Isolation and characterization of monopinocytotic vesicles containing polyomavirus from the cytoplasm of infected mouse kidney cells

Relocalization of junctional adhesion molecule A during inflammatory stimulation of brain endothelial cells

Junctional adhesion molecule A (JAM-A) is a unique tight junction (TJ) transmembrane protein that under basal conditions maintains endothelial cell-cell interactions but under inflammatory conditions acts as a leukocyte adhesion molecule. This study investigates the fate of JAM-A during inflammatory TJ complex remodeling and paracellular route formation in brain endothelial cells. The chemokine (C-C motif) ligand 2 (CCL2) induced JAM-A redistribution from the interendothelial cell area to the apical surface, where JAM-A played a role as a leukocyte adhesion molecule participating in transendothelial cell migration of neutrophils and monocytes. JAM-A redistribution was associated with internalization via macropinocytosis during paracellular route opening. A tracer study with dextran-Texas Red indicated that internalization occurred within a short time period (~10 min) by dextran-positive vesicles and then became sorted to dextran-positive/Rab34-positive/Rab5-positive vesicles and then Rab4-positive endosomes. By ~20 min, most internalized JAM-A moved to the brain endothelial cell apical membrane. Treatment with a macropinocytosis inhibitor, 5-(N-ethyl-N-isopropyl)amiloride, or Rab5/Rab4 depletion with small interfering RNA oligonucleotides prevented JAM-A relocalization, suggesting that macropinocytosis and recycling to the membrane surface occur during JAM-A redistribution. Analysis of the signaling pathways indicated involvement of RhoA and Rho kinase in JAM-A relocalization. These data provide new insights into the molecular and cellular mechanisms involved in blood-brain barrier remodeling during inflammation.

Dictyostelium dynamin B modulates cytoskeletal structures and membranous organelles

Dictyostelium discoideum cells produce five dynamin family proteins. Here, we show that dynamin B is the only member of this group of proteins that is initially produced as a preprotein and requires processing by mitochondrial proteases for formation of the mature protein. Our results show that dynamin B-depletion affects many aspects of cell motility, cell-cell and cell-surface adhesion, resistance to osmotic shock, and fatty acid metabolism. The mature form of dynamin B mediates a wide range and unique combination of functions. Dynamin B affects events at the plasma membrane, peroxisomes, the contractile vacuole system, components of the actin-based cytoskeleton, and cell adhesion sites. The modulating effect of dynamin B on the activity of the contractile vacuole system is unique for the Dictyostelium system. Other functions displayed by dynamin B are commonly associated with either classical dynamins or dynamin-related proteins.

BK polyoma virus allograft nephropathy: ultrastructural features from viral cell entry to lysis

BK virions must enter the host cell and target their genome to the nucleus in order to complete their life cycle. The mechanisms by which the virions accomplish these tasks are not known. In this morphological study we found that BK virions localized beneath the host cell cytoplasmic membrane in 60-70-nm, smooth (non-coated) monopinocytotic vesicles similar to, or consistent with, caveolae. In the cytoplasm, the monopinocytotic vesicles carrying virions appeared to fuse with a system of smooth, vesicles and tubules that communicated with the rough endoplasmic reticulum and was continuous with the Golgi system. Membrane-bound single virions and large tubulo-reticular complexes loaded with virions accumulated in paranuclear locations. Occasional nuclei displayed virions within the perinuclear cisterna in association to the perinuclear viral accumulations. Tubular cells with mature productive infection had large nuclei, distended by daughter virions, whereas they lacked significant numbers of cytoplasmic virions. In addition to virally induced cell necrosis, there was extensive tubular cell damage (apoptosis and necrosis) in morphologically non-infected tubules. The observed ultrastructural interactions between the BK virions and host cells are remarkably similar to viral cell entry and nuclear targeting described for SV40 virus.

