The G-patch domain of Mason-Pfizer monkey virus is a part of reverse transcriptase.

Mason-Pfizer monkey virus (M-PMV), like some other betaretroviruses, encodes a G-patch domain (GPD). This glycine-rich domain, which has been predicted to be an RNA binding module, is invariably localized at the 3' end of the pro gene upstream of the pro-pol ribosomal frameshift sequence of genomic RNAs of betaretroviruses. Following two ribosomal frameshift events and the translation of viral mRNA, the GPD is present in both Gag-Pro and Gag-Pro-Pol polyproteins. During the maturation of the Gag-Pro polyprotein, the GPD transiently remains a C-terminal part of the protease (PR), from which it is then detached by PR itself. The destiny of the Gag-Pro-Pol-encoded GPD remains to be determined. The function of the GPD in the retroviral life cycle is unknown. To elucidate the role of the GPD in the M-PMV replication cycle, alanine-scanning mutational analysis of its most highly conserved residues was performed. A series of individual mutations as well as the deletion of the entire GPD had no effect on M-PMV assembly, polyprotein processing, and RNA incorporation. However, a reduction of the reverse transcriptase (RT) activity, resulting in a drop in M-PMV infectivity, was determined for all GPD mutants. Immunoprecipitation experiments suggested that the GPD is a part of RT and participates in its function. These data indicate that the M-PMV GPD functions as a part of reverse transcriptase rather than protease.

In vitro assembly of virus-like particles of a gammaretrovirus, the murine leukemia virus XMRV.

Immature retroviral particles are assembled by self-association of the structural polyprotein precursor Gag. During maturation the Gag polyprotein is proteolytically cleaved, yielding mature structural proteins, matrix (MA), capsid (CA), and nucleocapsid (NC), that reassemble into a mature viral particle. Proteolytic cleavage causes the N terminus of CA to fold back to form a ß-hairpin, anchored by an internal salt bridge between the N-terminal proline and the inner aspartate. Using an in vitro assembly system of capsid-nucleocapsid protein (CANC), we studied the formation of virus-like particles (VLP) of a gammaretrovirus, the xenotropic murine leukemia virus (MLV)-related virus (XMRV). We show here that, unlike other retroviruses, XMRV CA and CANC do not assemble tubular particles characteristic of mature assembly. The prevention of ß-hairpin formation by the deletion of either the N-terminal proline or 10 initial amino acids enabled the assembly of ΔProCANC or Δ10CANC into immature-like spherical particles. Detailed three-dimensional (3D) structural analysis of these particles revealed that below a disordered N-terminal CA layer, the C terminus of CA assembles a typical immature lattice, which is linked by rod-like densities with the RNP.

Panurgines, novel antimicrobial peptides from the venom of communal bee Panurgus calcaratus (Hymenoptera: Andrenidae).

Three novel antimicrobial peptides (AMPs), named panurgines (PNGs), were isolated from the venom of the wild bee Panurgus calcaratus. The dodecapeptide of the sequence LNWGAILKHIIK-NH2 (PNG-1) belongs to the category of α-helical amphipathic AMPs. The other two cyclic peptides containing 25 amino acid residues and two intramolecular disulfide bridges of the pattern Cys8-Cys23 and Cys11-Cys19 have almost identical sequence established as LDVKKIICVACKIXPNPACKKICPK-OH (X=K, PNG-K and X=R, PNG-R). All three peptides exhibited antimicrobial activity against Gram-positive bacteria and Gram-negative bacteria, antifungal activity, and low hemolytic activity against human erythrocytes. We prepared a series of PNG-1 analogs to study the effects of cationicity, amphipathicity, and hydrophobicity on the biological activity. Several of them exhibited improved antimicrobial potency, particularly those with increased net positive charge. The linear analogs of PNG-K and PNG-R having all Cys residues substituted by α-amino butyric acid were inactive, thus indicating the importance of disulfide bridges for the antimicrobial activity. However, the linear PNG-K with all four cysteine residues unpaired, exhibited antimicrobial activity. PNG-1 and its analogs induced a significant leakage of fluorescent dye entrapped in bacterial membrane-mimicking large unilamellar vesicles as well as in vesicles mimicking eukaryotic cell membrane. On the other hand, PNG-K and PNG-R exhibited dye-leakage activity only from vesicles mimicking bacterial cell membrane.

