Isolation, characterization, and primary structure of a calcium-binding 63-kDa bone protein

Aberrant environment and PS-binding to calnuc C-terminal tail drives exosomal packaging and its metastatic ability

The characteristic features of cancer cells are aberrant (acidic) intracellular pH and elevated levels of phosphatidylserine. The primary focus of cancer research is concentrated on the discovery of biomarkers directed towards early diagnosis and therapy. It has been observed that azoxymethane-treated mice demonstrate an increased expression of calnuc (a multi-domain, Ca2+- and DNA-binding protein) in their colon, suggesting it to be a good biomarker of carcinogenesis. We show that culture supernatants from tumor cells have significantly higher amounts of secreted calnuc compared to non-tumor cells, selectively packaged into exosomes. Exosomal calnuc is causal for epithelial-mesenchymal transition and atypical migration in non-tumor cells, which are key events in tumorigenesis and metastasis. In vitro studies reveal a significant affinity for calnuc towards phosphatidylserine, specifically to its C-terminal region, leading to the formation of 'molten globule' conformation. Similar structural changes are observed at acidic pH (pH 4), which demonstrates the role of the acidic microenvironment in causing the molten globule conformation and membrane interaction. On a precise note, we propose that the molten globule structure of calnuc caused by aberrant conditions in cancer cells to be the causative mechanism underlying its exosome-mediated secretion, thereby driving metastasis.

NUCB1 Suppresses Growth and Shows Additive Effects With Gemcitabine in Pancreatic Ductal Adenocarcinoma via the Unfolded Protein Response

Pancreatic ductal adenocarcinoma (PDAC) is a highly aggressive cancer with poor patient prognosis. A cellular stress response mechanism called the unfolded protein response (UPR) has been implicated in PDAC progression. More recently, nucleobindin 1 (NUCB1), a calcium-binding protein, has been shown to control the UPR but its precise role in PDAC has not been explored. Here, we found that downregulation of NUCB1 was associated with poor prognosis in patients with PDAC. Functionally, NUCB1 overexpression suppressed pancreatic cancer cell proliferation and showed additive effects with gemcitabine (GEM) in vitro and in vivo. Moreover, by controlling ATF6 activity, NUCB1 overexpression suppressed GEM-induced UPR and autophagy. Last but not least, we uncovered METTL3-mediated m⁶A modification on NUCB1 5'UTR via the reader YTHDF2 as a mechanism for NUCB1 downregulation in PDAC. Taken together, our study revealed crucial functions of NUCB1 in suppressing proliferation and enhancing the effects of gemcitabine in pancreatic cancer cells and identified METTL3-mediated m⁶A modification as a mechanism for NUCB1 downregulation in PDAC.

A Change in Domain Cooperativity Drives the Function of Calnuc

With the increasing incidence of neurodegenerative disorders, there is an urgent need to understand the protein folding process. Examining the folding process of multidomain proteins remains a prime challenge, as their complex conformational dynamics make them highly susceptible to misfolding and/or aggregation. The presence of multiple domains in a protein can lead to interaction between the partially folded domains, thereby driving misfolding and/or aggregation. Calnuc is one such multidomain protein for which Ca²⁺ binding plays a pivotal role in governing its structural dynamics and stability and, presumably, in directing its interactions with other proteins. We demonstrate differential structural dynamics between the Ca²⁺-free and Ca²⁺-bound forms of calnuc. In the absence of Ca²⁺, full-length calnuc displays equilibrium structural transitions with four intermediate states, reporting a sum of the behavioral properties of its individual domains. Fragment-based studies illustrate the sequential events of structure adoption proceeding in the following order: EF domain followed by the NT and LZ domains in the apo state. On the other hand, Ca²⁺ binding increases domain cooperativity and enables the protein to fold as a single unit. Single-tryptophan mutant proteins, designed in a domain-dependent manner, confirm an increase in the number of interdomain interactions in the Ca²⁺-bound form as compared to the Ca²⁺-free state of the protein, thereby providing insight into its folding process. The attenuated domain crosstalk in apo-calnuc is likely to influence and regulate its physiologically important intermolecular interactions.

Nucleobindin 1 (NUCB1) is a Golgi-resident marker of neurons

Nucleobindin 1 (NUCB1; also known as CALNUC or NUC) is a putative DNA- and calcium-binding protein and exhibits significant structural homology with the protein nucleobindin 2 (NUCB2; also known as nesfatin). While NUCB2 has been mapped in detail in the brain and implicated in the hypothalamic control of energy metabolism, no study has to date addressed the presence of NUCB1 in the central nervous system. Here we have explored the expression and distribution of NUCB1 in the rat brain and spinal cord, using RT-PCR, immunofluorescence and in situ hybridization. NUCB1 mRNA and protein was found to be present in all brain regions, extending to the spinal cord and dorsal root ganglia. Double-staining for NUCB1 and NeuN, glial fibrillary acidic protein and myelin basic protein revealed that NUCB1 is exclusively found in neurons, and not in glial or ependymal cells. Notably, NUCB1-immunoreactivity was observed in all neurons examined, making no distinction between previously identified glutamatergic and GABAergic populations, including those that are known not to stain for NeuN. This included the markedly more restricted population of NUCB2-expressing neurons in the brain. The protein was detected in cell somata and proximal dendrites, but not in axons or terminal structures. Further examination of the subcellular distribution of NUCB1 using organelle-specific markers revealed its consistent presence in the Golgi apparatus. These findings identify NUCB1 as a novel pan-neuronal marker. Along with the recent demonstration of broad expression of the protein in endocrine cells, the present results suggest that NUCB1 may play a role in spatiotemporal calcium handling in signaling cells.

Nucleobindin-2 is a positive regulator for insulin-stimulated glucose transporter 4 translocation in fenofibrate treated E11 podocytes

The physiology of insulin signaling under normal and disease conditions is well studied in classical insulin target tissues, but not in podocytes. To examine insulin stimulation of podocyte GLUT4 translocation, we established a protocol involving treatment with the PPARα agonist fenofibrate to induce E11 podocyte differentiation within 48 hours rather than 7-10 days, which is required for differentiation under the reported protocol. This allowed us to transiently introduce GLUT4 reporter cDNA and RNAi and thereby to examine the regulatory pathway involved. Here we demonstrate that treatment with 200 μM fenofibrate for 36 hours following transfection had a dramatic effect on podocyte morphology, induced several podocyte specific protein expression markers (G protein-coupled receptor 137B, chloride intracellular channel 5, and nephrin) and resulted in insulin-stimulated GLUT4 translocation. In addition, Nucleobindin-2 was found to constitutively associate with Septin 7 (the repressor of GLUT4 translocation), and knockdown of Nucleobindin-2 was found to completely abrogate insulin-stimulated GLUT4 translocation. Together, these data suggest that Nucleobindin-2 may repress Septin7-induced inhibition of insulin-stimulated GLUT4 translocation in podocytes.

