Interaction of intracellular ion buffering with transmembrane-coupled ion transport

The role of the Na/Ca exchanger in the control of cellular excitability and tension development is a subject of current interest in cardiac physiology. It has been suggested that this coupled transporter is responsible for rapid changes in intracellular calcium activity during single beats, generation of plateau currents, which control action potential duration, and control of intracellular sodium during Na/K pump suppression, which may occur during terminal states of ischemia. The actual behavior of this exchanger is likely to be complex for several reasons. First, the exchanger transports two ionic species and thus its instantaneous flux rate depends on both intracellular sodium and calcium activity. Secondly, the alteration in intracellular calcium activity, which is caused by a given transmembrane calcium flux, and which controls the subsequent exchanger rate, is a complex function of available intracellular calcium buffering. The buffers convert the ongoing transmembrane calcium fluxes into changes in activity that are a small and variable fraction of the change in total calcium concentration. Using a number of simple assumptions, we model changes in intracellular calcium and sodium concentration under the influence of Na/Ca exchange, Na/K ATPase and Ca-ATPase pumps, and passive sodium and calcium currents during periods of suppression and reactivation of the Na/K ATPase pump. The goal is to see whether and to what extent general notions of the role of the Na/Ca exchanger used in planning and interpreting experimental studies are consistent with its function as derived from current mechanistic assumptions about the exchanger. We find, for example, that based on even very high estimates of intracellular calcium buffering, it is unlikely that Na/Ca exchange alone can control intracellular sodium during prolonged Na/K pump blockade. It is also shown that Na/Ca exchange can contaminate measurements of Na/K pump currents under a variety of experimental conditions. The way in which these and other functions are affected by the dissociation constants and total capacity of the intracellular calcium buffers are also explored in detail.     

Cell surface-binding motifs of L2 that facilitate papillomavirus infection

Human papillomavirus type 16 (HPV16) is the primary etiologic agent of cervical carcinoma, whereas bovine papillomavirus type 1 (BPV1) causes benign fibropapillomas. However, the capsid proteins, L1 and L2, of these divergent papillomaviruses exhibit functional conservation. A peptide comprising residues 1 to 88 of BPV1 L2 binds to a variety of cell lines, but not to the monocyte-derived cell line D32, and blocks BPV1 infection of mouse C127 cells. Residues 13 to 31 of HPV16 L2 and BPV1 L2 residues 1 to 88 compete for binding to the cell surface, and their binding, unlike that of HPV16 L1/L2 virus-like particles, is unaffected by heparinase or trypsin pretreatment of HeLa cells. A fusion of HPV16 L2 peptide 13-31 and GFP binds (K(d), approximately 1 nM) to approximately 45,000 receptors per HeLa cell. Furthermore, mutation of L2 residues 18 and 19 or 21 and 22 significantly reduces both the ability of the HPV16 L2 13-31-GFP fusion protein to bind to SiHa cells and the infectivity of HPV16 pseudovirions. Antibody to BPV1 L2 peptides comprising residues 115 to 135 binds to intact BPV1 virions, but fails to neutralize at a 1:10 dilution. However, deletion of residues 91 to 129 from L2 abolishes the infectivity of BPV1, but not their binding to the cell surface. In summary, L2 residues 91 to 129 contain epitopes displayed on the virion surface and are required for infection, but not virion binding to the cell surface. Upon the binding of papillomavirus to the cell surface, residues 13 to 31 of L2 interact with a widely expressed, trypsin- and heparinase-resistant cell surface molecule and facilitate infection.     

