The endoplasmic reticulum calcium-binding protein of 55 kDa is a novel EF-hand protein retained in the endoplasmic reticulum by a carboxyl-terminal His-Asp-Glu-Leu motif

Calcium-binding protein TgpCaBP regulates calcium storage of the zoonotic parasite Toxoplasma gondii

Toxoplasma gondii, the causative parasite of toxoplasmosis, is an apicomplexan parasite that infects warm-blooded mammals. The ability of the calcium-binding proteins (CBPs) to transport large amounts of Ca2+ appears to be critical for the biological activity of T. gondii. However, the functions of some members of the CBP family have not yet been deciphered. Here, we characterized a putative CBP of T. gondii, TgpCaBP (TGME49_229480), which is composed of four EF-hand motifs with Ca2+-binding capability. TgpCaBP was localized in the cytosol and ER of T. gondii, and parasites lacking the TgpCaBP gene exhibited diminished abilities in cell invasion, intracellular growth, egress, and motility. These phenomena were due to the abnormalities in intracellular Ca2+ efflux and ER Ca2+ storage, and the reduction in motility was associated with a decrease in the discharge of secretory proteins. Therefore, we propose that TgpCaBP is a Ca2+ transporter and signaling molecule involved in Ca2+ regulation and parasitization in the hosts.IMPORTANCECa2+ signaling is essential in the development of T. gondii. In this study, we identified a calcium-binding protein in T. gondii, named TgpCaBP, which actively regulates intracellular Ca2+ levels in the parasite. Deletion of the gene coding for TgpCaBP caused serious deficits in the parasite's ability to maintain a stable intracellular calcium environment, which also impaired the secretory protein discharged from the parasite, and its capacity of gliding motility, cell invasion, intracellular growth, and egress from host cells. In summary, we have identified a novel calcium-binding protein, TgpCaBP, in the zoonotic parasite T. gondii, which is a potential therapeutic target for toxoplasmosis.

Structural Plasticity Allows Calumenin-1 to Moonlight as a Ca2+-Independent Chaperone: Pb2+ Enables Probing Alternate Inhibitory Conformation

Human calumenin-1 (HsCalu-1) is an endoplasmic reticulum (ER) and Golgi-resident Ca2+-binding protein of the hepta-EF-hand superfamily that plays a vital role in maintaining the cytoplasmic Ca2+ concentration below toxic levels by interacting with Sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) and ryanodine receptors (RyR), indicating its role in Ca2+ homeostasis in the ER. HsCalu-1 seems to be able to exhibit structural plasticity to achieve its plethora of functions. In this study, we demonstrate that HsCalu-1 acts as a chaperone in both its intrinsically disordered state (apo form) and the structured state (Ca2+-bound form). HsCalu-1 chaperone activity is independent of Ca2+ and Pb2+ binding attenuating its chaperone-like activity. Incidentally, Pb2+ binds to HsCalu-1 with lower affinity (KD = 38.46 μM) (compared to Ca2+-binding), leading to the formation of a less-stable conformation as observed by a sharp drop in its melting temperature Tm from 67 °C in the Ca2+-bound form to 43 °C in the presence of Pb2+. The binding site for Pb2+ was mapped as being in the EF-Hand-234 domain of HsCalu-1, a region that overlaps with the Ca2+-dependent initiator of its functional fold. A change in the secondary and tertiary structure, leading to a less-stable but compact conformation upon Pb2+ binding, is the mechanism by which the chaperone-like activity of HsCalu-1 is diminished. Our results not only demonstrate the chaperone activity by a protein in its disordered state but also explain, using Pb2+ as a probe, that the multiple functions of calumenin are due to its ability to adopt a quasi-stable conformation.

Physiology of intracellular calcium buffering

Calcium signaling underlies much of physiology. Almost all the Ca2+ in the cytoplasm is bound to buffers, with typically only ∼1% being freely ionized at resting levels in most cells. Physiological Ca2+ buffers include small molecules and proteins, and experimentally Ca2+ indicators will also buffer calcium. The chemistry of interactions between Ca2+ and buffers determines the extent and speed of Ca2+ binding. The physiological effects of Ca2+ buffers are determined by the kinetics with which they bind Ca2+ and their mobility within the cell. The degree of buffering depends on factors such as the affinity for Ca2+, the Ca2+ concentration, and whether Ca2+ ions bind cooperatively. Buffering affects both the amplitude and time course of cytoplasmic Ca2+ signals as well as changes of Ca2+ concentration in organelles. It can also facilitate Ca2+ diffusion inside the cell. Ca2+ buffering affects synaptic transmission, muscle contraction, Ca2+ transport across epithelia, and the killing of bacteria. Saturation of buffers leads to synaptic facilitation and tetanic contraction in skeletal muscle and may play a role in inotropy in the heart. This review focuses on the link between buffer chemistry and function and how Ca2+ buffering affects normal physiology and the consequences of changes in disease. As well as summarizing what is known, we point out the many areas where further work is required.

Quantitative interactome proteomics identifies a proteostasis network for GABAA receptors

Gamma-aminobutyric acid type A (GABAA) receptors are the primary inhibitory neurotransmitter-gated ion channels in the mammalian central nervous system. Maintenance of GABAA receptor protein homeostasis (proteostasis) in cells utilizing its interacting proteins is essential for the function of GABAA receptors. However, how the proteostasis network orchestrates GABAA receptor biogenesis in the endoplasmic reticulum is not well understood. Here, we employed a proteomics-based approach to systematically identify the interactomes of GABAA receptors. We carried out a quantitative immunoprecipitation-tandem mass spectrometry analysis utilizing stable isotope labeling by amino acids in cell culture. Furthermore, we performed comparative proteomics by using both WT α1 subunit and a misfolding-prone α1 subunit carrying the A322D variant as the bait proteins. We identified 125 interactors for WT α1-containing receptors, 105 proteins for α1(A322D)-containing receptors, and 54 overlapping proteins within these two interactomes. Our bioinformatics analysis identified potential GABAA receptor proteostasis network components, including chaperones, folding enzymes, trafficking factors, and degradation factors, and we assembled a model of their potential involvement in the cellular folding, degradation, and trafficking pathways for GABAA receptors. In addition, we verified endogenous interactions between α1 subunits and selected interactors by using coimmunoprecipitation in mouse brain homogenates. Moreover, we showed that TRIM21 (tripartite motif containing-21), an E3 ubiquitin ligase, positively regulated the degradation of misfolding-prone α1(A322D) subunits selectively. This study paves the way for understanding the molecular mechanisms as well as fine-tuning of GABAA receptor proteostasis to ameliorate related neurological diseases such as epilepsy.

A mechanosensitive lipolytic factor in the bone marrow promotes osteogenesis and lymphopoiesis

Exercise can prevent osteoporosis and improve immune function, but the mechanism remains unclear. Here, we show that exercise promotes reticulocalbin-2 secretion from the bone marrow macrophages to initiate bone marrow fat lipolysis. Given the crucial role of lipolysis in exercise-stimulated osteogenesis and lymphopoiesis, these findings suggest that reticulocalbin-2 is a pivotal regulator of a local adipose-osteogenic/immune axis. Mechanistically, reticulocalbin-2 binds to a functional receptor complex, which is composed of neuronilin-2 and integrin beta-1, to activate a cAMP-PKA signaling pathway that mobilizes bone marrow fat via lipolysis to fuel the differentiation and function of mesenchymal and hematopoietic stem cells. Notably, the administration of recombinant reticulocalbin-2 in tail-suspended and old mice remarkably decreases bone marrow fat accumulation and promotes osteogenesis and lymphopoiesis. These findings identify reticulocalbin-2 as a novel mechanosensitive lipolytic factor in maintaining energy homeostasis in bone resident cells, and it provides a promising target for skeletal and immune health.