Secreted nucleobindin-2 inhibits 3T3-L1 adipocyte differentiation

Nucleobindin-2 is a 420 amino acid EF-hand Ca2+ binding protein that can be further processed to generate an 82 amino terminal peptide termed Nesfatin-1. To examine the function of secreted Nucleobindin-2 in adipocyte differentiation, cultured 3T3-L1 cells were incubated with either 0 or 100 nM of GST, GST-Nucleobindin-2, prior to and during the initiation of adipocyte differentiation. Nucleobindin-2 treatment decreased neutral lipid accumulation (Oil-Red O staining) and expression of several marker genes for adipocyte differentiation (PPARγ, aP2, and adipsin). When Nucleobindin-2 was constitutively secreted into cultured medium, cAMP content and insulin stimulated CREB phosphorylation were significantly reduced. On the other hand, intracellularly overexpressed Nucleobindin-2 failed to affect cAMP content and CREB phosphorylation. Taken together, these data indicate that secreted Nucleobindin-2 is a suppressor of adipocyte differentiation through inhibition of cAMP production and insulin signal.

Effect of Nesfatin-1 on gastric acid secretion in rats

AIM: To investigate the effect of Nesfatin-1 on gastric acid secretion in rats.

METHODS: Thirty-six male Sprague-Dawley rats were randomly and equally divided into six groups to receive intracerebroventricular injection of 0.05, 0.5 μg of Nesfatin-1 or sterile water and intravenous injection of 10, 50 μg/kg of Nesfatin-1 or sterile water. Gastric secretion was measured using the pylorus-ligation method. Three hours after treatment, rats were killed to remove the stomach and collect the gastric contents. The volume of gastric secretion was measured and the amount of gastric acid was determined by titration with NaOH. H⁺-K⁺-ATPase mRNA expression was detected by RT-PCR.

RESULTS: Intracerebroventricular infusion of 0.05 or 0.5 μg of Nesfatin-1 significantly reduced the volume of gastric juice (3.3 mL/3 h ± 0.3 mL/3 h vs 2.4 mL/3 h ± 0.3 mL/3 h; 3.3 mL/3 h ± 0.3 mL/3 h vs 2.5 mL/3 h ± 0.3 mL/3 h, both P < 0.05), inhibited gastric acid output (582.7 μmol/3 h ± 59.3 μmol/3 h vs 373.6 μmol/3 h ± 61.5 μmol/3h, 582.7 μmol/3 h ± 59.3 μmol/3h vs 380.0 μmol/3 h ± 55.8 μmol/3h, both P < 0.05), and decreased gastric H⁺-K⁺-ATPase mRNA expression (both P < 0.05). Intravenous injection of 10 or 50 μg/kg of Nesfatin-1 had no significant effect on the volume of gastric juice (3.7 mL/3 h ± 0.7 mL/3 h vs 3.3 mL/3 h ± 0.4 mL/3 h, 3.7 mL/3 h ± 0.7 mL/3 h vs 3.8 mL/3 h ± 0.5 mL/3 h, both P > 0.05), gastric acid output (647.6 μmol/3 h ± 102.8 μmol/3 h vs 573.8 μmol/3 h ± 97.4 μmol/3 h, 647.6 μmol/3 h ± 102.8 μmol/3 h vs 594.4 μmol/3 h ± 121.0 μmol/3 h, both P > 0.05) and gastric H⁺-K⁺-ATPase mRNA expression (both P >0.05 ).

CONCLUSION: Nesfatin-1 acts in the central nervous system to inhibit gastric acid secretion in rats.

Nucleobindins: bioactive precursor proteins encoding putative endocrine factors?

The nucleobindins, nucleobindin 1 (NUCB1) and nucleobindin 2 (NUCB2), are homologous multidomain calcium and DNA binding proteins. NUCB1 is a well-characterized Golgi protein found within the rat pituitary, liver and kidney with functions related to immunity, calcium homeostasis and G protein signaling. NUCB2 is found both in the hypothalamus and brain stem centers, as well as peripherally in the digestive tract. Renewed interest in the nucleobindins has been sparked by the recent discovery of nesfatin-1, an endocrine factor post-translationally processed from the N-terminal of NUCB2. Nesfatin-1 has quickly established itself as a novel regulator of appetite, insulin secretion, energy homeostasis and reproduction with important consequences to the etiology of metabolic diseases including diabetes and obesity. The discovery of nesfatin-1 and it endocrine functions attracted more attention to the nucleobindins that are already known to have important intracellular functions. From the sequence information available, it is possible that nucelobindins itself or nesfatin-1 like peptides within the NUCB1 could also elicit nesfatin-1-like biological functions. The research on nesfatin-1 in last 5years further adds to the importance of nucleobindins as potential endocrine precursors. This review aims to summarize some of the most recent findings on the functional significance of NUCB1, NUCB2, as well as encoded proteins and highlights the questions that remain unanswered.

Empirical evaluation of bone extraction protocols

The application of high-resolution analytical techniques to characterize ancient bone proteins requires clean, efficient extraction to obtain high quality data. Here, we evaluated many different protocols from the literature on ostrich cortical bone and moa cortical bone to evaluate their yield and relative purity using the identification of antibody-antigen complexes on enzyme-linked immunosorbent assay and gel electrophoresis. Moa bone provided an ancient comparison for the effectiveness of bone extraction protocols tested on ostrich bone. For the immunological part of this study, we focused on collagen I, osteocalcin, and hemoglobin because collagen and osteocalcin are the most abundant proteins in the mineralized extracellular matrix and hemoglobin is common in the vasculature. Most of these procedures demineralize the bone first, and then the remaining organics are chemically extracted. We found that the use of hydrochloric acid, rather than ethylenediaminetetraacetic acid, for demineralization resulted in the cleanest extractions because the acid was easily removed. In contrast, the use of ethylenediaminetetraacetic acid resulted in smearing upon electrophoretic separation, possibly indicating these samples were not as pure. The denaturing agents sodium dodecyl sulfate, urea, and guanidine HCl have been used extensively for the solubilization of proteins in non-biomineralized tissue, but only the latter has been used on bone. We show that all three denaturing agents are effective for extracting bone proteins. One additional method tested uses ammonium bicarbonate as a solubilizing buffer that is more appropriate for post-extraction analyses (e.g., proteomics) by removing the need for desalting. We found that both guanidine HCl and ammonium bicarbonate were effective for extracting many bone proteins, resulting in similar electrophoretic patterns. With the increasing use of proteomics, a new generation of scientists are now interested in the study of proteins from not only extant bone but also from ancient bone.