Lasiocepsin, a novel cyclic antimicrobial peptide from the venom of eusocial bee Lasioglossum laticeps (Hymenoptera: Halictidae).

In the venom of eusocial bee Lasioglossum laticeps, we identified a novel unique antimicrobial peptide named lasiocepsin consisting of 27 amino acid residues and two disulfide bridges. After identifying its primary structure, we synthesized lasiocepsin by solid-phase peptide synthesis using two different approaches for oxidative folding. The oxidative folding of fully deprotected linear peptide resulted in a mixture of three products differing in the pattern of disulfide bridges. Regioselective disulfide bond formation significantly improved the yield of desired product. The synthetic lasiocepsin possessed antimicrobial activity against both Gram-positive and -negative bacteria, antifungal activity against Candida albicans, and no hemolytic activity against human erythrocytes. We synthesized two lasiocepsin analogs cyclized through one native disulfide bridge in different positions and having the remaining two cysteines substituted by alanines. The analog cyclized through a Cys8-Cys25 disulfide bridge showed reduced antimicrobial activity compared to the native peptide while the second one (Cys17-Cys27) was almost inactive. Linear lasiocepsin having all four cysteine residues substituted by alanines or alkylated was also inactive. That was in contrast to the linear lasiocepsin with all four cysteine residues non-paired, which exhibited remarkable antimicrobial activity. The shortening of lasiocepsin by several amino acid residues either from the N- or C-terminal resulted in significant loss of antimicrobial activity. Study of Bacillus subtilis cells treated by lasiocepsin using transmission electron microscopy showed leakage of bacterial content mainly from the holes localized at the ends of the bacterial cells.

Lucifensin, a novel insect defensin of medicinal maggots: synthesis and structural study.

Recently, we identified a new insect defensin, named lucifensin that is secreted/excreted by the blowfly Lucilia sericata larvae into a wound as a disinfectant during the medicinal process known as maggot therapy. Here, we report the total chemical synthesis of this peptide of 40 amino acid residues and three intramolecular disulfide bridges by using three different protocols. Oxidative folding of linear peptide yielded a peptide with a pattern of disulfide bridges identical to that of native lucifensin. The synthetic lucifensin was active against Gram-positive bacteria and was not hemolytic. We synthesized three lucifensin analogues that are cyclized through one native disulfide bridge in different positions and having the remaining four cysteines substituted by alanine. Only the analogue cyclized through a Cys16-Cys36 disulfide bridge showed weak antimicrobial activity. Truncating lucifensin at the N-terminal by ten amino acid residues resulted in a drop in antimicrobial activity. Linear lucifensin having all six cysteine residues alkylated was inactive. Circular dichroism spectra measured in the presence of α-helix-promoting compounds showed different patterns for lucifensin and its analogues. Transmission electron microscopy revealed that Bacillus subtilis treatment with lucifensin induced significant changes in its envelope.

The noncanonical Gag domains p8 and n are critical for assembly and release of mouse mammary tumor virus.

The mouse mammary tumor virus (MMTV) Gag contains the unique domains pp21, p3, p8, and n. We investigated the contribution of these domains to particle assembly and found that the region spanning the p8 and n domains is critical for shape determination and assembly. Deletion of pp21 and p3 reduced the number of released particles, but deletion of the n domain resulted in frequent formation of aberrant particles, while deletion of p8 severely impaired assembly. Further investigation of p8 revealed that both the basic and the proline-rich motifs within p8 contribute to MMTV assembly.

Effect of dimerizing domains and basic residues on in vitro and in vivo assembly of Mason-Pfizer monkey virus and human immunodeficiency virus.