Lipopolysaccharide induces vascular endothelial cell pyroptosis via the SP1/RCN2/ROS signaling pathway

Endothelial cell pyroptosis is a novel cause of endothelial dysfunction in sepsis. Reticulocalbin-2 (RCN2) is involved in regulating vascular inflammation and plays an important role in the cardiovascular system. However, the role of RCN2 in inflammation-induced endothelial cell pyroptosis remains to be explored. Here, we found that RCN2 was upregulated after lipopolysaccharide (LPS) treatment in a concentration- and time-dependent manner. RCN2 knockdown resulted in a significant decrease in pyroptosis, reduced LDH and IL-1β release and ROS production and inhibited the expression of pyroptosis-related proteins (NLRP3, cleaved caspase-1, and cleaved GSDMD) (all p < 0.05). N-acetyl-L-cysteine (NAC) counteracted the effects of RCN2 on pyroptosis (all p < 0.01). The silencing of RCN2 antagonized the inhibitory effect of LPS on the phosphorylation of eNOS (p < 0.05). We predicted and confirmed that specificity protein-1(SP1) could directly bind to the RCN2 promoter and regulate RCN2. RCN2 overexpression rescued the inhibitory effect of SP1 inhibitor on HUVEC pyroptosis induced by LPS (all p < 0.05). These findings suggested that the activation of the SP1/RCN2/ROS signaling pathway could promote LPS-induced endothelial cell pyroptosis.

Endoplasmic reticulum & mitochondrial calcium homeostasis: The interplay with viruses

Calcium ions (Ca²⁺) act as secondary messengers in a plethora of cellular processes and play crucial role in cellular organelle function and homeostasis. The average resting concentration of Ca²⁺ is nearly 100 nM and in certain cells it can reach up to 1 µM. The high range of Ca²⁺ concentration across the plasma membrane and intracellular Ca²⁺ stores demands a well-coordinated maintenance of free Ca²⁺ via influx, efflux, buffering and storage. Endoplasmic Reticulum (ER) and Mitochondria depend on Ca²⁺ for their function and also serve as major players in intracellular Ca²⁺ homeostasis. The ER-mitochondria interplay helps in orchestrating cellular calcium homeostasis to avoid any detrimental effect resulting from Ca²⁺ overload or depletion. Since Ca²⁺ plays a central role in many biological processes it is an essential component of the virus-host interactions. The large gradient across membranes enable the viruses to easily modulate this buffered environment to meet their needs. Viruses exploit Ca²⁺ signaling to establish productive infection and evade the host immune defense. In this review we will detail the interplay between the viruses and cellular & ER-mitochondrial calcium signaling and the significance of these events on viral life cycle and disease pathogenesis.

Loss of reticulocalbin 2 lowers blood pressure and restrains ANG II-induced hypertension in vivo

Hypertension affects over 1 billion people worldwide and increases the risk for heart failure, stroke, and chronic kidney disease. Despite high prevalence and devastating impact, its etiology still remains poorly understood for most hypertensive cases. Rcn2, which encodes reticulocalbin 2, is a candidate gene for atherosclerosis that we have previously reported in mice. Here, we identified Rcn2 as a novel regulator of blood pressure in mice. Rcn2 was abundantly expressed in the endothelium and adventitia of normal arteries and was dramatically upregulated in the medial layer of the artery undergoing structural remodeling. Deletion of Rcn2 lowered basal blood pressure and attenuated ANG II-induced hypertension in C57BL/6 mice. siRNA knockdown of Rcn2 dramatically increased production of the nitric oxide (NO) breakdown products nitrite and nitrate by endothelial cells but not by smooth muscle cells. Isolated carotid arteries from Rcn2-/- mice showed an increased sensitivity to the ACh-induced NO-mediated relaxant response compared with arteries of Rcn2+/+ mice. Analysis of a recent meta-data set showed associations of genetic variants near RCN2 with blood pressure in humans. These data suggest that Rcn2 regulates blood pressure and contributes to hypertension through actions on endothelial NO synthase.

The Differential Response to Ca2+ from Vertebrate and Invertebrate Calumenin Is Governed by a Single Amino Acid Residue

Calumenin (Calu) is a well-conserved multi-EF-hand-containing Ca²⁺-binding protein. In this work, we focused on the alterations that calumenin has undergone during evolution. We demonstrate that vertebrate calumenin is significantly different from its invertebrate homologues with respect to its response to Ca²⁺ binding. Human calumenin (HsCalu1) is intrinsically unstructured in the Ca²⁺ free form and responds to Ca²⁺ with a dramatic gain in structure. Calumenin from Caenorhabditis elegans (CeCalu) is structured even in the apo form, with no conformational change upon binding of Ca²⁺. We decode this structural and functional distinction by identifying a single "Leu" residue-based switch located in the fourth EF-hand of HsCalu1, occupied by "Gly" in the invertebrate homologues. We demonstrate that replacing Leu with Gly (L150G) in HsCalu1 enables the protein to adopt a structural fold even in the Ca²⁺ free form, similar to CeCalu, leading to ligand compensation (adoption of structure in the absence of Ca²⁺). The fourth (of seven) EF-hand of HsCalu1 nucleates the structural fold of the protein depending on the switch residue (Gly or Leu). Our analyses reveal that the Leu that replaced Gly from fishes onward is absolutely conserved in higher vertebrates, while lower organisms have Gly, not only enlarging the scope of Ca²⁺-dependent structural transitions but also drawing a boundary between the invertebrate and vertebrate calumenin. The evolutionary selection of the switch residue strongly corroborates the change in the structure of the protein and its pleiotropic functions and seems like it can be extended to the presence or absence of a heart in that organism.

Phosphatidylinositol glycan anchor biosynthesis, class X containing complex promotes cancer cell proliferation through suppression of EHD2 and ZIC1, putative tumor suppressors

We identified phosphatidylinositol glycan anchor biosynthesis, class X (PIGX), which plays a critical role in the biosynthetic pathway of glycosylphosphatidylinositol (GPI)-anchor motif, to be upregulated highly and frequently in breast cancer cells. Knockdown of PIGX as well as reticulocalbin 1 (RCN1) and reticulocalbin 2 (RCN2), which we found to interact with PIGX and was indicated to regulate calcium-dependent activities, significantly suppressed the growth of breast cancer cells. We also identified PIGX to be a core protein in an RCN1/PIGX/RCN2 complex. Microarray analysis revealed that the expression of two putative tumor suppressor genes, Zic family member 1 (ZIC1) and EH-domain containing 2 (EHD2), were upregulated commonly in cells in which PIGX, RCN1, or RCN2 was knocked down, suggesting that this RCN1/PIGX/RCN2 complex could negatively regulate the expression of these two genes and thereby contribute to human breast carcinogenesis. Our results imply that PIGX may be a good candidate molecule for development of novel anticancer drugs for breast cancer.

Analysis of the Differentiation of Kenyon Cell Subtypes Using Three Mushroom Body-Preferential Genes during Metamorphosis in the Honeybee (Apis mellifera L.)