Phosphoproteomes of Strongylocentrotus purpuratus shell and tooth matrix: identification of a major acidic sea urchin tooth phosphoprotein, phosphodontin

BACKGROUND

Sea urchin is a major model organism for developmental biology and biomineralization research. However, identification of proteins involved in larval skeleton formation and mineralization processes in the embryo and adult, and the molecular characterization of such proteins, has just gained momentum with the sequencing of the Strongylocentrotus purpuratus genome and the introduction of high-throughput proteomics into the field.

RESULTS

The present report contains the determination of test (shell) and tooth organic matrix phosphoproteomes. Altogether 34 phosphoproteins were identified in the biomineral organic matrices. Most phosphoproteins were specific for one compartment, only two were identified in both matrices. The sea urchin phosphoproteomes contained several obvious orthologs of mammalian proteins, such as a Src family tyrosine kinase, protein kinase C-delta 1, Dickkopf-1 and other signal transduction components, or nucleobindin. In most cases phosphorylation sites were conserved between sea urchin and mammalian proteins. However, the majority of phosphoproteins had no mammalian counterpart. The most interesting of the sea urchin-specific phosphoproteins, from the perspective of biomineralization research, was an abundant highly phosphorylated and very acidic tooth matrix protein composed of 35 very similar short sequence repeats, a predicted N-terminal secretion signal sequence, and an Asp-rich C-terminal motif, contained in [Glean3:18919].

CONCLUSIONS

The 64 phosphorylation sites determined represent the most comprehensive list of experimentally identified sea urchin protein phosphorylation sites at present and are an important addition to the recently analyzed Strongylocentrotus purpuratus shell and tooth proteomes. The identified phosphoproteins included a major, highly phosphorylated protein, [Glean3:18919], for which we suggest the name phosphodontin. Although not sequence-related to such highly phosphorylated acidic mammalian dental phosphoproteins as phosphoryn or dentin matrix protein-1, phosphodontin may perform similar functions in the sea urchin tooth. More than half of the detected proteins were not previously identified at the protein level, thus confirming the existence of proteins only known as genomic sequences previously.

A novel endoplasmic reticulum export signal: proline at the +2-position from the signal peptide cleavage site

NUCB1 (nucleobindin 1) is a Golgi-localized soluble protein with a signal peptide and multiple functional domains. We reported recently that NUCB1 is a negative regulator of the unfolded protein response that activates various endoplasmic reticulum (ER)-originating signaling pathways. In that report, we also showed that Golgi localization of NUCB1 was essential to regulate the unfolded protein response. However, the localization mechanism of NUCB1 is still unknown. Here, we report that the proline residue at the +2-position (Pro(+2)) from the signal peptide cleavage site is the determinant of NUCB1 protein export from the ER and subsequent transport to the Golgi. Fusion of the N-terminal amino acids 1-35 peptide region, including both signal peptide (amino acids 1-26) and Pro(+2), was sufficient for enhanced green fluorescent protein to localize in the Golgi, whereas single amino acid mutation of Pro(+2) resulted in defective export from the ER without affecting the protein maturation process. Furthermore, we demonstrated that Pro(+2) was important for the enhanced green fluorescent protein fusion protein to concentrate at a transport vesicle formation site within the ER, often termed the ER exit site. Interestingly, such a Pro(+2) has also been functionally conserved in other Golgi-localized soluble proteins, Cab45 (Ca(2+)-binding protein of 45 kDa), reticulocalbin 1, and calumenin. Our findings indicate that Pro(+2) can function as a novel ER export signal of some Golgi proteins.

Molecular characterisation and expression analysis of SEREX-defined antigen NUCB2 in gastric epithelium, gastritis and gastric cancer

NUCB2 is an EF-hand Ca²⁺ binding protein that has been implicated in various physiological processes like calcium homeostasis, hypothalamic regulation of feeding and TNF receptor shedding. In our previous study we identified NUCB2 as a potential tumour antigen eliciting autoantibody responses in 5.4% of gastric cancer patients but not in the healthy individuals. The current study aimed to elucidate the molecular mechanism underlying NUCB2 immunogenicity and to gain an insight into the physiological functions of NUCB2 in the stomach. mRNA expression analysis demonstrated that NUCB2 is ubiquitously expressed in normal tissues, including lymphoid tissues, and downregulated in gastric tumours when compared with the adjacent relatively normal stomach tissues. The search for molecular alterations resulted in the identification of novel mRNA variants transcribed from an alternative promoter and expressed predominantly in gastric cancers. Western blot analysis demonstrated that the protein levels correspond to mRNA levels and revealed that NUCB2 is phosphorylated in gastric mucosa. Furthermore, a 55 kDa isoform, generated presumably by yet an unidentified post-translational modification was detected in gastric tumours and AGS gastric cancer cells but was absent in the relatively normal gastric mucosa and thereby might have served as a trigger for the immune response against NUCB2. Staining of stomach tissue microarray with anti-NUCB2 antibody revealed that it is expressed in the secretory granules of chief cells and in the cytoplasm of parietal cells in the functioning gastric glands which are lost in atrophic glands and tumour cells. Hence we propose that NUCB2 may be implicated in gastric secretion by establishing an agonist-releasable Ca²⁺ store in ER or Golgi apparatus, signalling via heterotrimeric G_α proteins and/or mediating the exocytosis of the secretory granules.

Calnuc plays a role in dynamic distribution of Galphai but not Gbeta subunits and modulates ACTH secretion in AtT-20 neuroendocrine secretory cells

In AtT-20 cells ACTH secretion is regulated by both Ca²⁺ and G proteins. We previously demonstrated that calnuc, an EF-hand Ca²⁺ binding protein which regulates Alzheimer's β-amyloid precursor protein (APP) biogenesis, binds both Ca²⁺ as well as Gα subunits. Here we investigate calnuc's role in G protein-mediated regulation of ACTH secretion in AtT-20 neuroendocrine secretory cells stably overexpressing calnuc-GFP. Similar to endogenous calnuc, calnuc-GFP is mainly found in the Golgi, on the plasma membrane (PM), and associated with regulated secretion granules (RSG). By deconvolution immunofluorescence, calnuc-GFP partially colocalizes with Gαi1/2 and Gαi3 at the PM and on RSG. Cytosolic calnuc(ΔSS)-CFP with the signal sequence deleted also partially colocalizes with RSG and partially cosediments with Gαi1/2 in fractions enriched in RSG. Overexpression of calnuc-GFP specifically increases the distribution of Gαi1/2 on the PM whereas the distribution of Gβ subunits and synaptobrevin 2 (Vamp 2) is unchanged. Overexpression of calnuc-GFP or cytosolic calnuc(ΔSS)-CFP enhances ACTH secretion two-fold triggered by mastoparan or GTPγS but does not significantly affect glycosaminoglycan (GAG) chain secretion along the constitutive pathway or basal secretion of ACTH. Calnuc's facilitating effects on ACTH secretion are decreased after introducing anti-Gαi1/2, Gαi3, Gβ or calnuc IgG into permeabilized cells but not when Gα12 or preimmune IgG is introduced. The results suggest that calnuc binds to Gα subunits on the Golgi and on RSG and that overexpression of calnuc causes redistribution of Gαi subunits to the PM and RSG, indicating that calnuc plays a role in dynamic distribution of only Gα but not Gβ subunits. Thus calnuc may connect G protein signaling and calcium signaling during regulated secretion.