Neurotrophin-induced transport of a beta-actin mRNP complex increases beta-actin levels and stimulates growth cone motility

Neurotrophin regulation of actin-dependent changes in growth cone motility may depend on the signaling of beta-actin mRNA transport. Formation of an RNP complex between the beta-actin mRNA zipcode sequence and Zipcode Binding Protein 1 (ZBP1) was required for its localization to growth cones. Antisense oligonucleotides to the zipcode inhibited formation of this RNP complex in vitro and the neurotrophin-induced localization of beta-actin mRNA and ZBP1 granules. Live cell imaging of neurons transfected with EGFP-ZBP1 revealed fast, bidirectional movements of granules in neurites that were inhibited by antisense treatment, as visualized by FRAP analysis. NT-3 stimulation of beta-actin protein localization was dependent on the 3'UTR and inhibited by antisense treatment. Growth cones exhibited impaired motility in the presense of antisense. These results suggest a novel mechanism to influence growth cone dynamics involving the regulated transport of mRNA.     

Protein flexibility directs DNA recognition by the papillomavirus E2 proteins

Although DNA flexibility is known to play an important role in DNA-protein interactions, the importance of protein flexibility is less well understood. Here, we show that protein dynamics are important in DNA recognition using the well-characterized human papillomavirus (HPV) type 6 E2 protein as a model system. We have compared the DNA binding properties of the HPV 6 E2 DNA binding domain (DBD) and a mutant lacking two C-terminal leucine residues that form part of the hydrophobic core of the protein. Deletion of these residues results in increased specific and non-specific DNA binding and an overall decrease in DNA binding specificity. Using (15)N NMR relaxation and hydrogen/deuterium exchange, we demonstrate that the mutation results in increased flexibility within the hydrophobic core and loop regions that orient the DNA binding helices. Stopped-flow kinetic studies indicate that increased flexibility alters DNA binding by increasing initial interactions with DNA but has little or no effect on the structural rearrangements that follow this step. Taken together these data demonstrate that subtle changes in protein dynamics have a major influence on protein-DNA interactions.     

Interaction of tSNARE syntaxin 18 with the papillomavirus minor capsid protein mediates infection

The papillomavirus capsid mediates binding to the cell surface and passage of the virion to the perinuclear region during infection. To better understand how the virus traffics across the cell, we sought to identify cellular proteins that bind to the minor capsid protein L2. We have identified syntaxin 18 as a protein that interacts with bovine papillomavirus type 1 (BPV1) L2. Syntaxin 18 is a target membrane-associated soluble N-ethylmaleimide-sensitive factor-attachment protein receptor (tSNARE) that resides in the endoplasmic reticulum (ER). The ectopic expression of FLAG-tagged syntaxin 18, which disrupts ER trafficking, blocked BPV1 pseudovirion infection. Furthermore, the expression of FLAG-syntaxin 18 prevented the passage of BPV1 pseudovirions to the perinuclear region that is consistent with the ER. Genetic studies identified a highly conserved L2 domain, DKILK, comprising residues 40 to 44 that mediated BPV1 trafficking through the ER during infection via an interaction with the tSNARE syntaxin 18. Mutations within the DKILK motif of L2 that did not significantly impact virion morphogenesis or binding at the cell surface prevented the L2 interaction with syntaxin 18 and disrupted BPV1 infection.     

Expression vector system based on the chicken beta-actin promoter directs efficient production of interleukin-5

We examined the promoter activity of the 1.3-kb chicken beta-actin gene sequence located between the 5' flanking region and the proximal region of the second exon. This promoter region showed higher promoter activity than the simian virus 40 (SV40) early promoter or the Rous sarcoma virus (RSV) long terminal repeat (LTR) as assayed by transient lacZ gene expression in mouse L cells. Furthermore, replacement of the 3' splice sequence in this promoter by that derived from the rabbit beta-globin gene resulted in a approximately 2.5-fold enhancement in the synthesis of beta-galactosidase (beta Gal). Introduction of the SV40 origin of DNA replication (ori) into the vector carrying this hybrid promoter, which we designate the AG promoter, markedly enhanced the production of beta Gal in an SV40 T antigen-producing cell, BMT10. We have constructed a useful vector containing the strong AG promoter, several unique restriction sites, a SV40 polyadenylation signal and the SV40 ori for transient expression of cDNA in BMT10 or COS cells. We demonstrate the use of this vector for efficient production of interleukin-5 in BMT10 cells.     