The adult honeybee (Apis mellifera L.) mushroom bodies (MBs, a higher center in the insect brain) comprise four subtypes of intrinsic neurons: the class-I large-, middle-, and small-type Kenyon cells (lKCs, mKCs, and sKCs, respectively), and class-II KCs. Analysis of the differentiation of KC subtypes during metamorphosis is important for the better understanding of the roles of KC subtypes related to the honeybee behaviors. In the present study, aiming at identifying marker genes for KC subtypes, we used a cDNA microarray to comprehensively search for genes expressed in an MB-preferential manner in the honeybee brain. Among the 18 genes identified, we further analyzed three genes whose expression was enriched in the MBs: phospholipase C epsilon (PLCe), synaptotagmin 14 (Syt14), and discs large homolog 5 (dlg5). Quantitative reverse transcription-polymerase chain reaction analysis revealed that expression of PLCe, Syt14, and dlg5 was more enriched in the MBs than in the other brain regions by approximately 31-, 6.8-, and 5.6-fold, respectively. In situ hybridization revealed that expression of both Syt14 and dlg5 was enriched in the lKCs but not in the mKCs and sKCs, whereas expression of PLCe was similar in all KC subtypes (the entire MBs) in the honeybee brain, suggesting that Syt14 and dlg5, and PLCe are available as marker genes for the lKCs, and all KC subtypes, respectively. In situ hybridization revealed that expression of PLCe is already detectable in the class-II KCs at the larval fifth instar feeding stage, indicating that PLCe expression is a characteristic common to the larval and adult MBs. In contrast, expression of both Syt14 and dlg5 became detectable at the day three pupa, indicating that Syt14 and dlg5 expressions are characteristic to the late pupal and adult MBs and the lKC specific molecular characteristics are established during the late pupal stages.

Regulation of mitochondrial calcium in plants versus animals

Ca(2+) acts as an important cellular second messenger in eukaryotes. In both plants and animals, a wide variety of environmental and developmental stimuli trigger Ca(2+) transients of a specific signature that can modulate gene expression and metabolism. In animals, mitochondrial energy metabolism has long been considered a hotspot of Ca(2+) regulation, with a range of pathophysiology linked to altered Ca(2+) control. Recently, several molecular players involved in mitochondrial Ca(2+) signalling have been identified, including those of the mitochondrial Ca(2+) uniporter. Despite strong evidence for sophisticated Ca(2+) regulation in plant mitochondria, the picture has remained much less clear. This is currently changing aided by live imaging and genetic approaches which allow dissection of subcellular Ca(2+) dynamics and identification of the proteins involved. We provide an update on our current understanding in the regulation of mitochondrial Ca(2+) and signalling by comparing work in plants and animals. The significance of mitochondrial Ca(2+) control is discussed in the light of the specific metabolic and energetic needs of plant and animal cells.

Ca(2+) homeostasis and endoplasmic reticulum (ER) stress: An integrated view of calcium signaling

Cellular Ca(2+) homeostasis is maintained through the integrated and coordinated function of Ca(2+) transport molecules, Ca(2+) buffers and sensors. These molecules are associated with the plasma membrane and different cellular compartments, such as the cytoplasm, nucleus, mitochondria, and cellular reticular network, including the endoplasmic reticulum (ER) to control free and bound Ca(2+) levels in all parts of the cell. Loss of nutrients/energy leads to the loss of cellular homeostasis and disruption of Ca(2+) signaling in both the reticular network and cytoplasmic compartments. As an integral part of cellular physiology and pathology, this leads to activation of ER stress coping responses, such as the unfolded protein response (UPR), and mobilization of pathways to regain ER homeostasis.

Fibulin-1C, C1 Esterase Inhibitor and Glucose Regulated Protein 75 Interact with the CREC Proteins, Calumenin and Reticulocalbin

Affinity purification, immunoprecipitation, gel electrophoresis and mass spectrometry were used to identify fibulin-1C, C1 esterase inhibitor and glucose regulated protein 75, grp75, as binding partners of the CREC proteins, calumenin and reticulocalbin. Surface plasmon resonance was used to verify the interaction of all three proteins with each of the CREC proteins. Fibulin-1C interacts with calumenin and reticulocalbin with an estimated dissociation constant around 50-60 nM. The interaction, at least for reticulocalbin, was not dependent upon the presence of Ca²⁺. C1 esterase inhibitor interacted with both proteins with an estimated dissociation constant at 1 μM for reticulocalbin and 150 nM for calumenin. The interaction, at least for calumenin, was dependent upon the presence of Ca²⁺ with strong interaction at 3.5 mM while no detectable interaction could be found at 0.1 mM. Grp75 binds with an affinity of approximately 3-7 nM with reticulocalbin as well as with calumenin. These interactions suggest functional participation of the CREC proteins in chaperone activity, cell proliferation and transformation, cellular aging, haemostasis and thrombosis as well as modulation of the complement system in fighting bacterial infection.

Mushroom body-preferential expression of proteins/genes involved in endoplasmic reticulum Ca(2+)-transport in the worker honeybee (Apis mellifera L.) brain

To identify the molecular characteristics specific to the mushroom body (MB, a higher processing centre) neurones in the honeybee brain, we previously used proteomics to identify proteins that are preferentially expressed in these MBs. Here we continued our proteomic analysis to show that reticulocalbin, which is involved in endoplasmic reticulum (ER) Ca(2+) transport, is also preferentially expressed in the MBs in the honeybee brain. Gene expression analysis revealed that reticulocalbin is preferentially expressed in the large-type Kenyon cells, which are MB-intrinsic neurones. In addition, the gene for the ryanodine receptor, which is also involved in ER Ca(2+) transport, was also preferentially expressed in the large-type Kenyon cells. In contrast, the expression of three other ER-related genes, protein disulphide isomerase, sec61 and erp60, was not enriched in the MBs. These findings further support the notion that the function of ER Ca(2+)-signalling, but not the mere intracellular density of ER, is specifically enhanced in the large-type Kenyon cells in the honeybee brain.

Inner nuclear envelope proteins SUN1 and SUN2 play a prominent role in the DNA damage response

The DNA damage response (DDR) and DNA repair are critical for maintaining genomic stability and evading many human diseases. Recent findings indicate that accumulation of SUN1, a nuclear envelope (NE) protein, is a significant pathogenic event in Emery-Dreifuss muscular dystrophy and Hutchinson-Gilford progeria syndrome, both caused by mutations in LMNA. However, roles of mammalian SUN proteins in mitotic cell division and genomic stability are unknown. Here we report that the inner NE proteins SUN1 and SUN2 may play a redundant role in DDR. Mouse embryonic fibroblasts from Sun1(-/-)Sun2(-/-) mice displayed premature proliferation arrest in S phase of cell cycle, increased apoptosis and DNA damage, and decreased perinuclear heterochromatin, indicating genome instability. Furthermore, activation of ATM and H2A.X, early events in DDR, were impaired in Sun1(-/-)Sun2(-/-) fibroblasts. A biochemical screen identified interactions between SUN1 and SUN2 and DNA-dependent protein kinase (DNAPK) complex that functions in DNA nonhomologous end joining repair and possibly in DDR. Knockdown of DNAPK reduced ATM activation in NIH 3T3 cells, consistent with a potential role of SUN1- and SUN2-DNAPK interaction during DDR. SUN1 and SUN2 could affect DDR by localizing certain nuclear factors to the NE or by mediating communication between nuclear and cytoplasmic events.