Nucleobindin co-localizes and associates with cyclooxygenase (COX)-2 in human neutrophils

The inducible cyclooxygenase isoform (COX-2) is associated with inflammation, tumorigenesis, as well as with physiological events. Despite efforts deployed in order to understand the biology of this multi-faceted enzyme, much remains to be understood. Nucleobindin (Nuc), a ubiquitous Ca²⁺-binding protein, possesses a putative COX-binding domain. In this study, we investigated its expression and subcellular localization in human neutrophils, its affinity for COX-2 as well as its possible impact on PGE₂ biosynthesis. Complementary subcellular localization approaches including nitrogen cavitation coupled to Percoll fractionation, immunofluorescence, confocal and electron microscopy collectively placed Nuc, COX-2, and all of the main enzymes involved in prostanoid synthesis, in the Golgi apparatus and endoplasmic reticulum of human neutrophils. Immunoprecipitation experiments indicated a high affinity between Nuc and COX-2. Addition of human recombinant (hr) Nuc to purified hrCOX-2 dose-dependently caused an increase in PGE₂ biosynthesis in response to arachidonic acid. Co-incubation of Nuc with COX-2-expressing neutrophil lysates also increased their capacity to produce PGE₂. Moreover, neutrophil transfection with hrNuc specifically enhanced PGE₂ biosynthesis. Together, these results identify a COX-2-associated protein which may have an impact in prostanoid biosynthesis.

Nucleobindin 1 Controls the Unfolded Protein Response by Inhibiting ATF6 Activation

In response to endoplasmic reticulum (ER) stress, activating transcription factor 6 (ATF6), an ER membrane-anchored transcription factor, is transported to the Golgi apparatus and cleaved by site-1 protease (S1P) to activate the unfolded protein response (UPR). Here, we identified nucleobindin 1 (NUCB1) as a novel repressor of the S1P-mediated ATF6 activation. NUCB1 is an ER stress-inducible gene with the promoter region having functional cis-elements for transcriptional activation by ATF6. Overexpression of NUCB1 inhibits S1P-mediated ATF6 cleavage without affecting ER-to-Golgi transport of ATF6, whereas knock-down of NUCB1 by siRNA accelerates ATF6 cleavage during ER stress. NUCB1 protein localizes in the Golgi apparatus, and disruption of the Golgi localization results in loss of the ATF6-inhibitiory activity. Consistent with these observations, NUCB1 can suppress physical interaction of S1P-ATF6 during ER stress. Together, our results demonstrate that NUCB1 is the first-identified, Golgi-localized negative feedback regulator in the ATF6-mediated branch of the UPR.

Calnuc binds to Alzheimer's beta-amyloid precursor protein and affects its biogenesis

Calnuc, a Golgi calcium binding protein, plays a key role in the constitution of calcium storage. Abnormal calcium homeostasis has been linked to Alzheimer's disease (AD). Excessive production and/or accumulation of beta-amyloid (Abeta) peptides that are proteolytically derived from the beta-amyloid precursor protein (APP) have been linked to the pathogenesis of AD. APP has also been indicated to play multiple physiological functions. In this study, we demonstrate that calnuc interacts with APP through direct binding to the carboxyl-terminal region of APP, possibly in a calcium-sensitive manner. Immunofluorescence study revealed that the two proteins co-localize in the Golgi in both cultured cells and mouse brains. Over-expression of calnuc in neuroblastoma cells significantly reduces the level of endogenous APP. Conversely, down-regulation of calnuc by siRNA increases cellular levels of APP. Additionally, we show that over-expression of calnuc down-regulates the APP mRNA level and inhibits APP biosynthesis, which in turn results in a parallel reduction of APP proteolytic metabolites, sAPP, CTFs and Abeta. Furthermore, we found that the level of calnuc was significantly decreased in the brain of AD patients as compared with that of age-matched non-AD controls. Our results suggest a novel function of calnuc in modulating the levels of APP and its proteolytic metabolites, which may further affect the patho/physiological functions of APP including AD pathogenesis.

A comprehensive proteomic analysis of the accessory sex gland fluid from mature Holstein bulls

The expression of proteins in accessory sex gland fluid (AGF) of proven, high use mature Holstein bulls was evaluated. Thirty-seven bulls with documented fertility based on their non-return rates were studied. AGF was obtained by artificial vagina after bulls were surgically equipped with cannulae in the vasa deferentia. Samples of AGF were evaluated by two-dimensional SDS-PAGE, gels stained with Coomassie blue and polypeptide maps analyzed by PDQuest software. A master gel generated by the software representing the best pattern of spots in the AGF polypeptide maps was used as a reference for protein identification. Proteins were identified by Western blots and capillary liquid chromatography-nanoelectrospray ionization tandem-mass spectrometry (CapLC-MS/MS). The product ion spectra were processed using Protein Lynx Global Server 2.1 prior to database search with both PLGS and MASCOT (Matrix Science) software. The entire NCBI database was considered for mass fingerprint matching. An average of 52+/-5 spots was detected in the AGF 2D gels, which corresponded to proteins potentially involved in capacitation (bovine seminal plasma protein-BSP-A1/A2 and A3, BSP 30 kDa, albumin); sperm membrane protection, prevention of oxidative stress, complement-mediated sperm destruction and anti-microbial activity (albumin, clusterin, acidic seminal fluid protein--aSFP, 5'-nucleotidase--5'-NT, phospholipase A2--PLA2); acrosome reaction and sperm-oocyte interaction (PLA2, osteopontin); interaction with the extracellular matrix (tissue inhibitor of metalloproteinase 2, clusterin) and sperm motility (aSFP, spermadhesin Z13, 5'-NT). The 20 spots distinguished in all gels were matched to proteins associated with these functions. Proteins identified by tandem mass spectrometry as ecto-ADP-ribosyltransferase 5 and nucleobindin, never described before in the accessory sex gland secretions, were also detected. In summary, we identified a diverse range of components in the accessory sex gland fluid of a select group of Holstein bulls with documented fertility. Known characteristics of these proteins suggest that they play important roles in sperm physiology after ejaculation.