Mouse polyomavirus utilizes recycling endosomes for a traffic pathway independent of COPI vesicle transport

Mouse polyomavirus enters host cells internalized, similar to simian virus 40 (SV40), in smooth monopinocytic vesicles, the movement of which is associated with transient actin disorganization. The major capsid protein (VP1) of the incoming polyomavirus accumulates on membranes around the cell nucleus. Here we show that unlike SV40, mouse polyomavirus infection is not substantially inhibited by brefeldin A, and colocalization of VP1 with beta-COP during early stages of polyomavirus infection in mouse fibroblasts was observed only rarely. Thus, these viruses obviously use different traffic routes from the plasma membrane toward the cell nucleus. At approximately 3 h postinfection, a part of VP1 colocalized with the endoplasmic reticulum marker BiP, and a subpopulation of virus was found in perinuclear areas associated with Rab11 GTPase and colocalized with transferrin, a marker of recycling endosomes. Earlier postinfection, a minor subpopulation of virions was found to be associated with Rab5, known to be connected with early endosomes, but the cell entry of virus was slower than that of transferrin or cholera toxin B-fragment. Neither Rab7, a marker of late endosomes, nor LAMP-2 lysosomal glycoprotein was found to colocalize with polyomavirus. In situ hybridization with polyomavirus genome-specific fluorescent probes clearly demonstrated that, regardless of the multiplicity of infection, only a few virions delivered their genomic DNA into the cell nucleus, while the majority of viral genomes (and VP1) moved back from the proximity of the nucleus to the cytosol, apparently for their degradation.

Scanning surface confocal microscopy for simultaneous topographical and fluorescence imaging: application to single virus-like particle entry into a cell

We have developed a method for simultaneous recording of high-resolution topography and cell surface fluorescence in a single scan which we call scanning surface confocal microscopy. The resolution of the system allows imaging of individual fluorescent particles in the nanometer range on fixed or live cells. We used this technique to record the interaction of single virus-like particles with the cell surface and demonstrated that single particles sink into the membrane in invaginations reminiscent of caveolae or pinocytic vesicles. This method provides a technique for elucidating the interaction of individual viruses and other nanoparticles, such as gene therapy vectors, with target cells. Furthermore, this technique should find widespread application for studying the relationship of fluorescently tagged molecules with components of the cell plasma membrane.

Analysis of mouse polyomavirus mutants with lesions in the minor capsid proteins

Polyomavirus mutants E, Q and H, expressing non-myristylated VP2, were generated by replacing the N-terminal glycine residue with glutamic acid, glutamine or histidine, respectively. Viruses mutated in either VP2 or VP3 translation initiation codons were also prepared. All mutated genomes, when transfected into murine host cells, gave rise to viral particles. Infectivity of VP2- and VP3- viruses, as measured by the number of cells expressing viral antigens, was dramatically diminished, indicative of defects in the early stages of infection. In contrast, the absence of a myristyl moiety on VP2 did not substantially affect the early steps of virus infection. No differences in numbers of cells expressing early or late viral antigens were observed between wild-type (wt) and E or Q myr- viruses during the course of a life cycle. Furthermore, no delay in virus DNA replication was detected. However, when cells were left for longer in culture, the number of infected cells, measured by typical virus bursts, was much lower when mutant rather than wt genomes were used. In situ, cell fractionation studies revealed differences in the interaction of viral particles with host cell structures. The infectivity of mutants was affected not only by loss of the myristyl group on VP2, but also, and to a greater extent, by alterations of the N-terminal amino acid composition.

A cell regulatory agent, CeReS-18, inhibits mouse 3T6 cell proliferation but not polyomavirus replication