Molecular and functional analyses of aspolin, a fish-specific protein extremely rich in aspartic acid

Aspolin is a muscular protein having unique structural characteristics where the most part of its primary structure is occupied by aspartic acid. Aspolin has been found exceptionally in fish muscle, suggesting its specific role in this tissue. However, biological functions of aspolin have remained unknown. In the present study, we cloned full-length cDNAs encoding zebrafish Danio rerio aspolins 1 and 2, revealed their genomic organization, and examined in vivo function using knockdown techniques. Genomic analysis clearly showed that aspolin is a paralog of the histidine-rich calcium binding protein gene, which encodes a calcium binding protein in sarcoplasmic reticulum (SR). Expression analysis showed that the transcripts and their translated products, aspolins 1 and 2, are distributed in myotomal skeletal muscle, but not in cardiac muscle. Injection of antisense morpholino oligo targeting both aspolins 1 and 2 increased the mRNA levels of calsequestrin 1, another calcium binding protein in SR. These lines of evidence suggest that aspolins regulate calcium concentrations in SR.

Identification and characterization of endonuclein binding proteins: evidence of modulatory effects on signal transduction and chaperone activity

Background

We have previously identified endonuclein as a cell cycle regulated WD-repeat protein that is up-regulated in adenocarcinoma of the pancreas. Now, we aim to investigate its biomedical functions.

Results

Using the cDNA encoding human endonuclein, we have expressed and purified the recombinant protein from Escherichia coli using metal affinity chromatography. The recombinant protein was immobilized to a column and by affinity chromatography several interacting proteins were purified from several litres of placenta tissue extract. After chromatography the eluted proteins were further separated by two-dimensional gel electrophoresis and identified by tandem mass spectrometry. The interacting proteins were identified as; Tax interaction protein 1 (TIP-1), Aα fibrinogen transcription factor (P16/SSBP1), immunoglobulin heavy chain binding protein (BiP), human ER-associated DNAJ (HEDJ/DNAJB11), endonuclein interaction protein 8 (EIP-8), and pregnancy specific β-1 glycoproteins (PSGs). Surface plasmon resonance analysis and confocal fluorescence microscopy were used to further characterize the interactions.

Conclusions

Our results demonstrate that endonuclein interacts with several proteins indicating a broad function including signal transduction and chaperone activity.

The rapidly expanding CREC protein family: members, localization, function, and role in disease

Although many aspects of the physiological and pathophysiological mechanisms remain unknown, recent advances in our knowledge suggest that the CREC proteins are promising disease biomarkers or targets for therapeutic intervention in a variety of diseases. The CREC family of low affinity, Ca2+-binding, multiple EF-hand proteins are encoded by five genes, RCN1, RCN2, RCN3, SDF4, and CALU, resulting in reticulocalbin, ER Ca2+-binding protein of 55 kDa (ERC-55), reticulocalbin-3, Ca2+-binding protein of 45 kDa (Cab45), and calumenin. Alternative splicing increases the number of gene products. The proteins are localized in the cytosol, in various parts of the secretory pathway, secreted to the extracellular space or localized on the cell surface. The emerging functions appear to be highly diverse. The proteins interact with several different ligands. Rather well-described functions are attached to calumenin with the inhibition of several proteins in the endoplasmic or sarcoplasmic reticulum membrane, the vitamin K(1) 2,3-epoxide reductase, the gamma-carboxylase, the ryanodine receptor, and the Ca2+-transporting ATPase. Other functions concern participation in the secretory process, chaperone activity, signal transduction as well as participation in a large variety of disease processes.

Analysis of Pax6 contiguous gene deletions in the mouse, Mus musculus, identifies regions distinct from Pax6 responsible for extreme small-eye and belly-spotting phenotypes

In the mouse Pax6 function is critical in a dose-dependent manner for proper eye development. Pax6 contiguous gene deletions were shown to be homozygous lethal at an early embryonic stage. Heterozygotes express belly spotting and extreme microphthalmia. The eye phenotype is more severe than in heterozygous Pax6 intragenic null mutants, raising the possibility that deletions are functionally different from intragenic null mutations or that a region distinct from Pax6 included in the deletions affects eye phenotype. We recovered and identified the exact regions deleted in three new Pax6 deletions. All are homozygous lethal at an early embryonic stage. None express belly spotting. One expresses extreme microphthalmia and two express the milder eye phenotype similar to Pax6 intragenic null mutants. Analysis of Pax6 expression levels and the major isoforms excluded the hypothesis that the deletions expressing extreme microphthalmia are directly due to the action of Pax6 and functionally different from intragenic null mutations. A region distinct from Pax6 containing eight genes was identified for belly spotting. A second region containing one gene (Rcn1) was identified for the extreme microphthalmia phenotype. Rcn1 is a Ca(+2)-binding protein, resident in the endoplasmic reticulum, participates in the secretory pathway and expressed in the eye. Our results suggest that deletion of Rcn1 directly or indirectly contributes to the eye phenotype in Pax6 contiguous gene deletions.

Distribution and variable expression of secretory pathway protein reticulocalbin in normal human organs and non-neoplastic pathological conditions

Reticulocalbin (RCN) is one member of the Ca(2+)-binding proteins in the secretory pathway and is localized in the endoplasmic reticulum. RCN may play a role in the normal behavior and life of cells, although its detailed role remains unknown. Overexpression of RCN may also play a role in tumorigenesis, tumor invasion, and drug resistance. The new antibody for human RCN is used in the distribution of RCN in normal human organs of fetuses and adults with or without inflammation. Immunohistochemical examination demonstrated a broad distribution of RCN in various organs of fetuses and adults, predominantly in the endocrine and exocrine organs. However, RCN expression was heterogeneous in each constituent cell of some organs. Among non-epithelial organs, vascular endothelial cells, testicular germ cells, neurons, and follicular dendritic cells showed strong staining. Plasma cells were the only RCN-positive cells among hematopoietic and lymphoid cells. In inflammatory conditions, RCN expression was enhanced in both epithelial and non-epithelial cells. Heterogeneous expression of RCN indicates that the amount of RCN needed for cell behavior and life may be variable, depending on each cell type and, therefore, RCN may be helpful in establishing the cell origin of neoplasms in some organs. However, further study is needed to establish the significance of RCN in tumorigenesis and in some peculiar features of neoplasms.

Cytobiological consequences of calcium-signaling alterations induced by human viral proteins

Since calcium-signaling regulates specific and fundamental cellular processes, it represents the ideal target of viral proteins, in order for the virus to control cellular functions and favour its persistence, multiplication and spread. A detailed analysis of reports focused on the impact of viral proteins on calcium-signaling has shown that virus-related elevations of cytosolic calcium levels allow increased viral protein expression (HIV-1, HSV-1/2), viral replication (HBx, enterovirus 2B, HTLV-1 p12(I), HHV-8, EBV), viral maturation (rotavirus), viral release (enterovirus 2B) and cell immortalization (EBV). Interestingly, virus-induced decreased cytosolic calcium levels have been found to be associated with inhibition of immune cells functions (HIV-1 Tat, HHV-8 K15, EBV LMP2A). Finally, several viral proteins are able to modulate intracellular calcium-signaling to control cell viability (HIV-1 Tat, HTLV-1 p13(II), HCV core, HBx, enterovirus 2B, HHV-8 K7). These data point out calcium-signaling as a key cellular target for viral infection and should stimulate further studies exploring new calcium-related therapeutic strategies.