Proteins of the accessory sex glands associated with the oocyte-penetrating capacity of cauda epididymal sperm from holstein bulls of documented fertility

We previously reported that accessory sex gland fluid (AGF) from high fertility (HF) bulls influenced the oocyte-penetrating capacity of cauda epididymal sperm from low fertility (LF) bulls, based on in vitro fertilization (IVF) assays. The present study determined if AGF proteins were associated with these effects. Nineteen IVF assays with 12 bulls were grouped as follows. Group I (n = 8): assays where sperm from LF bulls exposed to AGF from HF bulls had greater oocyte penetration than exposed to homologous AGF. Group II (n = 7): sperm from LF bulls to AGF from HF bulls versus homologous AGF showed no significant differences. Group III (n = 4): sperm from LF bulls treated with homologous AGF had greater fertility than sperm treated with AGF from HF bulls. Sire fertility was based on nonreturn rates (NNR) and AGF collected by artificial vagina from bulls with cannulated vasa deferentia. Two-dimensional SDS-PAGE maps of AGF were analyzed by PDQuest and proteins identified by tandem mass spectrometry and Western blots. Differences in spot intensity between AGF of HF and LF bulls were compared across groups of IVF assays (P < 0.05). The expression of BSP A1/A2 and A3, BSP 30 kDa, clusterin, albumin, phospholipase A(2) (PLA(2)), and osteopontin was greater in the AGF of HF bulls in Group I as compared to Groups II and III. Conversely, there was less nucleobindin in the AGF of HF bulls in Group I than in Groups II and III. This is the first report of nucleobindin (58 kDa/pI 5.6) in male reproductive fluids, using both immunoblots and mass spectrometry. Thus, the effect of AGF from HF bulls on epididymal sperm is likely the result of specific proteins expressed in the AGF.

Nucleobindin is produced by bone cells and secreted into the osteoid, with a potential role as a modulator of matrix maturation

Nucleobindin (Nuc), also known as CALNUC, is a Ca(2+)-binding protein, located in the nucleus, the Golgi apparatus and the endoplasmic reticulum (ER). The presence of a signal sequence in Nuc suggests secretion from the cell and it has been found in bone extracellular matrix. Within the present study, molecular biological and morphological methods were combined to evaluate the synthesis and distribution of Nuc in and around cells of rat metaphyseal and calvarial bone. Northern blot analysis and in situ hybridization of bone tissues confirmed that the protein was a product of bone cells. By electron microscopy, immunolabeling for Nuc was seen in osteoid of newly formed bone, on all surfaces facing the various bone cells and also in compact bone. Intracellularly, the gold particles were found in the rough ER of osteoblasts, which suggested synthesis of the protein by these cells. Compared to bone sialoprotein and osteopontin, Nuc demonstrated different localization pattern in bone trabeculae, with the majority of labeling restricted to nonmineralized osteoid. Moreover, the role of Nuc during the mineralization process was investigated in rat calvaria-derived primary osteoblasts grown under osteogenic conditions. Semiquantitative RT-PCR and Northern blot analysis showed Nuc expression to be low during cell proliferation, upregulated during differentiation and matrix maturation, but subsequently downregulated during mineralization. In summary, our data show that Nuc was synthesized by osteoblasts and osteocytes, and secreted into the osteoid, suggesting a role as a modulator of matrix maturation in the mineralization process in bone.

Calnuc, an EF-hand Ca(2+)-binding protein, is stored and processed in the Golgi and secreted by the constitutive-like pathway in AtT20 cells

Calnuc is an ubiquitous, EF-hand Ca(2+) binding protein found in the cytoplasm where it binds to Galphai3, in the Golgi lumen where it constitutes a Ca(2+) storage pool, and secreted outside the cell. Here we investigated the pathway of secretion of calnuc in AtT20 cells. We found by pulse-chase experiments that calnuc is synthesized in the endoplasmic reticulum, transported to the Golgi where it remains greater than 12 h and undergoes posttranslational modification (O-glycosylation and sulfation) followed by secretion into the culture medium. We examined if calnuc is secreted by the constitutive or regulated secretory pathway in AtT20 cells. By immunofluorescence and immunogold labeling, endogenous calnuc is found in immature secretion granules (ISG) but not mature regulated secretory granules (RSG), whereas overexpressed calnuc-green fluorescent protein (GFP) is found in both ISG and RSG, where it colocalizes with ACTH. Neither calnuc nor calnuc-GFP are released by the regulated secretory pathway, suggesting that endogenous calnuc and calnuc-GFP are progressively removed from ISG and RSG during granule maturation. We conclude that calnuc is secreted via the constitutive-like pathway and represents a useful endogenous marker for this pathway in AtT20 cells. Together, these observations indicate that calnuc has a unique itinerary as it is retained in the Golgi and is then constitutively secreted extracellularly where it may influence cell behavior via its Ca(2+)-binding properties.

Galpha i3 binding to calnuc on Golgi membranes in living cells monitored by fluorescence resonance energy transfer of green fluorescent protein fusion proteins

Galphai3 is found both on the plasma membrane and on Golgi membranes. Calnuc, an EF hand protein, binds both Galphai3 and Ca(2+) and is found both in the Golgi lumen and in the cytoplasm. To investigate whether Galphai3 binds calnuc in living cells and where this interaction takes place we performed fluorescence resonance energy transfer (FRET) analysis between Galphai3 and calnuc in COS-7 cells expressing Galphai3-yellow fluorescent protein (YFP) and calnuc-cyan fluorescent protein (CFP). The tagged proteins have the same localization as the endogenous, nontagged proteins. When Galphai3-YFP and calnuc-CFP are coexpressed, a FRET signal is detected in the Golgi region, but no FRET signal is detected on the plasma membrane. FRET is also seen within the Golgi region when Galphai3 is coexpressed with cytosolic calnuc(DeltaN2-25)-CFP lacking its signal sequence. No FRET signal is detected when Galphai3(DeltaC12)-YFP lacking the calnuc-binding region is coexpressed with calnuc-CFP or when Galphai3-YFP and calnuc(DeltaEF-1,2)-CFP, which is unable to bind Galphai3, are coexpressed. Galphai3(G2AC3A)-YFP lacking its lipid anchors is localized in the cytoplasm, and no FRET signal is detected when it is coexpressed with wild-type calnuc-CFP. These results indicate that cytosolic calnuc binds to Galphai3 on Golgi membranes in living cells and that Galphai3 must be anchored to the cytosolic surface of Golgi membranes via lipid anchors for the interaction to occur. Calnuc has the properties of a Ca(2+) sensor protein capable of binding to and potentially regulating interactions of Galphai3 on Golgi membranes.