We have purified a cell regulatory sialoglycopeptide, CeReS-18, from intact bovine cerebral cortex cells. This is an 18-kDa molecule that reversibly inhibits cellular DNA synthesis and the proliferation of a wide array of target cells. In the present study, the effect of CeReS-18 on mouse 3T6 host cell proliferation and polyomavirus replication was investigated. The results showed that CeReS-18 was able to inhibit 3T6 cell cycling in a concentration-dependent, calcium-sensitive, and reversible manner. Despite the inhibition of cell proliferation, CeReS-18 did not influence polyomavirus infection of 3T6 cells. Indirect immunofluorescent assays revealed that CeReS-18-treated, and cell cycle-arrested, 3T6 cells remained permissive to polyomavirus replication. Electron microscopy and immunogold labeling showed that new viral particles were assembled inside the nuclei of infected cells in the presence of CeReS-18 and during cell cycle arrest. The cellular requirements for the replication of polyomavirus DNA and the synthesis of viral proteins, as well as for the assembly of viral particles, therefore, remained available in CeReS-18-inhibited 3T6 cells. In addition, although polyomavirus infection can be mitogenic, infection of CeReS-18-treated 3T6 cells did not reverse the cell cycle arrest mediated by this cell cycle inhibitor.

Caveolae are involved in the trafficking of mouse polyomavirus virions and artificial VP1 pseudocapsids toward cell nuclei

Electron and confocal microscopy were used to observe the entry and the movement of polyomavirus virions and artificial virus-like particles (VP1 pseudocapsids) in mouse fibroblasts and epithelial cells. No visible differences in adsorption and internalization of virions and VP1 pseudocapsids ("empty" or containing DNA) were observed. Viral particles entered cells internalized in smooth monopinocytic vesicles, often in the proximity of larger, caveola-like invaginations. Both "empty" vesicles derived from caveolae and vesicles containing viral particles were stained with the anti-caveolin-1 antibody, and the two types of vesicles often fused in the cytoplasm. Colocalization of VP1 with caveolin-1 was observed during viral particle movement from the plasma membrane throughout the cytoplasm to the perinuclear area. Empty vesicles and vesicles with viral particles moved predominantly along microfilaments. Particle movement was accompanied by transient disorganization of actin stress fibers. Microfilaments decorated by the VP1 immunofluorescent signal could be seen as concentric curves, apparently along membrane structures that probably represent endoplasmic reticulum. Colocalization of VP1 with tubulin was mostly observed in areas close to the cell nuclei and on mitotic tubulin structures. By 3 h postinfection, a strong signal of the VP1 (but no viral particles) had accumulated in the proximity of nuclei, around the outer nuclear membrane. However, the vast majority of VP1 pseudocapsids did not enter the nuclei.

Caveolar endocytosis of simian virus 40 reveals a new two-step vesicular-transport pathway to the ER

Simian virus 40 (SV40) is unusual among animal viruses in that it enters cells through caveolae, and the internalized virus accumulates in a smooth endoplasmic reticulum (ER) compartment. Using video-enhanced, dual-colour, live fluorescence microscopy, we show the uptake of individual virus particles in CV-1 cells. After associating with caveolae, SV40 leaves the plasma membrane in small, caveolin-1-containing vesicles. It then enters larger, peripheral organelles with a non-acidic pH. Although rich in caveolin-1, these organelles do not contain markers for endosomes, lysosomes, ER or Golgi, nor do they acquire ligands of clathrin-coated vesicle endocytosis. After several hours in these organelles, SV40 is sorted into tubular, caveolin-free membrane vesicles that move rapidly along microtubules, and is deposited in perinuclear, syntaxin 17-positive, smooth ER organelles. The microtubule-disrupting agent nocodazole inhibits formation and transport of these tubular carriers, and blocks viral infection. Our results demonstrate the existence of a two-step transport pathway from plasma-membrane caveolae, through an intermediate organelle (termed the caveosome), to the ER. This pathway bypasses endosomes and the Golgi complex, and is part of the productive infectious route used by SV40.

Use of electron microscopic and immunogold labeling techniques to determine polyomavirus recombinant VP1 capsid-like particles entry into mouse 3T6 cell nucleus

Murine polyomavirus major structural protein VP1 could assemble into capsid-like particles when expressed in the baculovirus system. The recombinant capsid-like particles that were purified by CsCl density gradient centrifugation were capable of packaging host DNA. Electron microscopic and immunogold labeling techniques were used to study the entry of these VP1 recombinant capsid-like particles into mouse 3T6 cells. It was found that these VP1 recombinant capsid-like particles, which lack polyomavirus minor structural proteins (VP2 and VP3), use the same mechanism to enter mouse 3T6 cell cytoplasm and nucleus as that used by native polyomavirus virions.

