20 cell line does not cleave standard CP substrates. Northern blot analysis showed the highest levels of CPX-1 mRNA in testis and spleen and lower levels in salivary gland, brain, heart, lung, and kidney. In situ hybridization of CPX-1 mRNA in embryonic and fetal mouse tissue showed expression throughout the head and thorax, with abundance in primordial cartilage and skeletal structures. In the head, high levels of CPX-1 mRNA were associated with the nasal mesenchyme, primordial cartilage structures in the ear, and the meninges. In the thorax, CPX-1 mRNA was expressed in multiple developing skeletal structures, including chondrocytes and perichondrial cells of the rib, vertebral, and long-bone primordia. Taken together, these findings suggest that it is unlikely that CPX-1 functions in the processing of neuroendocrine peptides. Instead, CPX-1 may have a role in development, possibly mediating cell interactions via its discoidin domain.

Homology models of the C domains of blood coagulation factors V and VIII: a proposed membrane binding mode for FV and FVIII C2 domains

We present homology models of the C domains of coagulation factors V (FV) and VIII (FVIII). Using a threading approach, we identified the binding domain of galactose oxidase as an appropriate template for each C domain. The C1 and C2 domains of FV associate to form an elongated cylinder of 80A long and 30A diameter. The folding unit is a beta-sandwich with a long axis of 40A and a diameter of 30A. The current model allows us to propose a membrane binding mode for the C2 domains of FV and FVIII with three major characteristics: 1) solvent-exposed hydrophobic side chains from three loops at one end of the beta-sandwich are buried in the hydrophobic layer of the outer phospholipid leaflet; 2) a crown of positively charged residues is located in the polar zone of the phospholipid head groups; and 3) the long axis of the beta-sandwich of the C2 domain is perpendicular to the plane of the membrane. This proposal satisfies experimentally observed characteristics of membrane binding for the C2 domain and the light chain of FVa.

Identification of mouse CPX-2, a novel member of the metallocarboxypeptidase gene family: cDNA cloning, mRNA distribution, and protein expression and characterization

A novel member of the metallocarboxypeptidase gene family was identified from its homology with carboxypeptidase E and has been designated CPX-2. The cDNA of 2500 nucleotides encodes a protein of 764 amino acids that contains an N-terminal signal peptide-like sequence, a 158-residue discoidin domain, and a 400-residue carboxypeptidase domain. The 400-residue metallocarboxypeptidase domain has 59% amino acid identity with a protein designated AEBP-1; 44% to 46% identity with carboxypeptidases E, N, and Z; and lower homology with other members of the metallocarboxypeptidase gene family. The discoidin domain of CPX-2 has 22% amino acid identity with the carbohydrate-binding domain of discoideum-I, 29% to 34% identity with the phospholipid-binding domain of human factors V and VIII, and 59% identity with the discoidin-like domain on AEBP-1. CPX-2 is missing several of the predicted active-site residues that are conserved in most other members of the metallocarboxypeptidase gene family and which are thought to be required for enzyme activity. Expression of CPX-2 using the baculovirus system produced several forms of protein, from 80 to 105 kDa, but no detectable activity toward a variety of carboxypeptidase substrates. A shorter 50-kDa form of CPX-2, which contains the carboxypeptidase domain but not the discoidin domain, was also inactive when expressed in the baculovirus system. CPX-2 is able to bind to Sepharose-Arg; this binding is blocked by 10 mM Arg. Northern blot analysis showed CPX-2 mRNA in mouse brain, liver, kidney, and lung. In situ hybridization analysis of brain revealed a broad distribution. Areas that are enriched in CPX-2 include the hippocampus, cerebral cortex, median eminence, and choroid plexus. Taken together, these data suggest a widespread function for CPX-2, possibly as a binding protein rather than an active carboxypeptidase.

The discoidin domain family revisited: new members from prokaryotes and a homology-based fold prediction

Members of the discoidin (DS) domain family, which includes the C1 and C2 repeats of blood coagulation factors V and VIII, occur in a great variety of eukaryotic proteins, most of which have been implicated in cell-adhesion or developmental processes. So far, no three-dimensional structure of a known example of this extracellular module has been determined, limiting the usefulness of identifying a new sequence as member of this family. Here, we present results of a recent search of the protein sequence database for new DS domains using generalized profiles, a sensitive multiple alignment-based search technique. Several previously unrecognized DS domains could be identified by this method, including the first examples from prokaryotic species. More importantly, we present statistical, structural, and functional evidence that the D1 domain of galactose oxidase whose three-dimensional structure has been determined at 1.7 A resolution, is a distant member of this family. Taken together, these findings significantly expand the concept of the DS domain, by extending its taxonomic range and by implying a fold prediction for all its members. The proposed alignment with the galactose oxidase sequence makes it possible to construct homology-based three-dimensional models for the most interesting examples, as illustrated by an accompanying paper on the C1 and C2 domains of factor V.