Golgi retention of human protein NEFA is mediated by its N-terminal Leu/Ile-rich region

The subcellular localization of the human Ca(2+)-binding EF-hand/leucine zipper protein NEFA was studied in HeLa cells by immunofluorescence microscopy. Double immunostaining using mouse anti-NEFA monoclonal antibody 1H8D12 and rabbit anti-ERD2 polyclonal antibody proved that NEFA is localized in the Golgi apparatus. The result was confirmed by the expression of NEFA-green fluorescent protein (GFP) fusion protein in the Golgi in the same cell line. Cycloheximide treatment proved NEFA to be a Golgi-resident protein. Seven NEFA deletion mutants were constructed to ascertain the peptide region relevant for Golgi retention. The expression of each NEFA-GFP variant was detected by fluorescence microscopy and immunoblotting. Only the DeltaN mutant, lacking the N-terminal Leu/Ile-rich region, failed to be retained in the Golgi after cycloheximide treatment. The other six deletion mutants in which either the basic region, the complete EF-hand pair domain, the two EF-hand motifs separately, the leucine zipper and the leucine zipper plus the C-terminal region is deleted, were localized to the Golgi. The peptide sequence within the Leu/Ile-rich region is discussed as a novel Golgi retention motif.

Calnuc, an EF-hand Ca(2+) binding protein, specifically interacts with the C-terminal alpha5-helix of G(alpha)i3

Calnuc (nucleobindin) was previously shown to be present both in the cytosol and in the Golgi and to be the major Golgi Ca(2+) binding protein. In this study we verified the existence of the cytosolic pool of calnuc and investigated its interaction with G(alpha)i3. Cytosolic calnuc was released by mild digitonin permeabilization. In pulse-chase experiments, the two pools of calnuc had different mobilities, suggesting different posttranslational modifications. That calnuc interacts with G(alpha)i3 in vivo was verified by the finding that G(alpha)i3 could be crosslinked intracellularly to calnuc and co-immunoprecipitated from NIH 3T3 cells stably overexpressing either activated (Q204L) or inactivated (G203A) G(alpha)i3. Binding was Ca(2+) and Mg(2+)-dependent. Calnuc and G(alpha)i3-GFP codistributed primarily in the Golgi region. By yeast two-hybrid analysis, the binding site on G(alpha)i3 for calnuc was mapped to the C-terminal region because removal of the last 12 amino acids (but not 11) abolished the interaction. Peptide competition indicated that calnuc, with its coiled-coil domain constituted by the two EF-hands, binds to G(alpha)i3's C-terminal alpha5-helix. These results demonstrate that calnuc may play an important role in G protein- and Ca(2+)-regulated signal transduction events.

Tissue distribution of a novel cell binding protein, osteoadherin, in the rat

Osteoadherin is a cell binding keratan sulfate proteoglycan which was recently isolated from mineralized bovine bone and subsequently cloned and sequenced. For studies of osteoadherin expression in rat tissues we isolated and sequenced a 1.3-kbp partial cDNA covering most of the coding region using a rat calvaria cDNA library. The most 5' end of the cDNA was obtained by reverse transcription PCR from the bone total RNA preparation. The deduced, translated protein sequence containing 423 amino acid residues shows high sequence identity to mouse, bovine and human osteoadherin except in the very acidic C-terminal region. However, the rat counterpart showed a similarly high content of acidic amino acid residues. Ribonuclease protection assay showed osteoadherin mRNA to be expressed in femoral bone and calvaria tissues, while no expression was detected in cartilage, tendon or liver. Using very sensitive nested RT-PCR, however, message was detected in femoral head, rib, tendon and bone marrow total RNA preparations. An antiserum specific for the rat C-terminal region of osteoadherin was generated and used for studies of protein distribution by immunohistochemistry during femoral head development. Osteoadherin was primarily present in bone trabeculae and no staining was seen in cartilage. In situ hybridization showed the strongest expression in osteoblasts close to the cartilage/bone interface of the growth plate and lower expression in diaphyseal osteoblasts. On maturation of the femoral head on day 60 some expression was detected immediately below the forming articular cartilage. Our data indicated that osteoadherin is primarily expressed by osteoblasts and might have a role in regulation of mineralization.

NUCB1, the Drosophila melanogaster homolog of the mammalian EF-hand proteins NEFA and nucleobindin

Mammalian NEFA and nucleobindin are calcium-binding proteins containing a signal peptide, two EF-hand motifs, acidic and basic regions and a leucine-zipper motif. Although they have been discussed to be involved in autoimmunity, apoptosis and calcium homeostasis in the Golgi apparatus and bone matrix, their exact role remains unknown. Here we report the cloning of their Drosophila homolog, nucb1, as well as the analysis of its expression pattern during embryogenesis and the subcellular localization of the NUCB1 protein. The nucb1 mRNA and the NUCB1 protein were found to be expressed maternally and zygotically, and they accumulate ubiquitously at low levels during all embryonic stages due to a maternal component. From stage 11 onward, high levels of zygotic expression can be detected specifically in the salivary glands and their placodes. In contrast to the known mammalian family members, the NUCB1 protein localizes in a subpattern of cytoplasmic substructures, probably the Golgi apparatus.

Heterologous overexpression of human NEFA and studies on the two EF-hand calcium-binding sites

Human NEFA is an EF-hand, leucine zipper protein containing a signal sequence. To confirm the calcium binding capacity of NEFA, recombinant NEFA analogous to the mature protein and mutants with deletions in the EF-hand domain were expressed in Pichia pastoris and secreted into the culture medium at high yield. The calcium binding activity of each purified protein was measured by a modified equilibrium dialysis using the fluorescent Ca2+ indicator FURA-2 and atomic absorption spectroscopy. A stoichiometry of 2 mol Ca2+/mol NEFA was determined. The Ca2+ binding constants were resolved by intrinsic fluorescence spectroscopy. Fluorescence titration exhibited two classes of Ca2+ binding sites with Kd values of 0.08 microM and 0.2 microM. Circular dichroism (CD) spectroscopy showed an increase from 30 to 43% in the amount of alpha-helix in NEFA after addition of calcium ions. Limited proteolytic digestion indicated a Ca2+ dependent conformational change accompanied by an altered accessibility to the enzyme.