Murine polyomavirus infection of 3T6 mouse cells shows evidence of predominant necrosis as well as limited apoptosis

The current study was developed to determine if polyomavirus infected 3T6 mouse cells evoked an apoptotic or a necrotic mechanism during infection. Infected cells were analyzed by flow cytometry, transmission electron microscopy (TEM), DNA electrophoresis and by measuring caspase-3 enzymatic activity. Infected cells that were analyzed at 72 h post-infection showed the following: flow cytometry analysis revealed a 5% increase in apoptotic cells and a 46% increase in necrotic cells when compared to uninfected cells; electron microscopy showed 10% cells with characteristic apoptotic morphology and 40% with necrotic appearance; caspase-3 activity was found to increase two fold when compared to uninfected cells and DNA fragmentation (laddering) was clearly evident late in infection. It was concluded that infected cells predominantly showed necrosis, although some cells showed apoptosis in late infection. Recombinant capsid-like particles composed of the polyomavirus structural proteins were not able to induce cell death.

Use of the confocal microscope to determine polyomavirus recombinant capsid-like particle entry into mouse 3T6 cells

The structural protein genes of polyomavirus were expressed in the baculovirus system, and the proteins were found to assemble into capsid-like particles capable of packaging insect cell DNA. Recombinant capsid-like particles could be produced that were composed of the various structural proteins (VP1, VP1/2, VP1/3 and VP1/2 + VP3). Laser scanning confocal microscopy was used to determine if the various capsid-like particles could infect (enter) mouse 3T6 cells. Each of the various capsid-like particles was equally capable of cell entry as determined by indirect immunofluorescence confocal microscopy.

An Update on Non-clathrin-coated Endocytosis

Recent evidence has proved that in addition to the well-documented clathrin-mediated endocytic route (vesicles of 100-150 nm), at least three distinct non-clathrin-coated endocytic pathways function at the surface of mammalian cells. Endocytosis via these pathways is initiated by caveolae (50-80 nm), macropinosomes (500-2000 nm) and micropinosomes (95-100 nm). The current state of knowledge about these non-clathrin coated endocytic routes is presented and evidence that endocytic routes other than via clathrin-coated vesicles are utilised by viruses is discussed. The recent advances in these areas have provided us with tools to investigate the entry of those viruses which appear to enter cells via endocytosis into non-clathrin-coated vesicles. Data indicate that these four endocytic pathways differ in the absence, presence and/or type of coat on the vesicles, the size of the vesicles, their sensitivity to a variety of inhibitors, and in the ligands endocytosed. A historical perspective of the discovery of these non-clathrin-coated endocytic pathways is provided and recent information is summarised and discussed. The entry of viruses via non-clathrin-coated pits is destined to be an exciting new area of viral-cell entry, as has been indicated recently by the finding that entry of simian virus type 40 into cells occurs via caveolae. Copyright 1997 by John Wiley & Sons, Ltd.

Mutations in the putative calcium-binding domain of polyomavirus VP1 affect capsid assembly

Calcium ions appear to play a major role in maintaining the structural integrity of the polyomavirus and are likely involved in the processes of viral uncoating and assembly. Previous studies demonstrated that a VP1 fragment extending from Pro-232 to Asp-364 has calcium-binding capabilities. This fragment contains an amino acid stretch from Asp-266 to Glu-277 which is quite similar in sequence to the amino acids that make up the calcium-binding EF hand structures found in many proteins. To assess the contribution of this domain to polyomavirus structural integrity, the effects of mutations in this region were examined by transfecting mutated viral DNA into susceptible cells. Immunofluorescence studies indicated that although viral protein synthesis occurred normally, infective viral progeny were not produced in cells transfected with polyomavirus genomes encoding either a VP1 molecule lacking amino acids Thr-262 through Gly-276 or a VP1 molecule containing a mutation of Asp-266 to Ala. VP1 molecules containing the deletion mutation were unable to bind 45Ca in an in vitro assay. Upon expression in Escherichia coli and purification by immunoaffinity chromatography, wild-type VP1 was isolated as pentameric, capsomere-like structures which could be induced to form capsid-like structures upon addition of CaCl2, consistent with previous studies. However, although VP1 containing the point mutation was isolated as pentamers which were indistinguishable from wild-type VP1 pentamers, addition of CaCl2 did not result in their assembly into capsid-like structures. Immunogold labeling and electron microscopy studies of transfected mammalian cells provided in vivo evidence that a mutation in this region affects the process of viral assembly.