Calumenin, a multiple EF-hands Ca2+-binding protein, interacts with ryanodine receptor-1 in rabbit skeletal sarcoplasmic reticulum

Calumenin is a multiple EF-hand Ca2+-binding protein located in endo/sarcoplasmic reticulum of mammalian tissues. In the present study, we cloned two rabbit calumenin isoforms (rabbit calumenin-1 and -2, GenBank Accession Nos. SY225335 and AY225336, respectively) by RT-PCR. Both isoforms contain a 19 aa N-terminal signal sequence, 6 EF-hand domains, and a C-terminal ER/SR retrieval signal, HDEF. Both calumenin isoforms exist in rabbit cardiac and skeletal muscles, but calumenin-2 is the main isoform in skeletal muscle. Presence of calumenin in rabbit sarcoplasmic reticulum (SR) was identified by Western blot analysis. GST-pull down and co-immunoprecipitation experiments showed that ryanodine receptor 1 (RyR1) interacted with calumenin-2 in millimolar Ca2+ concentration range. Experiments of gradual EF-hand deletions suggest that the second EF-hand domain is essential for calumenin binding to RyR1. Adenovirus-mediated overexpression of calumenin-2 in C2C12 myotubes led to increased caffeine-induced Ca2+ release, but decreased depolarization-induced Ca2+ release. Taken together, we propose that calumenin-2 in the SR lumen can directly regulate the RyR1 activity in Ca2+-dependent manner.

Versatility of the endoplasmic reticulum protein folding factory

The endoplasmic reticulum (ER) is dedicated to import, folding and assembly of all proteins that travel along or reside in the secretory pathway of eukaryotic cells. Folding in the ER is special. For instance, newly synthesized proteins are N-glycosylated and by default form disulfide bonds in the ER, but not elsewhere in the cell. In this review, we discuss which features distinguish the ER as an efficient folding factory, how the ER monitors its output and how it disposes of folding failures.

Characterization of isoforms and genomic organization of mouse calumenin

Calumenin is a multiple EF-hand protein located in endo/sarcoplasmic reticulum of mammalian heart and other tissues [J. Biol. Chem. 272 (1997) 18232; Genomics 49 (1998) 331; Biochim. Biophys. Acta 1386 (1998) 121]. In the present study, a new isoform of mouse calumenin (mouse calumenin 2) was cloned by RT-PCR and genomic DNA PCR. The deduced amino acid sequence of mouse calumenin 2 is 315 aa long with the calculated MW of 37,064 and pI of 4.26. It has 92% aa sequence identity to previously identified mouse calumenin [J. Biol. Chem. 272 (1997) 18232] (mouse calumenin 1). The difference in the aa sequence was restricted to the first two EF-hand regions (residues 74-138). Northern blot analysis shows that mouse calumenin 2 is highly expressed in heart, lung, testis and unpregnant uterus. The expression of mouse calumenin 2 appears to decrease when fetal development is progressed. Genomic DNA PCR, sequencing and data mining of mouse genome database were utilized to examine the exon-intron boundaries of mouse calumenin genes. Both mouse calumenin 1 and 2 genes encompass six exons, and five of them (Exon1, 3, 4, 5 and 6) are identical. However, mouse calumenin 1 contains Exon2-1, whereas mouse calumenin 2 contains a neighboring Exon2-2. The calumenin genes are localized on mouse chromosome 6 having conserved synteny with human chromosome 7q32. For comparison, the genomic organization of human calumenin was also examined using the published human genome database (UCSC Genome Bioinformatics at ). Like mouse calumenin genes, two human calumenin genes also consist of five identical exons (Exon1, 3, 4, 5 and 6) and a different Exon2. The present study suggests that the genomic organization of calumenin genes is well conserved between human and mouse.

Reduction of the endoplasmic reticulum accompanies the oxidative damage of diabetes mellitus

The endoplasmic reticulum (ER), similary to other subcompartments of the eukaryotic cell possesses a relatively oxidizing environment. The special milieu of ER lumen is important for many ER-specific processes (redox protein folding, glycoprotein synthesis, quality control of secreted proteins, antigen presentation, etc.). Despite of the vital importance of redox regulation in the ER, we have a surprisingly fragmented knowledge about the mechanisms responsible for the ER redox balance. Moreover, new observations on disulfide bridge synthesis and on glutathione functions urge us to revise our recent theories based on many indirect and in vitro results. We have also very little information about the effects of different pathological conditions on the thiol metabolism and redox folding in the ER. Examining the role of molecular chaperones in the cellular pathology of diabetes mellitus we found that the ER redox environment shifted to a more reducing state, which was followed by changes of the thiol metabolism and structural-functional changes of the protein machinery involved in the redox folding process in diabetes. The possible consequences of these unexpected changes are also discussed.

Study of therapy resistance in cancer cells with functional proteome analysis

Different types of cancer are naturally resistant to many anticancer drugs. Additionally, these tumours develop acquired drug resistance, which includes the classical multidrug resistance (MDR) accompanied by the synthesis of P-glycoprotein, a member of the superfamily of ATP-binding cassette (ABC) transporters. Furthermore, atypical MDR is mediated by several different, some unknown, mechanisms. To overcome chemoresistance problems, antineoplastic drugs are often combined with other modes of therapy, e.g. hyperthermia, where good response has been reported in several experimental tumour models and in advanced cancer patients. The success of this combined anticancer treatment may be limited by an increase in chemoresistance and thermoresistance. A model system to study resistance phenomena is the use of chemoresistant and thermoresistant cancer cell lines. We have established chemoresistant cancer cell lines (gastric and pancreatic carcinoma, fibrosarcoma, melanoma) and now thermoresistant cell lines derived from gastric and pancreatic carcinoma cells and their counterparts that were resistant towards daunorubicin (classical MDR) and mitoxantrone (atypical MDR). Using proteomics, in this paper we evaluate the drug resistance of chemoresistant melanoma cells (parental cell line MeWo and sublines exhibiting drug resistance towards etoposide, cisplatin, fotemustine and vindesine) as a paradigm for analysis of drug resistance phenomena. Additionally, we investigate heat resistance and the interaction of chemoresistance and thermoresistance to identify common pathways using the parental and drug resistant stomach cancer cell lines EPG85-257, EPG85-257RNOV, EPG85-257RDB and their thermoresistant counterparts. Possible implications of differential protein expression will be discussed.

The human papillomavirus E6 protein and its contribution to malignant progression

The human papillomavirus (HPV) E6 protein is one of three oncoproteins encoded by the virus. It has long been recognized as a potent oncogene and is intimately associated with the events that result in the malignant conversion of virally infected cells. In order to understand the mechanisms by which E6 contributes to the development of human malignancy many laboratories have focused their attention on identifying the cellular proteins with which E6 interacts. In this review we discuss these interactions in the light of their respective contributions to the malignant progression of HPV transformed cells.