Overexpression of CALNUC (nucleobindin) increases agonist and thapsigargin releasable Ca2+ storage in the Golgi

We previously demonstrated that CALNUC, a Ca2+-binding protein with two EF-hands, is the major Ca2+-binding protein in the Golgi by 45Ca2+ overlay (Lin, P., H. Le-Niculescu, R. Hofmeister, J.M. McCaffery, M. Jin, H. Henneman, T. McQuistan, L. De Vries, and M. Farquhar. 1998. J. Cell Biol. 141:1515-1527). In this study we investigated CALNUC's properties and the Golgi Ca2+ storage pool in vivo. CALNUC was found to be a highly abundant Golgi protein (3.8 microg CALNUC/mg Golgi protein, 2.5 x 10(5) CALNUC molecules/NRK cell) and to have a single high affinity, low capacity Ca2+-binding site (Kd = 6.6 microM, binding capacity = 1.1 micromol Ca2+/micromol CALNUC). 45Ca2+ storage was increased by 2.5- and 3-fold, respectively, in HeLa cells transiently overexpressing CALNUC-GFP and in EcR-CHO cells stably overexpressing CALNUC. Deletion of the first EF-hand alpha helix from CALNUC completely abolished its Ca2+-binding capability. CALNUC was correctly targeted to the Golgi in transfected cells as it colocalized and cosedimented with the Golgi marker, alpha-mannosidase II (Man II). Approximately 70% of the 45Ca2+ taken up by HeLa and CHO cells overexpressing CALNUC was released by treatment with thapsigargin, a sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA) (Ca2+ pump) blocker. Stimulation of transfected cells with the agonist ATP or IP3 alone (permeabilized cells) also resulted in a significant increase in Ca2+ release from Golgi stores. By immunofluorescence, the IP3 receptor type 1 (IP3R-1) was distributed over the endoplasmic reticulum and codistributed with CALNUC in the Golgi. These results provide direct evidence that CALNUC binds Ca2+ in vivo and together with SERCA and IP3R is involved in establishment of the agonist-mobilizable Golgi Ca2+ store.

The binding of calcium to the B-repeat segment of SdrD, a cell surface protein of Staphylococcus aureus

In the Sdr family of Staphylococcus aureus cell surface proteins, three recently cloned members (Josefsson, E., McCrea, K., Ni Eidhin, D., O'Connell, D., Cox, J. A., Hook, M., and Foster, T. (1998) Microbiology, in press) display variable numbers of B-repeats, i.e. segments of 110-113 residues that probably make up one folding unit. Each B-repeat contains one conserved EF-hand motif and two acidic stretches. Equilibrium dialysis revealed that segment B1-B5 of SrdD contains 14 Ca2+-binding sites with high affinity ([Ca2+]0.5, 4 microM), whereas flow dialysis yielded 5 sites of high affinity (class I) and 10 of low affinity (class II). The discrepancy could be explained by the slow induction of high affinity in the class II sites. Kinetic experiments using fluorescent Ca2+ indicators corroborated slow binding of Ca2+ at the latter sites. Circular dichroism and Trp fluorescence showed that, whereas the Ca2+ form is well folded, the metal-free form seems strongly disorganized. The Ca2+-induced conformational changes comprise both fast and slow steps, giving thus a structural support for the induction of class II Ca2+-binding sites. The B-repeats may act as rulers or springs that modulate the distance between the interactive A region and the bacterial cell surface.

The mammalian calcium-binding protein, nucleobindin (CALNUC), is a Golgi resident protein

We have identified CALNUC, an EF-hand, Ca2+-binding protein, as a Golgi resident protein. CALNUC corresponds to a previously identified EF-hand/calcium-binding protein known as nucleobindin. CALNUC interacts with Galphai3 subunits in the yeast two-hybrid system and in GST-CALNUC pull-down assays. Analysis of deletion mutants demonstrated that the EF-hand and intervening acidic regions are the site of CALNUC's interaction with Galphai3. CALNUC is found in both cytosolic and membrane fractions. The membrane pool is tightly associated with the luminal surface of Golgi membranes. CALNUC is widely expressed, as it is detected by immunofluorescence in the Golgi region of all tissues and cell lines examined. By immunoelectron microscopy, CALNUC is localized to cis-Golgi cisternae and the cis-Golgi network (CGN). CALNUC is the major Ca2+-binding protein detected by 45Ca2+-binding assay on Golgi fractions. The properties of CALNUC and its high homology to calreticulin suggest that it may play a key role in calcium homeostasis in the CGN and cis-Golgi cisternae.

Upregulation of nucleobindin expression in human-activated lymphocytes and non-Hodgkin's lymphoma

Nucleobindin (Nuc) was originally found to be an enhancement factor of anti-DNA antibody production secreted by a lymphoid cell line derived from a lymphoproliferative MRL/lpr mouse. It has been shown that Nuc has a unique structure containing a DNA- and two calcium-binding domains, and a leucine zipper motif, but its biological roles have not yet been fully elucidated. Expression of Nuc was first studied in human lymphocytes. Expression of Nuc mRNA in normal peripheral blood mononuclear cells was significantly increased upon mitogen stimulation. Anti-human Nuc monoclonal antibody H-1D8 immunoprecipitated Nuc protein in the nuclear extract of Molt-4 cells. Furthermore, in the immunohistochemical staining of tumor specimens from 108 patients with non-Hodgkin's lymphoma (NHL) with H-1D8, H-1D8-positive cells were observed in nearly all cases in varying frequency. According to the Working Formulation, the percentage of cases in which more than 90% of the tumor cells were stained with H-1D8 was 65% in the high grade of the histological malignancy, 54% in the intermediate grade, and 22% in the low grade; however, normal cells surrounding the tumor cells were virtually negative for H-1D8. These results showed that the level of Nuc expression in human lymphocytes reflects the status of activation or proliferation of the cells, thus providing a clue for the further investigation into biological roles of Nuc. In addition, it might be applicable to the clinicopathological estimation of NHL as a novel indicator.

Structural and functional aspects of calcium binding in extracellular matrix proteins

Ca2+ ions play crucial roles in many matrix-matrix, cell-matrix and cell-cell contacts. Recent X-ray and NMR structure determinations have revealed an intriguing diversity of Ca(2+)-binding sites in extracellular proteins, ranging from the stabilization of isolated domains to intimate involvement in the superstructure of macromolecular assemblies. The central role of Ca2+ in extracellular proteins is illustrated by the molecular characterization of hereditary connective tissue disorders in humans. Point mutations of Ca(2+)-binding residues in fibrillin and cartilage oligomeric matrix protein are responsible for Marfan syndrome and pseudoachondroplasia, respectively. We also discuss the possibility that structure and function of extracellular proteins may be regulated by physiologically relevant Ca2+ gradients.

Identification and cDNA cloning of a novel human mosaic protein, LGN, based on interaction with G alpha i2

We have used the yeast two-hybrid system to identify proteins that interact with the alpha-subunit of the heterotrimeric GTP-binding protein, Gi2. We screened a human B cell cDNA library with full-length G alpha i2 and isolated four positive colonies, one of which expressed the 44-kDa COOH terminus of a previously unrecognized 677-amino acid (aa) protein. A full-length clone was isolated from a HeLa cell cDNA library. The deduced protein contains 10 Leu-Gly-Asn repeats, and thus we named it LGN. Computer analysis indicates that LGN is a mosaic protein with seven repeated sequences of about 40 aa in length at its N-terminal end, and four repeated sequences of about 34 aa at its C-terminal end. Each of the two repeat regions shows substantial similarity to proteins found in other organisms. RT-PCR analysis of human tissues showed that the mRNA of LGN was ubiquitously expressed. The specificity of interaction between G alpha i2 and LGN was confirmed by an in vitro binding assay using recombinant proteins. These data indicate that the yeast two-hybrid system can identify novel proteins, such as LGN, that interact with G alpha proteins. As a mosaic protein, LGN shows similarity with portions of proteins from many species and thus may define a new protein family.