The Arg-Gly-Asp motif in the cell adhesion molecule L1 promotes neurite outgrowth via interaction with the alphavbeta3 integrin

The cell adhesion molecule L1 is a potent inducer of neurite outgrowth and it has been implicated in X-linked hydrocephalus and related neurological disorders. To investigate the mechanisms of neurite outgrowth stimulated by L1, attempts were made to identify the neuritogenic sites in L1. Fusion proteins containing different segments of the extracellular region of L1 were prepared and different neuronal cells were assayed on substrate-coated fusion proteins. Interestingly, both immunoglobulin (Ig)-like domains 2 and 6 (Ig2, Ig6) promoted neurite outgrowth from dorsal root ganglion cells, whereas neural retinal cells responded only to Ig2. L1 Ig2 contains a previously identified homophilic binding site, whereas L1 Ig6 contains an Arg-Gly-Asp (RGD) sequence. The neuritogenic activity of Ig6 was abrogated by mutations in the RGD site. The addition of RGD-containing peptides also inhibited the promotion of neurite outgrowth from dorsal root ganglion cells by glutathione S-transferase-Ig6, implicating the involvement of an integrin. The monoclonal antibody LM609 against alphavbeta3 integrin, but not an anti-beta1 antibody, inhibited the neuritogenic effects of Ig6. These data thus provide the first evidence that the RGD motif in L1 Ig6 is capable of promoting neurite outgrowth via interaction with the alphavbeta3 integrin on neuronal cells.

Cloning and characterization of developmental endothelial locus-1: an embryonic endothelial cell protein that binds the alphavbeta3 integrin receptor

We have taken advantage of an enhancer trap event in a line of transgenic mice to identify a unique developmentally regulated endothelial cell locus (Del1). The protein encoded in this locus contains three EGF-like repeats homologous to those in Notch and related proteins, including an EGF-like repeat that contains an RGD motif, and two discoidin I-like domains. Del1 is shown to be a matrix protein and to promote adhesion of endothelial cells through interaction with the alphavbeta3 integrin receptor. Embryonic endothelial-like yolk sac cells expressing recombinant Del1 protein, or grown on an extracellular matrix containing Del1 protein, are inhibited from forming vascular-like structures. Expression of Del1 protein in the chick chorioallantoic membrane leads to loss of vascular integrity and promotes vessel remodeling. Del1 is thus a new ligand for the alphavbeta3 integrin receptor and may function to regulate vascular morphogenesis or remodeling in embryonic development.

An orphan receptor tyrosine kinase family whose members serve as nonintegrin collagen receptors

Mammalian cells constantly monitor and respond to a myriad of extracellular signals, often by using cell surface receptors. Two important classes of cell surface receptors include the receptor tyrosine kinases, which recognize peptide growth factors such as insulin, and the integrins, which most often mediate binding to components of the extracellular matrix. We report that the collagens serve as ligands for the previously orphan family of discoidin domain-containing receptor-like tyrosine kinases. The unexpected realization that an extracellular matrix molecule can directly serve as a ligand for receptor tyrosine kinases provides an example of ligands shared by integrins and receptor tyrosine kinases, and this finding seems likely to change prevailing views about the mechanisms by which cells perceive and respond to the extracellular matrix.

Ricin toxin contains at least three galactose-binding sites located in B chain subdomains 1 alpha, 1 beta, and 2 gamma

Ricin toxin, the heterodimeric 65 kDa glycoprotein synthesized in castor bean seeds, consists of a cell binding lectin subunit (RTB) disulfide linked to an rRNA N-glycosidase protein synthesis inactivating subunit (RTA). While X-ray crystallography and equilibrium dialysis suggested two sugar-combining sites located in subdomains 1 alpha and 2 gamma, biochemical and mutational analyses suggested the existence of a third lectin site. We performed oligonucleotide-directed mutagenesis on RTB cDNA to create mutants with modifications in subdomains 1 alpha, 2 gamma, and either 1 beta or 2 alpha. The triple-site mutant RTBs were expressed in insect cells. Partially purified recombinant proteins obtained from infected cell extracts and cell supernatants were characterized for asialofetuin and cell binding, immunoreactivites, ability to reassociate with RTA, and recombinant heterodimer cell cytotoxicity. Yields of both triple-site mutants were similar to the parent double-site mutant. Both mutants showed immunoreactivity with a panel of anti-RTB monoclonal and polyclonal antibodies. The triple-site mutant with modification of amino acid residues in subdomains 1 alpha, 2 alpha, and 2 gamma bound asialofetuin and cells similarly to the parent 1 alpha, 2 gamma, subdomain mutant. In contrast, the 1 alpha, 1 beta, 2 gamma subdomain triple-site mutant had a one and one-half log decrease in asialofetuin and cell binding relative to the parent double-site mutant. The 1 alpha, 2 alpha, 2 gamma triple-site mutant and 1 alpha, 2 gamma parent protein had sugar binding which was inhibited by 3-27-fold by lactose and asialofetuin. Both triple-site mutants reassociated well with RTA. The 1 alpha, 2 alpha, 2 gamma triple-site mutant-RTA was equally cytotoxic to mammalian cells as the double-site mutant-RTA heterodimer. In contrast, the 1 alpha, 1 beta, 2 gamma triple-site mutant-RTA was 25 times less toxic than the double mutant and 20 times more toxic than RTA alone. These data support a model for at least three lectin-binding subdomains in RTB.