A gene expression profile of Alzheimer's disease

Postmortem analysis of brains of patients with Alzheimer's disease (AD) has led to diverse theories about the causes of the pathology, suggesting that this complex disease involves multiple physiological changes. In an effort to better understand the variety and integration of these changes, we generated a gene expression profile for AD brain. Comparing affected and unaffected brain regions in nine controls and six AD cases, we showed that 118 of the 7050 sequences on a broadly representative cDNA microarray were differentially expressed in the amygdala and cingulate cortex, two regions affected early in the disease. The identity of these genes suggests the most prominent upregulated physiological correlates of pathology involve chronic inflammation, cell adhesion, cell proliferation, and protein synthesis (31 upregulated genes). Conversely, downregulated correlates of pathology involve signal transduction, energy metabolism, stress response, synaptic vesicle synthesis and function, calcium binding, and cytoskeleton (87 downregulated genes). The results support several separate theories of the causes of AD pathology, as well as add to the list of genes associated with AD. In addition, approximately 10 genes of unknown function were found to correlate with the pathology.

Characterization of the differential protein expression associated with thermoresistance in human gastric carcinoma cell lines

Resistance to chemotherapeutic agents is one of the major problems faced during palliative therapy of tumor cells. Thus, chemotherapy is frequently combined with other modes of therapy such as radiation therapy and/or hyperthermia. Tumor cells respond to heat stress with development of thermotolerance and the interactions between chemo- and thermoresistance phenomena are not clearly understood. In this paper, we analyze the differential protein expression in vitro in human stomach cancer cells, their chemoresistant and thermoresistant counterparts using proteomics. The immediate aim was to identify sets of proteins that may lead to the development of thermoresistance. Based on these results, we aim to develop functional tests and methods for the modulation of thermoresistance and chemoresistance phenomena that may assist the therapy of inoperable cancers.

Two distinct domains of the beta-subunit of glucosidase II interact with the catalytic alpha-subunit

Recent purification and cDNA cloning of the endoplasmic reticulum processing enzyme glucosidase II have revealed that it is composed of two soluble proteins: a catalytic alpha-subunit and a beta-subunit of unknown function, both of which are highly conserved in mammals. Since the beta-subunit, which contains a C-terminal His-Asp-Glu-Leu (HDEL) motif, may function to link the catalytic subunit to the KDEL receptor as a retrieval mechanism, we sought to map the regions of the mouse beta-subunit protein responsible for mediating the association with the alpha-subunit. By screening a panel of recombinant beta-subunit glutathione S-transferase fusion proteins for the ability to precipitate glucosidase II activity, we have identified two non-overlapping interaction domains (ID1 and ID2) within the beta-subunit. ID1 encompasses 118 amino acids at the N-terminus of the mature polypeptide, spanning the cysteine-rich element in this region. ID2, located near the C-terminus, is contained within amino acids 273-400, a region occupied in part by a stretch of acidic residues. Variable usage of 7 alternatively spliced amino acids within ID2 was found not to influence the association of the two sub-units. We theorize that the catalytic subunit of glucosidase II binds synergistically to ID1 and ID2, explaining the high associative stability of the enzyme complex.

Vacuolar system-associated protein-60: a protein characterized from bovine granulosa and luteal cells that is associated with intracellular vesicles and related to human 80K-H and murine beta-glucosidase II

It has been suggested that proteins of molecular size 56-58 kDa play an important role in bovine ovarian follicular development and oocyte maturation. A polyclonal antibody was raised against a 56- to 58-kDa protein band purified from bovine granulosa cells and was used to screen granulosa or luteal cell cDNA expression libraries. This work resulted in the identification of a cDNA encoding for a protein of 60.1 kDa with a signal peptide of 13 residues. The bovine 60.1-kDa protein shared an overall 86.7% and 81.8% identity with, respectively, the human 80K-H protein and the mouse putative beta subunit of glucosidase II (beta-GII), and was named vacuolar system-associated protein-60 (VASAP-60). Marked differences in sequence identity were noted in a putative molecular adapter domain containing a tandem D and E amino acid stretch flanked by proline-rich sequences presenting the minimal PXXP SH3 motif. VASAP-60 was shown to be unglycosylated using endoglycosidase H treatment and was found mainly in a cellular membrane fraction of bovine corpus luteum. VASAP-60 was localized in a rat hepatic Golgi/endosome fraction and in wheat germ agglutinin (WGA) affinity chromatographic eluates, thereby suggesting the presence of interactions with membrane glycoproteins. A polyclonal antibody was raised against the putative adapter domain of the recombinant VASAP-60; this was shown to recognize a major 88-kDa and two minor 58-kDa and 50-kDa proteins, suggesting that the major 88-kDa protein band represents the complete VASAP-60 protein whereas the 58-kDa and the 50-kDa bands represent its proteolytic fragments. Northern blot analysis demonstrated the presence of a single 2.3-kilobase transcript in all the bovine tissues analyzed with variation in the steady state level between tissues. Immunohistochemical observations showed that VASAP-60 was widely distributed in bovine tissues and was localized in pericytoplasmic and perinuclear membranes. In epithelial cells, the staining presented a basolateral or apical polarity associated with intracellular vacuoles. In conclusion, we have characterized a novel acidic membrane protein, associated with organelles of the vacuolar system, that is widely and histospecifically expressed in bovine tissues. VASAP-60 represents either the bovine ortholog or a new family member of the previously characterized human 80K-H and murine beta-GII proteins. Our results suggest that VASAP-60 presents characteristics of a molecular adaptor protein with functions in membrane-trafficking events.

Export of parasite proteins to the erythrocyte cytoplasm: secretory machinery and traffic signals

To the malaria parasite, the prospect of setting up residence within a human erythrocyte represents a formidable challenge. The mature human erythrocyte is essentially a bag of haemoglobin with no internal organelles and no protein synthesis machinery. The parasite needs, therefore, to assemble all the essential amenities--foundations, plumbing and furnishings--from scratch. The parasite remodels its adopted home by exporting proteins to the erythrocyte membrane. To reach their final destinations, the exported proteins must cross the parasite plasma membrane, the parasitophorous vacuole membrane and the erythrocyte cytosol. To further understand this unusual and complex trafficking pathway, we have searched for proteins that may form part of the trafficking machinery of the infected erythrocyte. We have identified an ER-located, calcium-binding homologue of reticulocalbin (PfERC) that co-localizes with the ER molecular chaperone, PfGRP. We have also identified a homologue of the GTP-binding protein, Sar1p, a small GTPase that, in other eukaryotic cells, is thought to play a crucial role in trafficking proteins between the ER and the Golgi. PfSar1p is located in discrete structures near the periphery of the parasite cytoplasm that may represent specialized export compartments. PfSar1p is exported to structures outside the parasite in the erythrocyte cytoplasm. The malaria parasite appears to be capable of elaborating components of the 'classical' vesicle mediated trafficking machinery outside the boundaries of its own plasma membrane.

The CREC family, a novel family of multiple EF-hand, low-affinity Ca(2+)-binding proteins localised to the secretory pathway of mammalian cells

The CREC family consists of a number of recently discovered multiple (up to seven) EF-hand proteins that localise to the secretory pathway of mammalian cells. At present, the family includes reticulocalbin, ERC-55/TCBP-49/E6BP, Cab45, calumenin and crocalbin/CBP-50. Similar proteins are found in quite diverse invertebrate organisms such as DCB-45 and SCF in Drosophila melanogaster, SCF in Bombyx mori, CCB-39 in Caenorhabditis elegans and Pfs40/PfERC in Plasmodium falciparum. The Ca(2+) affinity is rather low with dissociation constants around 10(-4)-10(-3) M. The proteins may participate in Ca(2+)-regulated activities. Recent evidence has been obtained that some CREC family members are involved in pathological activities such as malignant cell transformation, mediation of the toxic effects of snake venom toxins and putative participation in amyloid formation.