Assembly of immature retroviral particles is a complex process involving interactions of several specific domains of the Gag polyprotein localized mainly within capsid protein (CA), spacer peptide (SP), and nucleocapsid protein (NC). In the present work we focus on the contribution of NC to the oligomerization of CA leading to assembly of Mason-Pfizer monkey virus (M-PMV) and HIV-1. Analyzing in vitro assembly of substitution and deletion mutants of DeltaProCANC, we identified a "spacer-like" sequence (NC(15)) at the M-PMV NC N terminus. This NC(15) domain is indispensable for the assembly and cannot be replaced with oligomerization domains of GCN4 or CREB proteins. Although the M-PMV NC(15) occupies a position analogous to that of the HIV-1 spacer peptide, it could not be replaced by the latter one. To induce the assembly, both M-PMV NC(15) and HIV-1 SP1 must be followed by a short peptide that is rich in basic residues. This region either can be specific, i.e., derived from the downstream NC sequence, or can be a nonspecific positively charged peptide. However, it cannot be replaced by heterologous interaction domains either from GCN4 or from CREB. In summary, we report here a novel M-PMV spacer-like domain that is functionally similar to other retroviral spacer peptides and contributes to the assembly of immature-virus-like particles.

HMGB1 interacts with human topoisomerase IIalpha and stimulates its catalytic activity.

DNA topoisomerase IIalpha (topo IIalpha) is an essential nuclear enzyme and its unique decatenation activity has been implicated in many aspects of chromosome dynamics such as chromosome replication and segregation during mitosis. Here we show that chromatin-associated protein HMGB1 (a member of the large family of HMG-box proteins with possible functions in DNA replication, transcription, recombination and DNA repair) promotes topo IIalpha-mediated catenation of circular DNA, relaxation of negatively supercoiled DNA and decatenation of kinetoplast DNA. HMGB1 interacts with topo IIalpha and this interaction, like the stimulation of the catalytic activity of the enzyme, requires both HMG-box domains of HMGB1. A mutant of HMGB1, which cannot change DNA topology stimulates DNA decatenation by topo IIalpha indistinguishably from the wild-type protein. Although HMGB1 stimulates ATP hydrolysis by topo IIalpha, the DNA cleavage is much more enhanced. The observed abilities of HMGB1 to interact with topo IIalpha and promote topo IIalpha binding to DNA suggest a mechanism by which HMGB1 stimulates the catalytic activity of the enzyme via enhancement of DNA cleavage.

Characterization of four clones derived from human adenocarcinoma cell line, HT29, and analysis of their response to sodium butyrate.

The differentiation of colorectal cancer cells is associated with the arrest of tumor growth and tumor regression. However, the mechanism of such tumor cell differentiation has not yet been elucidated. Several adenocarcinoma cell lines, including HT29 which differentiates only upon stimulation with a differentiation agent, have been used for the study of colorectal cells. Since we had previously obtained variable results during analyses of these cells, we selected several clones of this cell line. In this study, four clones of the parental HT29 cells, H8, G9, G10 and A3, were characterized. All of them differentiated upon treatment with sodium butyrate as the differentiation agent but they appeared different in their response regarding some of the markers of differentiation. As revealed by ultrastructural analysis, H8 and G10 clones formed numerous intercellular cysts with microvilli whereas these structures were found only occasionally in G9 and A3 clones. An elevated level of the indicator of cell differentiation, CEACAM 1, was found in H8 and G10 clones and the activity of alkaline phosphatase, another important marker of colorectal cell differentiation, was up-regulated and highly increased upon butyrate treatment in the H8 clone. Phosphorylation of p38 MAPK was increased in H8 and A3 butyrate-treated clones. According to the levels of cleaved PARP and activated caspase-3, the apoptotic response to butyrate appeared similar in all four clones, while electronoptic analysis revealed that clones G9 and A3 were more perceptive to butyrate-induced apoptosis. In conclusion, our data showed considerable heterogeneity in morphology and some enzymatic activity of the cloned cells. This fact may contribute to the evidence that many HT29 cells possess multipotent information similar to that of stem cells of the normal intestinal crypt.

Response of HT115, a highly invasive human colorectal adenocarcinoma cell line, to sodium butyrate treatment and glucose deprivation.