Myristylated polyomavirus VP2: role in the life cycle of the virus

The double-stranded genome of the small DNA tumor virus, polyomavirus, is enclosed in a capsid composed of a major protein, VP1, which associates as pentameric capsomeres into an icosahedral structure, and two minor proteins, VP2 and VP3, whose functions and positions within the structure are unknown. The N-terminal glycine of the VP2 coat protein has been shown to be cotranslationally acylated with myristic acid. To study the function of this modification and the role of VP2 in the life cycle of polyomavirus, the N-terminal glycine, critical to the myristylation consensus sequence, has been altered to a glutamic acid or a valine residue by site-directed oligonucleotide mutagenesis. The glycine----glutamic acid mutant DNA has been further studied. When transfected into cells permissive for the polyomavirus full lytic life cycle, this mutant DNA replicated at levels comparable to those of wild-type viral DNA, and small amounts of nonrevertant (mutant) virus could be harvested from the cultures. The virus particles viewed by electron microscopy appeared slightly distorted, but the ratio of full to empty particles was similar to that produced in a wild-type viral infection. Mutant virus was capable of reinfecting permissive cells but with a considerably reduced efficiency.

Endocytosis of simian virus 40 into the endoplasmic reticulum

The endocytosis of SV-40 into CV-1 cells we studied using biochemical and ultrastructural techniques. The half-time of binding of [35S]methionine-radiolabeled SV-40 to CV-1 cells was 25 min. Most of the incoming virus particles remained undegraded for several hours. Electron microscopy showed that some virus entered the endosomal/lysosomal pathway via coated vesicles, while the majority were endocytosed via small uncoated vesicles. After infection at high multiplicity, one third of total cell-associated virus was observed to enter the ER, starting 1-2 h after virus application. The viruses were present in large, tubular, smooth membrane networks generated as extentions of the ER. The results describe a novel and unique membrane transport pathway that allows endocytosed viral particles to be targeted from the plasma membrane to the ER.

Synthesis, posttranslational modifications, and nuclear transport of polyomavirus major capsid protein VP1

Polyomavirus major capsid protein VP1 synthesis was studied in infected primary baby mouse kidney cells. A standard curve of VP1 protein was used to quantitate VP1 in the cytoplasm and nucleus of infected cells during the time course of infection. Polyomavirus VP1 continued to be accumulated in the cytoplasm of the cells until 27 h postinfection, at which time the synthesis of VP1 leveled off. VP1 continued to accumulate in the nucleus of the infected cells throughout the course of infection. The presence of the six isospecies, A to F, of polyomavirus VP1 was also studied to determine the relative quantity of each species during the time course of infection. All six species were found in the cytoplasm and nucleus of infected cells at various times postinfection. However, the relative quantity of each species was different at early as compared with later times of infection. In addition, phosphorylated VP1 was found in isolated polyribosomes of infected cells, suggesting that phosphorylation of VP1 is a cotranslational modification. Examination of the effect of macromolecular synthesis on the transport of VP1 into the nucleus of infected baby mouse kidney cells as well as the rate of its nuclear accumulation during and after protein synthesis inhibition revealed that the continual transport and accumulation of VP1 in the nucleus required protein synthesis.