Immunization with a pentameric L1 fusion protein protects against papillomavirus infection

The prophylactic papillomavirus vaccines currently in clinical trials are composed of viral L1 capsid protein that is synthesized in eukaryotic expression systems and purified in the form of virus-like particles (VLPs). To evaluate whether VLPs are necessary for effective vaccination, we expressed the L1 protein as a glutathione S-transferase (GST) fusion protein in Escherichia coli and assayed its immunogenic activity in an established canine oral papillomavirus (COPV) model that previously validated the efficacy of VLP vaccines. The GST-COPV L1 fusion protein formed pentamers, but these capsomere-like structures did not assemble into VLPs. Despite the lack of VLP formation, the GST-COPV L1 protein retained its native conformation as determined by reactivity with conformation-specific anti-COPV antibodies. Most importantly, the GST-COPV L1 pentamers completely protected dogs from high-dose viral infection of their oral mucosa. L1 fusion proteins expressed in bacteria represent an economical alternative to VLPs as a human papillomavirus vaccine.     

Interactions between papillomavirus L1 and L2 capsid proteins

The human papillomavirus (HPV) capsid consists of 360 copies of the major capsid protein, L1, arranged as 72 pentamers on a T=7 icosahedral lattice, with substoichiometric amounts of the minor capsid protein, L2. In order to understand the arrangement of L2 within the HPV virion, we have defined and biochemically characterized a domain of L2 that interacts with L1 pentamers. We utilized an in vivo binding assay involving the coexpression of recombinant HPV type 11 (HPV11) L1 and HPV11 glutathione S-transferase (GST) L2 fusion proteins in Escherichia coli. In this system, L1 forms pentamers, GST=L2 associates with these pentamers, and L1+L2 complexes are subsequently isolated by using the GST tag on L2. The stoichiometry of L1:L2 in purified L1+L2 complexes was 5:1, indicating that a single molecule of L2 interacts with an L1 pentamer. Coexpression of HPV11 L1 with deletion mutants of HPV11 L2 defined an L1-binding domain contained within amino acids 396 to 439 near the carboxy terminus of L2. L2 proteins from eight different human and animal papillomavirus serotypes were tested for their ability to interact with HPV11 L1. This analysis targeted a hydrophobic region within the L1-binding domain of L2 as critical for L1 binding. Introduction of negative charges into this hydrophobic region by site-directed mutagenesis disrupted L1 binding. L1-L2 interactions were not significantly disrupted by treatment with high salt concentrations (2 M NaCl), weak detergents, and urea concentrations of up to 2 M, further indicating that L1 binding by this domain is mediated by strong hydrophobic interactions. L1+L2 protein complexes were able to form virus-like particles in vitro at pH 5.2 and also at pH 6.8, a pH that is nonpermissive for assembly of L1 protein alone. Thus, L1/L2 interactions are primarily hydrophobic, encompass a relatively short stretch of amino acids, and have significant effects upon in vitro assembly.     

Positively charged termini of the L2 minor capsid protein are necessary for papillomavirus infection

Coexpression of bovine papillomavirus L1 with L2 mutants lacking either eight N-terminal or nine C-terminal amino acids that encode positively charged domains resulted in wild-type levels of viral genome encapsidation. Despite wild-type binding to the cell surface, the resulting virions were noninfectious. An L2 mutant encoding a scrambled version of the nine C-terminal residues restored infectivity, in contrast to an L2 mutant encoding a scrambled version of the N-terminal residues.     