Sodium butyrate or glucose deprivation induce a more differentiated phenotype in many cancer cells. The aim of this study was to determine whether the induction effect of butyrate and/or glucose deprivation is dependent, in some way, on the differentiation state of individual cell lines. Sodium butyrate enhanced alkaline phosphatase activity and induced formation of an ultrastructurally more differentiated phenotype in both HT29 and HT115 cell lines. Interestingly, the more invasive HT115 cells responded more strongly to butyrate treatment. On the other hand, the differentiation effect of glucose deprivation was much less prominent in the HT115 cell line in comparison with HT29 cells. Our data confirm the influence of the malignant potential of the cells on their response to treatment with differentiation and apoptosis-inducing agents. Butyrate treatment also enhanced the adhesiveness of HT115 cells. Since E-cadherin was not found in these cells, while the level of CEACAM1 was increased, it is obvious that the CEACAM1 molecules are involved in HT115 cell-cell adhesion.

Alterations in mitochondria function and morphology in HT29 cells upon conditions inducing differentiation and apoptosis.

Morphological and biochemical studies of HT29 cells treated with sodium butyrate and/or glucose-deprived revealed both apoptotic and differentiation response. The main apoptotic response was accompanied with an increase of floating cells. However, the ultrastructural analysis of adherent cells showed the typical apoptotic character of the nucleus in some of them. In addition, remarkable changes of mitochondria, assumed as early stages starting the apoptotic cascade, were observed. These changes were represented not only by alterations of mitochondrial morphology, but also by the number of mitochondria and their localization.

Delivery of recombinant adeno-associated virus by jet injection.

The jet-injection technology was used for delivery of recombinant adeno-associated virus (rAAV). Although AAV-based vectors are an attractive tool in gene therapy, some methodological and technical problems of their targeted delivery remain to be solved. We tried to address some of these cell-targeting problems by using a new low-volume needleless injection device the Swiss Injector. First we tested, by electron microscopy, whether jet-injection would have any detrimental effect on rAAV particle integrity. Second, we compared transgene expression after infection of 293T cells with fired or control (non-fired) rAAV that expressed the green fluorescent protein (GFP), beta-galactosidase (beta-gal), the B7.1 molecule, and interleukin 2 (IL2). Third, an rAAV carrying the genes coding for beta-gal was jet-injected into mouse subcutaneous (s.c.) tumours. The staining of tumour cryosections revealed beta-gal expression 72 h after the delivery. Our study demonstrated the applicability of the Swiss Injector for the delivery of rAAV into tumour tissue without either vector particle integrity or the level of expression of the transgenes, as tested in vitro, being affected. The jet-injection technology could improve the distribution of vector particles in the tumour mass without leakage of liquid from the injection site.

Response of HT29 cells to butyrate depends on time of exposure and glucose deprivation.

The high level of alkaline phosphatase activity in HT29 cells induced after 2 or 5 days of butyrate treatment was decreased during their prolonged exposure (about 30 days) to this agent together with a decrease of sensitivity to apoptosis. However, an enormous additive effect on alkaline phosphatase activity was found after butyrate treatment of glucose-starved cells. In concert with this finding, the substructural analysis revealed a dense brush border, tendency to polarization and morphologically normal mitochondria. It can be concluded that prolonged butyrate treatment of HT29 cells attenuated their response to this agent. On the other hand, glucose deprivation, as another inductor of differentiation, was found to increase the sensitivity of HT29 cells to butyrate.

Overexpression of v-myb oncogene or c-myb proto-oncogene in insect cells: characterization of newly induced nucleolus-like structures accumulating Myb protein.

The oncoprotein v-Myb induces myeloid leukemia and its cellular counterpart c-Myb is involved in the regulation of hematopoiesis. Although intensively studied, their precise subcellular localization is not known. In order to expand our knowledge in this respect, we used an artificial system overexpressing these proteins. We investigated the subcellular localization of Myb proteins in cultured non-synchronized insect cells transfected with recombinant baculoviruses overexpressing either v-myb oncogene or c-myb proto-oncogene. The cell expressing Myb proteins underwent extensive nuclear changes and exhibited distinct nuclear structures resembling nucleoli. The bulk of v-Myb and c-Myb proteins accumulated in such nucleolus-like structures which, according to the nucleolar nomenclature, we classified to three types: compact of enlarged size (type I), large ring-shaped (type II) and with nucleolonemas (type III). We investigated these structures for the presence of important nucleolar macromolecules in order to establish whether they were compatible with the function in the production of ribosomes. Strikingly, our results indicated that the different forms of these structures did not represent genuine nucleoli. They rather reflected progressive changes, induced by the virus infection and high expression of v-myb genes, accompanied by the formation of these prominent nucleolus-like structures highly enriched in Myb protein. Gradual changes in number of individual nucleolus-like forms during infection, increasing amount of Myb protein and DNA localized in them together with decreasing amount of RNA and their different interaction with viral particles indicate that the nucleolus-like structure of type I is a precursor of the type II and finally of the type III.