Modification of membrane permeability by animal viruses

Animal viruses modify membrane permeability during lytic infection. There is a co-entry of macromolecules and virion particules during virus penetration and a drastic change in transport and membrane permeability at the late stages of the lytic cycle. Both events are of importance to understand different molecular aspects of viral infection, as virus entry into the cell and the interference of virus infection with cellular metabolism. Other methods of cell permeabilization of potential relevance to understand the mechanism of viral damage of the membrane are also discussed.

Early events in polyomavirus infection: fusion of monopinocytotic vesicles containing virions with mouse kidney cell nuclei

The entry of polyomavirus enclosed in monopinocytotic vesicles into mouse kidney cell nuclei was studied and evidence for a fusion mechanism was obtained. In vivo studies using the fluorescent lipophilic dye diI-C16(3) as a plasma membrane label showed that polyomavirus-infected nuclei accumulate plasma membrane, while uninfected or polyoma capsid-infected nuclei do not. Further evidence for fusion was obtained with electron microscopy of thin sections of infected mouse kidney cells. These specimens showed accumulation of plasma membrane in the outer nuclear membrane as well as evidence of recent fusion events. The polyoma virions (capsid proteins) were seen to accumulate on the inner nuclear membrane and in the nucleus and were identified by immunogold staining of the thin sections. The combined results of the in vivo dye studies and thin section immunoelectron microscopy studies provide evidence for a fusion mechanism for polyomavirus entry into mouse kidney cell nuclei.

Localization of calcium on the polyomavirus VP1 capsid protein

Our laboratory has previously shown that the divalent cation Ca2+ is an integral part of the polyomavirus and plays a major role in stabilizing the intact virion structure. In this report, we show that calcium is sequestered on the major capsid protein VP1 of polyomavirus. The virion structural proteins were separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis before being transferred to nitrocellulose and probed with 45CaCl2. Autoradiography revealed 45Ca binding exclusively to VP1. Increasing the amount of VP1 transferred to the nitrocellulose resulted in a concomitant increase in 45Ca binding. 45Ca binding to VP1 could be reduced by competition with an excess of unlabeled CaCl2. Separation of the species of VP1 by two-dimensional gel electrophoresis before electroblotting and probing with 45CaCl2 revealed that all six species (A to F) bind the radiolabeled calcium. Formic acid cleavage of the 43-kilodalton (kDa) VP1 protein into 29-, 18-, and 16-kDa fragments before 45Ca-binding analysis revealed that only the 18- and 16-kDa carboxyl-terminal fragments of this protein bind 45Ca.

Effects of inhibitors of the cytoplasmic structures and functions on the early phase of infection of cultured cells with simian virus 40

To obtain information about cytoplasmic structures and functions involving the entry of simian virus 40 virions into cells, we examined whether the inhibitors that affect the functions and/or structure of lysosomes, cell membrane, and cytoskeletons inhibit expression of nuclear T antigen in the SV40-inoculated rat 3Y1 and monkey CV-1 cells. Chloroquine, methylamine, and butylamine did not inhibit T-antigen expression, suggesting that lysosomal acidification is not required for establishment of infection. Cytochalasin B had no effect, suggesting that microfilaments are not involved. Monensin, colcemid, and amantadine each inhibited T-antigen expression at doses causing no obvious cytotoxicity. Maximal inhibition was seen when these inhibitors were added to the cultures within 1 hr (monensin), within 4 hr (colcemid), or within 12 hr (amantadine) after virion adsorption to the cell surface. When the inhibitor was present in the virus-inoculated cultures for 24 hr and then removed, nuclear T antigen began to be expressed at 4 hr (monensin), 9 hr (colcemid), or 1 hr (amantadine) after removal of the inhibitors. Results of SDS-PAGE analysis of immunoprecipitated radiolabeled proteins of infected cells revealed that amantadine inhibited synthesis of large and small T antigens as well as general protein synthesis. Inhibition by colcemid may be due to disruption of microtubules, because other microtubule-disrupting agents (colchicine, vinblastine, nocodazole, and podophyllotoxin) also inhibited appearance of nuclear T antigen but lumicolchicine and taxol did not. Electron microscopy revealed that, in the presence of colcemid, although the adsorbed virions were readily internalized to form pinosomes, vectorial movement of the pinosomes to the nucleus appeared to be inhibited. Results of electron microscopy also suggest that inhibition by monensin may occur mainly in internalization of adsorbed virions and that the inhibition is leaky such that the early steps of infection proceed slowly in the presence of monensin. We conclude that monensin, colcemid, and amantadine interfere with mutually different early events of SV40 infection.