Calumenin interacts with serum amyloid P component

We recently reported the identification of human calumenin, a novel Ca(2+) binding, transformation-sensitive and secreted protein [Vorum et al. (1998) Biochim. Biophys. Acta 1386, 121-131; Vorum et al. (1999) Exp. Cell Res. 248, 473-481] belonging to the family of multiple EF-hand proteins of the secretory pathway that include reticulocalbin, ERC-55, Cab45 and crocalbin. In order to further investigate the extracellular functions of calumenin we immobilized the recombinant protein to a column. After application of a placental tissue extract we were able to elute one protein that interacts with calumenin in the presence of Ca(2+). Amino acid sequencing identified this protein as serum amyloid P component (SAP). Furthermore, we verified and characterized the calumenin-SAP interaction by the surface plasmon resonance technique. The findings indicate that calumenin may participate in the immunological defense system and could be involved in the pathological process of amyloidosis that leads to formation of amyloid deposits seen in different types of tissues.

Calcium homeostasis and signaling in the blood-stage malaria parasite

The nature of the mechanisms underlying Ca2+ homeostasis in malaria parasites has puzzled investigators for almost two decades. This review summarizes the current knowledge about Ca2+ homeostasis in Plasmodium spp and highlights some key aspects of this process that are specific to this parasite. Plasmodium spp are exposed, during their intracellular stage, not to the usual millimolar concentrations of Ca2+ found in body fluids, but rather to the very low Ca2+ environment of the host cell cytoplasm. Two crucial questions then arise: (1) how is Ca2+ homeostasis achieved by these protozoa; and (2) do they use Ca2+-based signaling pathways? By critically reviewing the recent literature in the field, Célia Garcia here provides at least some partial answers to these questions.

Biochemical analysis of mouse FKBP60, a novel member of the FKPB family

We have identified mouse and human FKBP60, a new member of the FKBP gene family. FKBP60 shares strongest homology with FKBP65 and SMAP. FKBP60 contains a hydrophobic signal peptide at the N-terminus, 4 peptidyl-prolyl cis/trans isomerase (PPIase) domains and an endoplasmic reticulum retention motif (HDEL) at the C-terminus. Immunodetection of HA-tagged FKBP60 in NIH-3T3 cells suggests that FKBP60 is segregated to the endoplasmic reticulum. Northern blot analysis shows that FKBP60 is predominantly expressed in heart, skeletal muscle, lung, liver and kidney. With N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide as a substrate, recombinant GST-FKBP60 is shown to accelerate effectively the isomerization of the peptidyl-prolyl bond. This isomerization activity is inhibited by FK506. mFKBP60 binds Ca2+ in vitro, presumably by its C-terminal EF-hand Ca2+ binding motif, and is phosphorylated in vivo. hFKBP60 has been mapped to 7p12 and/or 7p14 by fluorescence in situ hybridization (FISH).

Multiple functions of human papillomavirus type 16 E6 contribute to the immortalization of mammary epithelial cells

The E6 proteins from cervical cancer-associated human papillomavirus (HPV) types such as HPV type 16 (HPV-16) induce proteolysis of the p53 tumor suppressor protein through interaction with E6-AP. We have previously shown that human mammary epithelial cells (MECs) immortalized by HPV-16 E6 display low levels of p53. HPV-16 E6 as well as other cancer-related papillomavirus E6 proteins also binds the cellular protein E6BP (ERC-55). To explore the potential functional significance of these interactions, we created and analyzed a series of E6 mutants for their ability to interact with E6-AP, p53, and E6BP in vitro. While there was a similar pattern of binding among these E6 targets, a subset of mutants differentiated E6-AP binding, p53 binding, and p53 degradation activities. These results demonstrated that E6 binding to E6-AP is not sufficient for binding to p53 and that E6 binding to p53 is not sufficient for inducing p53 degradation. The in vivo activity of these HPV-16 E6 mutants was tested in MECs. In agreement with the in vitro results, most of these p53 degradation-defective E6 mutants were unable to reduce the p53 level in early-passage MECs. Interestingly, several mutants that showed severely reduced ability for interacting with E6-AP, p53, and E6BP in vitro efficiently immortalized MECs. These immortalized cells exhibited low p53 levels at late passage. Furthermore, mutants defective for p53 degradation but able to immortalize MECs were also identified, and the immortal cells retained normal levels of p53 protein. These results imply that multiple functions of HPV-16 E6 contribute to MEC immortalization.

Human papillomavirus type 16 E6-enhanced susceptibility of L929 cells to tumor necrosis factor alpha correlates with increased accumulation of reactive oxygen species

Human papillomavirus type 16 (HPV-16) E6 has been shown to prevent or enhance apoptosis depending on the stimulus and cell type. Here we present evidence that HPV-16 E6 sensitized murine fibrosarcoma L929 cells to tumor necrosis factor alpha (TNF)-induced cytolysis. The E6-enhanced cytolysis correlated with a precedent increase in reactive oxygen species (ROS) level and antioxidant treatment could completely block the E6-dependent sensitization. These findings represent the first demonstration of a link between a viral oncogene-sensitized cytolysis and ROS. Previous studies have shown conflicting results regarding whether TNF-induced cytolysis of L929 cells is through necrosis or apoptosis. Here we report that, although L929 cells underwent DNA fragmentation after exposure to TNF, they retained the morphology of intact nuclei while gaining permeability to propidium iodide, features characteristic of necrosis rather than apoptosis. We confirmed that the broad spectrum caspase inhibitor benzyloxycarbonyl-Val-Ala-Asp(OMe)-fluoromethylketone markedly increased the susceptibility of L929 cells to TNF, and further demonstrated that E6 enhanced this susceptibility, which again correlated with increased ROS accumulation. We showed that the expression of E6 in L929 cells did not alter the stability of p53, and the cells retained a p53 response to actinomycin D. Furthermore, two E6 mutants defective for p53 degradation in other systems exhibited differential effects on TNF sensitization. These results suggest that the enhancement of TNF-induced L929 cytolysis by E6 is independent of p53 degradation. We also found that TNF-induced activation of NF-kappaB did not account for the enhanced TNF susceptibility by E6.

New concepts on the role of human papillomavirus in cell cycle regulation

Human papillomaviruses (HPVs) are strictly host-specific and also show a distinct tropism to squamous epithelial cells. Upon HPV infection, only a portion of the virus reaching the nucleus seems to undergo replication, suggesting that HPV replication remains confined to a small number of cells. HPVs critically depend on the cellular machinery for the replication of their genome. Viral replication is restricted to differentiated keratinocytes that are normally growth arrested. Hence, HPVs have developed strategies to subvert cellular growth regulatory pathways and are able to uncouple cellular proliferation and differentiation. Endogenous growth factors and cellular oncogenes modify HPV E (early) and L (late) gene expression and influence on the pathogenesis of HPV infections. HPV oncoproteins (E5, E6, E7) are important proteins not only in cell transformation but also in the regulation of the mitotic cycle of the cell, thus allowing the continuous proliferation of the host cells. Cyclins are important regulators of cell cycle transitions through their ability to bind cyclin-dependent kinases (cdks). Cdks have no kinase activity unless they are associated with a cyclin. Several classes of cyclins exist which are thought to coordinate the timing of different events necessary for cell cycle progression. Each cdk catalytic subunit can associate with different cyclins, and the associated cyclin determines which proteins are phosphorylated by the cdk-cyclin complex. The effects of HPVs on the cell cycle are mediated through the inhibition of antioncogens (mostly p53 and retinoblastoma) and through interference with the cyclins and cdks, resulting in target cell proliferation, their delayed differentiation, and as a side-effect, in malignant transformation.