Minor capsid proteins of mouse polyomavirus are inducers of apoptosis when produced individually but are only moderate contributors to cell death during the late phase of viral infection.

Minor structural proteins of mouse polyomavirus (MPyV) are essential for virus infection. To study their properties and possible contributions to cell death induction, fusion variants of these proteins, created by linking enhanced green fluorescent protein (EGFP) to their C- or N-termini, were prepared and tested in the absence of other MPyV gene products, namely the tumor antigens and the major capsid protein, VP1. The minor proteins linked to EGFP at their C-terminus (VP2-EGFP, VP3-EGFP) were found to display properties similar to their nonfused, wild-type versions: they killed mouse 3T3 cells quickly when expressed individually. Carrying nuclear localization signals at their common C-terminus, the minor capsid proteins were detected in the nucleus. However, a substantial subpopulation of both VP2 and VP3 proteins, as well as of the fusion proteins VP2-EGFP and VP3-EGFP, was detected in the cytoplasm, co-localizing with intracellular membranes. Truncated VP3 protein, composed of 103 C-terminal amino acids, exhibited reduced affinity for intracellular membranes and cytotoxicity. Biochemical studies proved each of the minor proteins to be a very potent inducer of apoptosis, which was dependent on caspase activation. Immuno-electron microscopy showed the minor proteins to be associated with damaged membranes of the endoplasmic reticulum, nuclear envelope and mitochondria as soon as 5 h post-transfection. Analysis of apoptotic markers and cell death kinetics in cells transfected with the wild-type MPyV genome and the genome mutated in both VP2 and VP3 translation start codons revealed that the minor proteins contribute moderately to apoptotic processes in the late phase of infection and both are dispensable for cell destruction at the end of the virus replication cycle.

Premature processing of mouse mammary tumor virus Gag polyprotein impairs intracellular capsid assembly.

Mouse mammary tumor virus (MMTV) is the prototypical member of the Betaretrovirus genus, but the processes of its morphogenesis are poorly characterized. In this report, we describe an unusual intracellular processing of MMTV Gag polyprotein in human 293T cells transiently expressing MMTV from heterologous promoter. The same specific cleavage products of the viral protease were seen for the wild type as well as for nonmyristylated mutant of MMTV Gag polyprotein completely defective in the particle release. Inactivation of the viral protease resulted in more stable Gag polyprotein and in accumulation of intracytoplasmic particles for nonmyristylated Gag. The intracellular processing of nonmyristylated MMTV Gag indicates that protease activation in betaretrovirus can occur independently of budding.

The effect of point mutations within the N-terminal domain of Mason-Pfizer monkey virus capsid protein on virus core assembly and infectivity.

Retroviral capsid protein (CA) mediates protein interactions driving the assembly of both immature viral particles and the core of the mature virions. Structurally conserved N-terminal domains of several retroviruses refold after proteolytic cleavage into a beta-hairpin, stabilized by a salt bridge between conserved N-terminal Pro and Asp residues. Based on comparison with other retroviral CA, we identified Asp50 and Asp57 as putative interacting partners for Pro1 in Mason-Pfizer monkey virus (M-PMV) CA. To investigate the importance of CA Pro1 and its interacting Asp in M-PMV core assembly and infectivity, P1A, P1Y, D50A, T54A and D57A mutations were introduced into M-PMV. The P1A and D57A mutations partially blocked Gag processing and the released viral particles exhibited aberrant cores and were non-infectious. These data indicate that the region spanning residues Asp50-Asp57 plays an important role in stabilization of the beta-hairpin and that Asp57 likely forms a salt-bridge with P1 in M-PMV CA.