Differences in biological activity and structural protein VP1 phosphorylation of polyomavirus progeny resulting from infection of primary mouse kidney and primary mouse embryo cell cultures

Both primary mouse kidney and primary mouse embryo cells in culture were used for polyomavirus progeny production. Examination of polyomavirus virion structural integrity revealed that mouse embryo cell progeny contained a threefold greater population of unstable particles when compared with mouse kidney cell progeny. Differences in biological activity between these two progeny virion types were also shown. Mouse kidney cell progeny compared with mouse embryo cell progeny exhibited a 10-fold greater ability to agglutinate guinea pig erythrocytes, a 3-fold lower ability to become internalized into monopinocytotic vesicles, and a 2-fold lower ability to initiate a productive infection based on positive nuclear immunofluorescence when mouse embryo host cell cultures were used. The mouse kidney progeny were also found to bind to host cells less specifically than the mouse embryo cell progeny. When these two progeny virion types were labeled in vivo with 32P and subjected to isoelectric focusing followed by sodium dodecyl sulfate-polyacrylamide gel electrophroesis in the second dimension, differences in the phosphorylation pattern of the major virus-encoded structural protein VP1 species were observed. It was revealed that species D and E of mouse kidney cell progeny were phosphorylated to the same degree, while mouse embryo cell progeny species E and F were phosphorylated equally. These data suggest that the host cells play a role in modulating the biological activity of the virus by affecting the degree and site-specific phosphorylation of the major capsid protein VP1 which may influence the recognition of virus attachment proteins for specific cellular receptors.

Octyl-beta-D-glucopyranoside extracts polyomavirus receptor moieties from the surfaces of mouse kidney cells

Polyomavirus receptor moieties were extracted from the surfaces of mouse kidney cells with the nonionic detergent octyl-beta-D-glucopyranoside. Following extraction with this detergent, mouse kidney cells were refractory to polyomavirus infection. Binding studies demonstrated that this loss of susceptibility resulted from extraction of a peripheral membrane protein or proteins required for proper virus attachment to and infection of mouse kidney cells. Infection of extracted mouse kidney cells returned following a 2-h recovery period. However, the presence of cycloheximide or tunicamycin in the recovery media interfered with recovery from infection. Cells could be infected immediately after extraction by supplying them with the extracted moieties prior to or concomitant with infection. A complex of polyomavirus and the extracted receptor protein was formed by in vitro incubation and was stable in sucrose gradient analysis. Functional receptor moieties were prepared in the form of liposomes from the detergent extract. The virus-receptor complex was immunoprecipitated with anti-polyomavirus immunoglobulin G, and the portion of the complex contributed by the cell was identified. Immunoblot analysis of the mouse kidney cell detergent extract with a receptor-specific 125I-labeled anti-idiotypic antibody or 125I-labeled polyomavirus demonstrated several reactive proteins. Attachment of polyomavirus to mouse kidney cells, followed by extraction of the virus-receptor complex, identified polyomavirus-binding proteins similar to those observed in in vitro binding. Proteins with molecular weights of approximately 95,000, 50,000 and 25,000 to 30,000 were consistently observed in all receptor assays. The relationship between these proteins and their possible involvement as the cell receptor for polyomavirus are discussed.