Isolation of a calcium-binding phosphoprotein from the oocytes and hemolymph of the blood-sucking insect Rhodnius prolixus

A novel calcium-binding phosphoprotein was isolated from the oocytes of the blood-sucking bug Rhodnius prolixus. This protein exhibits an apparent molecular mass of 18 kDa on gel filtration, but migrates as an 8-kDa band on N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine/SDS-polyacrylamide gels. It has a high content of serine (24% of the total number of residues), and phosphoserine is the sole amino acid phosphorylated in vivo. A similar protein was partially purified from the hemolymph. It resembles the oocyte form of the protein in its NH2-terminal sequence and its ability to be taken up by growing ovaries. 45Ca binding to the oocyte phosphoprotein was determined after SDS-polyacrylamide gel electrophoresis followed by blotting on nitrocellulose membranes. Titration of Ca2+-binding sites shows a high capacity (approximately 50 mol/mol of protein), but a low affinity (K0.5 congruent with 10(-3) M). Based on these characteristics, we have named this protein Rhodnius calcium-binding phosphoprotein. It resembles phosvitin, a phosphoprotein present in the oocytes of nonmammalian vertebrates.

Effect of cortical perforations of both graft and host bed on onlay incorporation to the rat skull

The mechanisms involved in the integration of autogeneic bone grafts still attract much interest due to their clinical importance. The purpose of this study was to obtain data on the effects of a combination of cortical bone perforations both at the recipient site and the inner layer of a bicortical graft. 12 adult rats obtained femoral or tibial bicortical bone grafts from isogeneic donors to the tibia, one on each leg. On the experimental side, both the recipient bed and the inner cortical graft layer received multiple perforations (0.25 mm in diameter), while on the control side, only perforations of the recipient cortical bed were made. The findings were assessed by routine histology and immunohistochemical analysis for some bone and cartilage matrix proteins after 4 and 20 weeks. The combined cortical perforations of the graft and the host bed induced a locally improved bony incorporation of the graft and a corticalization of the graft marrow, implying improved mechanical stability. The graft height persistence was similar between groups. Intense labelling of the bone matrix proteins was apparent in all bone tissue and by diversified intensity at its various components, demonstrating ongoing remodeling activities. PRELP and fibromodulin mainly outlined the soft tissues surrounding the graft and compact bone sealing off the graft marrow. Immunolabelling contributed a more delicate picture of the mechanisms involved in graft incorporation.

Extracellular calcium-binding proteins

Point mutations in Ca2+-binding sites of extracellular matrix proteins have been identified as the cause of human disorders such as Marfansyndrome and pseudoachondroplasia. Although the modes of Ca2+ binding and the effects of point mutations are not yet understood in these two cases, new insight was recently gained by X-ray and NMR structure determinations of several other extracellular proteins; these studies revealed a diversity of functions of Ca2+ ions. Ca2+ may induce a profound conformational change within a single domain, may bridge adjacent domains and thus direct the relative domain orientation and supramolecular structure, or may be involved in carbohydrate and membrane binding.

Interaction of cyclooxygenases with an apoptosis- and autoimmunity-associated protein

Cyclooxygenases (COXs) 1 and 2 are 72-kDa, intralumenal residents of the endoplasmic reticulum (ER) and nuclear envelope, where they catalyze the rate-limiting steps in the conversion of arachidonate to the physiologically dynamic prostanoids. Recent studies, including the generation of knockout mice, show COX-1 and COX-2 to have biologically distinct roles within cells and organisms. Also apparent is that arachidonate substrate is selectably metabolized by COX-2 after mitogen stimulation in many cells that contain both isoforms. Because COX-1 and COX-2 are highly conserved in all residues needed for catalysis and in their purified forms have almost identical kinetic properties, we have searched for COX-interacting ER proteins that might mediate these unique isoenzymic properties. Using COXs as bait in the yeast two-hybrid system, we identified autoimmunity- and apoptosis-associated nucleobindin (Nuc) as a protein that specifically interacts with both isoenzymes. COX-Nuc binding was substantiated by immunoprecipitation experiments, which showed that COX-1 and, to a lesser extent, COX-2 form complexes with Nuc in vitro. When overexpressed in COS-1 cells, Nuc was found to be extracellularly released. However, when Nuc was co-overexpressed with COX-1 or COX-2, its release was reduced by >80%. This finding suggests that COX isoenzymes participate in the retention of Nuc within the lumen of the ER, where COX may regulate the release of Nuc from the cell. It also identifies Nuc as a potential regulator of COXs through this interaction.

Structure of a novel extracellular Ca(2+)-binding module in BM-40

The EF-hand is a highly conserved Ca(2+)-binding motif found in many cytosolic Ca(2+)-modulated proteins. Here we report the crystal structure at 2.0 A resolution of the carboxy-terminal domain of human BM-40 (SPARC, osteonectin), an extracellular matrix protein containing an EF-hand pair. The two EF-hands interact canonically but their detailed structures are unusual. In the first EF-hand a one-residue insertion is accommodated by a cis-peptide bond and by substituting a carboxylate by a peptide carbonyl as a Ca2+ ligand. The second EF-hand is stabilized by a disulphide bond. The EF-hand pair interacts tightly with an amphiphilic amino-terminal helix, reminiscent of target peptide binding by calmodulin. The present structure defines a novel protein module occurring in several other extracellular proteins.

Interaction of the protein nucleobindin with G alpha i2, as revealed by the yeast two-hybrid system

The heterotrimeric G protein, G alpha i2, transduces signals from seven membrane spanning receptors to effectors such as adenylyl cyclase and ion channels. The purpose of this study was to identify these or other cellular proteins that interact with G alpha i2 by use of the yeast two-hybrid system. A human B cell cDNA library was screened by this system using full length G alpha i2. Four positive colonies were obtained. Two of the four were identified as nucleobindin, a calcium binding protein and a putative antigen to which anti-nuclear antibodies are generated in mice with a disorder that resembles systemic lupus erythematosus. Nucleobindin has a leucine zipper, EF hands, and a signal peptide sequence and is thought to localize to the nucleus as well as being secreted. The specificity of intehraction between G alpha i2 and nucleobindin was confirmed by an in vitro binding assay using recombinant proteins. Transfection of G alpha i2 and nucleobindin in COS cells increased G alpha i2 expression relative to cells transfected with G alpha i2 and mock vector. Our results indicate that the yeast two-hybrid system provides a means to identify novel proteins that interact with G alpha proteins. Nucleobindin appears to represent one of those proteins.