Human calumenin localizes to the secretory pathway and is secreted to the medium

Calumenin belongs to a family of multiple EF-hand proteins that include reticulocalbin, ERC-55, and Cab45. Reticulocalbin and ERC-55 localize to the ER due to a C-terminal HDEL retrieval signal. Cab45 contains a HEEF C-terminal sequence and is localized to the Golgi apparatus. The murine homologue of calumenin is reported to be present in the ER due to a C-terminal HDEF retrieval signal. The human homologue differs from the murine at 7 amino acid positions but the HDEF signal is conserved. However, in the cultured human cell lines, HaCaT keratinocytes, normal and transformed MRC-5 fibroblasts, as well as in transfected COS-1 cells, human calumenin could be demonstrated in the ER as well as in the Golgi complex. Especially in MRC-5 cells, a certain heterogeneity was observed, with some of the cells having calumenin localized solely to the ER while in other cells calumenin could be demonstrated in the ER as well as in the Golgi complex. Immunoelectron microscopy of placental syncytiotrophoblast cells showed that a substantial fraction of calumenin is localized in close association with the ER membrane. In addition, the protein may be recovered from the medium of cultured cells in an endoglycosidase H-resistant form, suggesting that the glycosylated protein has been further modified in the Golgi apparatus and secreted to the medium.

Crocalbin: a new calcium-binding protein that is also a binding protein for crotoxin, a neurotoxic phospholipase A2

Utilizing Marathon-ready cDNA library and a gene-specific primer corresponding to a partial amino acid sequence determined previously, the complete nucleotide sequence for the cDNA of crocalbin, which binds crotoxin (a phospholipase A2) and Ca2+, was obtained by polymerase chain reaction. The open reading frame of the cDNA encodes a novel polypeptide of 315 amino acid residues, including a signal sequence of 19 residues. This protein contains six potential Ca(2+)-binding domains, one N-glycosylation site, and a large amount of acidic amino acid residues. The ability to bind Ca2+ has been ascertained by calcium overlay experiment. Evidenced by sequence similarity in addition, it is concluded that crocalbin is a new member of the reticulocalbin family of calcium-binding proteins.

The bovine papillomavirus type 1 E6 oncoprotein sensitizes cells to tumor necrosis factor alpha-induced apoptosis

Expression of viral proteins may result in susceptibility of cells to the cytotoxic effect of Tumor Necrosis Factor Alpha (TNF). While murine C127 cells containing the bovine papillomavirus type 1 (BPV-1) genome were reported to exhibit increased TNF sensitivity, the gene(s) responsible was not identified. The BPV-1 E6 oncoprotein induces tumorigenic transformation of murine C127 cells and stimulates transcription when targeted to a promoter. BPV-1 E6 was introduced into C127 cells (PBE6) by retroviral infection and stable clones were isolated. These cells showed increased apoptosis in response to TNF, as measured by several criteria. TNF-induced apoptosis in PBE6 cells was accompanied by increased release of arachidonic acid, indicating that phospholipase A2 was activated. We also provide evidence that BPV-1 E6 mediated-sensitization of cells to TNF-induced apoptosis can occur in the absence of p53.

Molecular cloning, characterization, and chromosomal localization of FKBP23, a novel FK506-binding protein with Ca2+-binding ability

We have identified and characterized a cDNA encoding a novel FK506-binding protein (FKBP), named FKBP23, from mouse heart by the signal sequence trap method. The deduced amino acid sequence has significant homology to other FKBP family members around the peptidylprolyl cis-trans-isomerase motifs. FKBP23 also has two Ca2+-binding (EF-hand) motifs, and purified FKBP23 protein was shown to have Ca2+-binding ability. This is the first report of a Ca2+-binding FKBP. FKBP23 is a glycoprotein retained in the endoplasmic reticulum by its carboxyl-terminal tetrapeptide His-Asp-Glu-Leu, as demonstrated by immunostaining, retention, and deglycosylation assays. FKBP23 mRNA is expressed most strongly in heart, lung, and testis, beginning at day 8.5 of embryonic development. The FKBP23 gene was mapped to mouse chromosome 2.

DNA supercoiling factor localizes to puffs on polytene chromosomes in Drosophila melanogaster

DNA supercoiling factor (SCF) was first identified in silkworm as a protein that generates negative supercoils in DNA in conjunction with eukaryotic topoisomerase II. To analyze the in vivo role of the factor, we cloned a cDNA encoding Drosophila melanogaster SCF. Northern analysis revealed 1.6- and 1.8-kb mRNAs throughout development. The longer mRNA contains an open reading frame that shares homology with mouse reticulocalbin whereas the shorter one encodes a truncated version lacking the N-terminal signal peptide-like sequence. An antibody against SCF detected a 45-kDa protein in the cytoplasmic fraction and a 30-kDa protein in the nuclear fraction of embryonic extracts. Immunoprecipitation suggests that the 30-kDa protein interacts with topoisomerase II in the nucleus, and hence that it is a functional form of SCF. Immunostaining of blastoderm embryos showed that SCF is present in nuclei during interphase but is excluded from mitotic chromosomes. In larvae, the antibody stained the nuclei of several tissues including a posterior part of the salivary gland. This latter staining was associated with natural or ecdysteroid-induced puffs on polytene chromosomes. Upon heat treatment of larvae, the staining on the endogenous puffs disappeared, and strong staining appeared on heat shock puffs. These results implicate SCF in gene expression.

Molecular targets for human papillomaviruses: prospects for antiviral therapy

A substantial medical need exists for the development of antiviral medicines for the treatment of diseases associated with infection by human papillomaviruses (HPVs). HPVs are associated with various benign and malignant lesions including benign genital condyloma, common skin warts, laryngeal papillomas and anogenital cancer. Since treatment options are limited and typically not very satisfactory, the development of safe and effective antiviral drugs for HPV could have substantial clinical impact. In the last few years, exciting advances have been made in our understanding of papillomavirus replication and the effects that the virus has on growth of the host cell. Although still somewhat rudimentary, techniques have been developed for limited virion production in vitro offering the promise of more rapid advances in the dissection and understanding of the virus life cycle. Of the 8-10 HPV gene products that are made during infection, only one encodes enzymatic activities, the E1 helicase. Successful antiviral therapies have traditionally targeted viral enzymes such as polymerases, kinases and proteases. In contrast, macromolecular interactions which mediate the functions of E6, E7 and E2 are thought to be more difficult targets for small molecule therapy.

The papillomavirus E6 proteins

Specific types of human papillomaviruses (HPV) are strongly associated with the development of cervical cancer. The E6 gene from cancer-related HPVs has exhibited functions in tumorigenesis, regulation of transcription, telomerase, and apoptosis. Cancer-related HPVs E6 proteins bind the tumor suppressor p53 and promotes its degradation through an ubiquitin-dependent pathway. Several additional cellular E6-binding proteins have recently been identified and implicated in playing roles in p53-independent functions of E6.