Cross-linking of a polyomavirus attachment protein to its mouse kidney cell receptor

We used photoaffinity cross-linking with the heterobifunctional cross-linker N-hydroxysuccinimidyl 4-azidobenzoate (HSAB) to covalently link polyomavirus to a mouse kidney cell surface component. The virus-HSAB combination was adsorbed to the cells and then cross-linked and isolated in monopinocytotic vesicles from the cells after endocytosis. The cross-linked product was identified on sodium dodecyl sulfate-polyacrylamide gels by the presence of a new band carrying 125I-labeled virion protein with a higher molecular mass than the normal virion protein bands. A single new band, with an apparent molecular mass of 120 kilodaltons (120 kDa), was identified by this procedure. This band was formed only in the presence of the HSAB cross-linker when virions were bound to the cells. The band also copurified with cross-linked virions when virion-containing vesicles were treated with detergent to remove the cell membrane. Antibody treatments that blocked up to 100% of virus binding and internalization also blocked cross-linking, as measured by the formation of the 120-kDa band. The 120-kDa band was characterized by preparation of antibody against the excised band from the gel. This antibody was shown to have the expected dual specificity for polyomavirus VP1 sequences and plasma membrane proteins, as analyzed on Western blots. The anti-120-kDa antibody was also shown by immunofluorescence to bind to the surface of mouse kidney cells. These data have demonstrated that molecules of possible biological significance in the binding of polyomavirus to mouse kidney cells have been cross-linked and that cell surface molecules have been identified that may be characterized further for possible receptor function in polyomavirus attachment.

Production and characterization of monoclonal antibodies to polyomavirus major capsid protein VP1

Four hybridoma cell lines producing monoclonal antibodies against intact polyoma virions were produced and characterized. These antibodies were selected for their ability to react with polyoma virions in an enzyme-linked immunosorbent assay. The antibodies immunoprecipitated polyoma virions and specifically recognized the major capsid protein VP1 on an immunoblot. Distinct VP1 isoelectric species were immunoprecipitated from dissociated virion capsomere preparations. Two-dimensional gel electrophoresis demonstrated antibody reactivity with specific VP1 species. Monoclonal antibodies E7 and G9 recognized capsomeres containing VP1 species D, E, and F, while monoclonal antibodies C10 and D3 recognized capsomeres containing species B and C. Two of the monoclonal antibodies, E7 and G9, were capable of neutralizing viral infection and inhibiting hemagglutination. The biological activity of the monoclonal antibodies correlated well with the biological function of the species with which they reacted.

Infection of B lymphocytes by a human herpesvirus, Epstein-Barr virus, is blocked by calmodulin antagonists

Epstein-Barr virus (EBV) is a human herpesvirus that selectively binds to and infects human B lymphocytes (B cells). In the studies presented here, we found that several phenothiazines, including trifluoperazine, chlorpromazine, prochlorpromazine, and promethazine, blocked EBV infectivity of isolated adult human B cells as measured either by outgrowth of transformed cell colonies or by [3H]thymidine incorporation. Trifluoperazine, chlorpromazine, and prochlorpromazine were equally effective with 20 microM fully inhibiting infectivity, whereas 100 microM promethazine was required for a comparable effect. Inhibition by trifluoperazine was partially reversible. Studies with radiolabeled EBV demonstrated that the inhibitors did not impair virus binding to B cells. Electron microscopic examination of B lymphocytes revealed that trifluoperazine reduced the number of large uncoated cell vacuoles and the number of membrane microvilli, indicating that this agent interfered with cell pinocytosis. This process was accompanied by inhibition of EBV endocytosis into B cells. Phenothiazines bind to and inhibit calmodulin, an intracellular calcium-binding protein that regulates several key enzymes, some of which directly affect cytoskeletal elements, although they also may interact nonspecifically with other cellular constituents. In this regard, haloperidol, a non-phenothiazine calmodulin antagonist, and R24571, a derivative of the antimycotic miconazole, which is a potent and highly specific calmodulin inhibitor, also blocked EBV infection. These studies suggest that calmodulin or a calmodulin-regulated cellular enzyme(s) is involved in normal cellular endocytic processes in B lymphocytes and thereby in the early stages of EBV infection.