A novel developmental stage-specific lectin of the basidiomycete Pleurotus cornucopiae

A novel lectin was isolated from mycelia of the basidiomycete Pleurotus cornucopiae grown on solid medium. The lectin was purified to homogeneity by mucin-Sepharose affinity chromatography. The molecular mass of the lectin was 40 kDa under reducing conditions, but the subunits were polymerized through disulfide bridges under physiological conditions. Hemagglutinating activity of this lectin was completely inhibited by 2-mercaptoethanol, indicating that the multimer is active. The activity was also inhibited by EDTA, and restored by CaCl2. N-Acetyl-D-galactosamine was the most potent hapten inhibitor. N-terminal amino acid sequence analysis revealed that the mycelial lectin was different from the fruit body lectin of this organism. The mycelial lectin appeared prior to fruit body formation and disappeared during the formation of fruit bodies. The lectin was localized on the surface of solid-medium-grown mycelia, and only dikaryotic, and not monokaryotic, mycelia produced the lectin. These results suggest that the appearance of this lectin is associated with fruit body formation.

The genomic structure of discoidin receptor tyrosine kinase

The discoidin domain receptor (DDR) is a new class of receptor tyrosine kinase that is distinguished by a unique extracellular domain homologous to the lectin Discoidin I found Dictyostelium discoideum. A cosmid was isolated from a human chromosome 6 cosmid library containing the DDR gene. A complete genomic contig of the DDR gene was constructed from seven subclones of the cosmid. The cosmid fragments were analyzed by PCR, sequencing, and comparison of genomic/cDNA sequence. The DDR gene is composed of 17 exons, ranging in size from 96 to 1014 bp, distributed along approximately 12 kb of genomic DNA. The extracellular domain is encoded by 8 exons of which three code for the discoidin domain. The transmembrane domain is encoded by 1 exon, the juxtamembrane by 3 exons, and the catalytic domain by 5 exons. The generation of the two splice variants of DDR, EDDR1 and EDDR2 are explained by the genomic structure. Exon 11 (111 bp in the juxtamembrane domain) is present in DDR and absent in the splice variant EDDR1. An inverted repeat of 20 bp was identified at the 3' exon-intron junction of exon 11, which results in a lariat loop-like secondary structure. EDDR2 is generated because of a cryptic splice acceptor site that results in an extra 18 bp (6 amino acids) inserted 5' of exon 14 in the catalytic domain. A polymorphic (GT)17 repeat was identified in intron 5 with a heterozygosity of 0.71. The exon-intron structure of the DDR gene will be helpful in further understanding of its function and explains the possible structural basis for the two splice variants.

A eukaryotic transcriptional repressor with carboxypeptidase activity

Adipocyte differentiation involves the transcriptional activation of several genes in triglyceride metabolism, including the adipose P2 (aP2 or 422) gene that encodes the adipocyte lipid-binding protein ALBP. Within the mouse aP2 promoter region, the AE-1 sequence functions as either a positive or a negative element in the regulation of aP2 gene expression. The AE-1 sequence is the binding site for the positive murine (3T3) adipocyte factor C/EBP-alpha, several human preadipocyte factors, and a 3T3 preadipocyte factor(s) that has been implicated as a repressor of aP2 gene expression. Here we report the cloning of new complementary DNAs that encode the 3T3 preadipocyte factor (termed AEBP1) and demonstrate that AEBP1 expression is abolished during adipocyte differentiation. Furthermore, we show that an activity of a carboxypeptidase associated with AEBP1 is important in the transcriptional repression function of AEBP1. Thus AEBP1 might represent a new type of transcription factor that regulates transcription by cleavage of factors involved in transcription.

The effect of endogenous oestradiol levels on protein S concentration during a menstrual cycle and after GnRH analogues and gonadotropin therapy

We studied two groups of females to investigate the effect of endogenous oestradiol levels on total and free protein S (tPS, fPS) plasma concentrations. One group (group I) consisted of 12 healthy volunteers who were studied throughout one menstrual cycle; the other group (group II) consisted of 16 young women who were treated with GnRH analogues and gonadotropins before undergoing in vitro fertilization. Neither tPS nor fPS varied significantly with respect to the physiological changes of oestradiol or to the very low and high levels of oestradiol, achieved after GnRH analogues suppression and gonadotropin stimulation. These results indicate that endogenous oestradiol does not affect PS concentration.