Arrangement of L2 within the papillomavirus capsid

Papillomaviruses are a family of nonenveloped DNA tumor viruses. Some sexually transmitted human papillomavirus (HPV) types, including HPV type 16 (HPV16), cause cancer of the uterine cervix. Papillomaviruses encode two capsid proteins, L1 and L2. The major capsid protein, L1, can assemble spontaneously into a 72-pentamer icosahedral structure that closely resembles native virions. Although the minor capsid protein, L2, is not required for capsid formation, it is thought to participate in encapsidation of the viral genome and plays a number of essential roles in the viral infectious entry pathway. The abundance of L2 and its arrangement within the virion remain unclear. To address these questions, we developed methods for serial propagation of infectious HPV16 capsids (pseudoviruses) in cultured human cell lines. Biochemical analysis of capsid preparations produced using various methods showed that up to 72 molecules of L2 can be incorporated per capsid. Cryoelectron microscopy and image reconstruction analysis of purified capsids revealed an icosahedrally ordered L2-specific density beneath the axial lumen of each L1 capsomer. The relatively close proximity of these L2 density buttons to one another raised the possibility of homotypic L2 interactions within assembled virions. The concept that the N and C termini of neighboring L2 molecules can be closely apposed within the capsid was supported using bimolecular fluorescence complementation or "split GFP" technology. This structural information should facilitate investigation of L2 function during the assembly and entry phases of the papillomavirus life cycle.     

Hexadecylphosphocholine interferes with the intracellular transport of cholesterol in HepG2 cells

We have shown, in a previous publication, that nontoxic concentrations of hexadecylphosphocholine exert an antiproliferative effect on HepG2 cells. Hexadecylphosphocholine also interferes with the biosynthesis of cholesterol and phosphatidylcholine. We have now extended our studies to try to establish the molecular mechanism by which hexadecylphosphocholine disrupts cholesterol homeostasis. Using radiolabelled substrates we determined the effect of hexadecylphosphocholine on cholesterol synthesis, the destiny of cholesterol from low-density lipoprotein and the transport of cholesterol between the plasma membrane and the endoplasmic reticulum. Protein levels and gene expression of the main proteins involved in cholesterol homeostasis were analysed by western blotting and RT-PCR, respectively. HepG2 cells exposed to hexadecylphosphocholine showed an increase in cholesterol biosynthesis when acetate, but not mevalonate, was used as a substrate. The activity of 3-hydroxy-3-methylglutaryl-CoA reductase (EC 1.1.1.34) and low-density lipoprotein receptor, as well as the corresponding mRNA expression, increased after 24 h of treatment with hexadecylphosphocholine. Cholesteryl linoleate in low-density lipoprotein uptake and further hydrolysis of these esters increased but the cholesterol esterification was reduced after 6 h of treatment with alkylphosphocholine. Cholesterol transport from the plasma membrane to the endoplasmic reticulum was impaired by hexadecylphosphocholine. In conclusion, hexadecylphosphocholine interfered with the transport of cholesterol from the cell surface to the endoplasmic reticulum, leading to a depletion of cholesterol in the endoplasmic reticulum and a deregulation of cholesterol biosynthesis. The accumulation of cholesterol within the cell and the reduction in phosphatidylcholine synthesis produces an alteration in the phosphatidylcholine/cholesterol ratio that may well be responsible for the antiproliferative activity exhibited by hexadecylphosphocholine in HepG2 cells.     

Sequences responsible for intracellular localization of beta-actin messenger RNA also affect cell phenotype

We have characterized the structure and function of RNA sequences that direct beta-cytoplasmic actin mRNA to the cell periphery were mapped to two segments of 3'-untranslated region by expression of LacZ/beta-actin chimeric mRNAs in chicken embryo fibroblasts (CEFs). A 54-nt segment, the "RNA zipcode," and a homologous but less active 43-nt segment each localized beta-galactosidase activity to the leading lamellae. This zipcode contains the full activity, and mutations or deletions within it reduce, but do not eliminate, its activity, indicating that several motifs contribute to the activity. Two of these motifs, when multimerized, can regenerate almost full activity. These sequences are highly conserved in evolution, since the human beta-actin zipcode, positioned identically in the 3'UTR localizes equally well in chicken cells. Complementary phosphorothioate oligonucleotides against the zipcode delocalized endogenous beta-actin mRNA, whereas those complementary to the region just outside the zipcode, or sense oligonucleotides, did not. Actin mRNA or protein levels were unaffected by the antisense treatments, but a dramatic change in lamellipodia structure, and actin stress fiber organization was observed using the same antizipcode oligonucleotides which delocalized the mRNA. Hence, discrete 3'UTR sequences direct beta-actin isoform synthesis to the leading lamellae and affect cell morphology, presumably through the actin cytoskeleton.     