Assemblages of simian virus 40 capsid proteins and viral DNA visualized by electron microscopy.

SV40 assembles in the nucleus by addition of capsid proteins to the minichromosome. The VP15VP2/3 capsomer is composed of a pentamer of the major protein VP1 complexed with a monomer of a minor protein, VP2 or VP3. In the capsid, the capsomers are bound together via their flexible carboxy-terminal arms. Our previous studies suggested that the capsomers are recruited to the packaging signal ses via avid interaction with Sp1. During assembly Sp1 is displaced, allowing chromatin compaction. Here we investigated the interactions in vitro of VP1(5)VP2/3 capsomers with the entire SV40 genome, using mutant VP1 deleted in the carboxy-arm that cannot assemble, but retains DNA-binding capacity. EM revealed that VP1(5)VP2/3 complexes bind non-specifically at random locations around the DNA. Sp1 was absent from mature virions. The findings suggest that multiple capsomers attach simultaneously to the viral genome, increasing their local concentration, facilitating rapid, concerted assembly reaction and removal of Sp1.

Mouse polyomavirus enters early endosomes, requires their acidic pH for productive infection, and meets transferrin cargo in Rab11-positive endosomes.

Mouse polyomavirus (PyV) virions enter cells by internalization into smooth monopinocytic vesicles, which fuse under the cell membrane with larger endosomes. Caveolin-1 was detected on monopinocytic vesicles carrying PyV particles in mouse fibroblasts and epithelial cells (33). Here, we show that PyV can be efficiently internalized by Jurkat cells, which do not express caveolin-1 and lack caveolae, and that overexpression of a caveolin-1 dominant-negative mutant in mouse epithelial cells does not prevent their productive infection. Strong colocalization of VP1 with early endosome antigen 1 (EEA1) and of EEA1 with caveolin-1 in mouse fibroblasts and epithelial cells suggests that the monopinocytic vesicles carrying the virus (and vesicles containing caveolin-1) fuse with EEA1-positive early endosomes. In contrast to SV40, PyV infection is dependent on the acidic pH of endosomes. Bafilomycin A1 abolished PyV infection, and an increase in endosomal pH by NH4Cl markedly reduced its efficiency when drugs were applied during virion transport towards the cell nucleus. The block of acidification resulted in the retention of a fraction of virions in early endosomes. To monitor further trafficking of PyV, we used fluorescent resonance energy transfer (FRET) to determine mutual localization of PyV VP1 with transferrin and Rab11 GTPase at a 2- to 10-nm resolution. Positive FRET between PyV VP1 and transferrin cargo and between PyV VP1 and Rab11 suggests that during later times postinfection (1.5 to 3 h), the virus meets up with transferrin in the Rab11-positive recycling endosome. These results point to a convergence of the virus and the cargo internalized by different pathways in common transitional compartments.

Point mutation in calcium-binding domain of mouse polyomavirus VP1 protein does not prevent virus-like particle formation, but changes VP1 interactions with Saccharomyces cerevisiae cell structures.

The mouse polyomavirus gene for the major structural protein, VP1, with point mutation in the calcium-binding pocket (VP1(Ala)), was expressed in Saccharomyces cerevisiae and in a baculovirus expression system. Surprisingly, VP1(Ala) forms virus-like particles (VLPs) in nuclei of both yeast and insect cells. VP1(Ala)-VLPs produced in S. cerevisiae are unstable and, unlike wild-type VP1 (VP1(wt))-VLPs, they disassemble during the purification procedure and storage. In contrast to VP1(wt), VP1(Ala) does not interact with the yeast mitotic spindle. Nevertheless, both wild-type and mutated VP1 inhibit yeast cell growth. The inhibition is cAMP-dependent. The production of VP1(Ala) and VP1(wt)-VLPs in insect cells also revealed differences in their interactions with cellular protein(s). Thus, the mutation in the VP1 calcium pocket alters the stability and surface conformation of VLPs rather than the ability of VP1 to self-assemble.