Interaction of human papillomavirus (HPV) type 16 capsid proteins with HPV DNA requires an intact L2 N-terminal sequence.

Encapsidation of papillomavirus DNA involves DNA-protein and protein-protein interactions. We sought to define the role of each human papillomavirus (HPV) capsid protein in HPV DNA encapsidation. HPV16 major (L1) and minor (L2) capsid proteins purified from recombinant vaccinia virus-infected cells were compared for their ability to bind nucleic acids. L2 protein, but not L1 protein, could bind HPV DNA. To map the DNA-binding region of L2, a series of truncated or point-mutated L2 protein open reading frames were used to show that only the N terminal of L2 was required for L2-DNA binding. This interaction depends critically on charged amino acids (Lys or Arg) in the first 12 amino acids of the N terminal of the protein. Several techniques were used to show that L2 interaction with DNA did not require specific DNA sequences. We propose that HPV L2 protein may play a major role in papillomavirus capsid assembly by introducing HPV DNA to the virus particles formed by the self assembly of the L1 major structural protein.     

Specific inhibitors of intracellular Ca2+ transport ATPases

Support from the National Institutes of Health and the American Heart Association is gratefully acknowledged.     

Interaction of porcine circovirus type 2 replication with intracellular redox status in vitro

Abstract

Objectives

Redox status influences replication of some viruses but its effect on porcine circovirus type 2 (PCV2), the primary causative agent of the emerging swine disease post-weaning multisystemic wasting syndrome is not known. The interaction of PCV2 replication with intracellular redox status in PK15 cells was examined in this study.

Methods

Intracellular glutathione (GSH) was measured spectrophotometrically by reaction with 5, 5′-dithiobis (2-nitrobenzoic acid). Total superoxide dismutase activity (SOD) was assayed by inhibition of oxyamine oxidation by the xanthine oxidase system. Malondialdehyde (MDA) was assayed spectrophotometrically using the thiobarbituric acid reaction. Both quantification of PCV2 DNA by real-time polymerase chain reaction and indirect immunofluorescence of PCV2-infected cells were used to evaluate the replication of PCV2.

Results

Both GSH and SOD decreased significantly at 48 hours after PCV2 infection, whereas MDA concentration increased significantly after 48 hour post-infection. Furthermore, PCV2 replication in PK15 cells was significantly impaired after the elevation of intracellular GSH through treatment with the antioxidant N-acetyl-l-cysteine (NAC), a precursor in GSH synthesis. In contrast, PCV2 replication in PK15 cells was enhanced after reduction of GSH levels through H₂O₂-mediated oxidation. In addition, NAC treatment blocked the increase of virus replication induced by H₂O₂.

Conclusions

This study suggests that PCV2 infection induces oxidative stress and that intracellular redox status influences PCV2 replication in PK15 cells.     

Properties of intracellular bovine papillomavirus chromatin.

Episomal nucleoprotein complexes of bovine papillomavirus type 1 (BPV-1) in transformed cells were exposed to DNase I treatment to localize hypersensitive regions. Such regions, which are indicative for gene expression, were found within the noncoding part of the genome, coinciding with the origin of replication and the 5' ends of most of the early mRNAs. However, there were also regions of hypersensitivity within the structural genes. These intragenic perturbations of the chromatin structure coincide with regulatory sequences at the DNA level. One of these regions maps in close proximity to a Z-DNA antibody-binding site which is located near the putative BPV-1 enhancer sequence.