Leucine zipper motif extends

Development of cell culture based peste des petits ruminants (PPR) virus vaccine candidate from Bangladeshi isolates

This study was conducted to develop a cell culture based PPR virus vaccine candidate using recent Bangladeshi strain of peste des petits ruminant's (PPR) virus. PPR virus was isolated from field outbreaks, confirmed by RT-PCR and used as viral inoculum for serial passaging in Vero cells for adaptation and attenuation. 60th serial passage had completed and RT-PCR and real time RT-PCR were done in every 5 passages for confirmation of PPR virus in tissue culture fluid (TCF). To assess the adaptation and attenuation cytopathology, virus titration, sequencing of both F and N genes and live animal experimentation were done. Different cellular alterations produced by PPR virus in infected Vero cells including syncytia formation, development of both intranuclear and intra cytoplasmic inclusion bodies and finally cell degradation are the indications of adaptation. The virus titre was found 2.5, 3.31, 3.55, 4.44, 4.71 and 6.5 Log₁₀ TCID₅₀/ml at 10th, 20th, 30th, 40th, 50th and 60th passages level respectively. In F gene sequence analysis it has been observed that few nucleotide (nt) and mino acid (aa) has been substituted as the effects of serial passaging of PPR virus in Vero cells. TCF at 60th passage level was found effective to produced protective antibody (Ab) titre in live animal experimentation. It is concluded that serially passaged and Vero cells adapted PPR virus TCF could be used as a vaccine candidate for further use to develop a potent & effective vaccine against PPR diseases.

A review emphasizing on utility of heptad repeat sequence as a tool to design pharmacologically safe peptide-based antibiotics

Extensive usage of antibiotics has created an unprecedented scenario of the rapid emergence of many drug-resistant bacteria, which has become an alarming public health concern around the globe. Search for better alternatives that are as efficacious as antibiotics led to the discovery of antimicrobial peptides (AMPs). These small cationic amphiphilic peptides have emerged as a promising option as antimicrobial agents, owing to their multifaceted implications against varied pathogens. Recent years have witnessed tremendous growth in research on AMPs resulting in them being tested in clinical trials of which six got approved for topical application. The relatively less successful outcome has been attributed to the poor cell selectivity shown by most of the naturally occurring AMPs. This drawback needs to be circumvented by identifying strategies to design safe and effective peptides. In the present review, we have emphasized the importance of heptad repeat sequence (leucine and/or phenylalanine zipper motif) as a tool that has shown great promise in remodeling the toxic AMPs to safe antimicrobial agents.

Mutations in the Leucine Zipper-Like Motif of the Human Parainfluenza Virus 3 Fusion Protein Impair Fusion Activity

Objective

To investigate the effect of the leucine zipper-like motif between HRA and HRB of the human parainfluenza virus 3 fusion protein on fusion activity.

Methods

Site-directed mutagenesis was utilized to substitute the heptadic residues at 257, 264, 271, 278, 285, 292, and 299 in this motif with alanine. Additionally, 3 middle heptadic leucine residues at 271, 278, and 285 were replaced with alanine singly or in combination. A vaccinia virus-T7 RNA polymerase transient expression system was employed to express the wild-type or mutated fusion (F) proteins. Three different types of membrane fusion assays were performed to analyze the fusogenic activity, fluorescence-activated cell sorting (FACS) analysis was executed to examine the cell surface expression level, and a coimmunoprecipitation assay was conducted to probe the hemagglutinin-neuraminidase (HN)-F interaction at the cell surface.

Results

All of the substitutions in this motif exhibited diminished or even lost fusion activity in all stages of fusion, although they all had no effect on cell surface expression. In the coimmunoprecipitation assay, all mutants resulted in decreased detection of the HN-F complexes compared with that of the wild-type F protein.

Conclusions

This motif has an important influence on fusion activity, and its integrality is indispensable for membrane fusion.

Newcastle disease virus: macromolecules and opportunities

Over the past two decades, enormous advances have occurred in the structural and biological characterization of Newcastle disease virus (NDV). As a result, not only the complete sequence of the viral genome has been fully determined, but also a clearer understanding of the viral proteins and their respective roles in the life cycle has been achieved. This article reviews the progress in the molecular biology of NDV with emphasis on the new technologies. It also identifies the fundamental problems that need to be addressed and attempts to predict some research opportunities in NDV that can be realized in the near future for the diagnosis, prevention and treatment of disease(s).

The ArcB leucine zipper domain is required for proper ArcB signaling

The Arc two-component system modulates the expression of numerous genes in response to respiratory growth conditions. This system comprises ArcA as the response regulator and ArcB as the sensor kinase. ArcB is a tripartite histidine kinase whose activity is regulated by the oxidation of two cytosol-located redox-active cysteine residues that participate in intermolecular disulfide bond formation. Here, we report that the ArcB protein segment covering residues 70-121, fulfills the molecular characteristics of a leucine zipper containing coiled coil structure. Also, mutational analyses of this segment reveal three different phenotypical effects to be distributed along the coiled coil structure of ArcB, demonstrating that this motif is essential for proper ArcB signaling.

Genomic sequence of an avian paramyxovirus type 1 strain isolated from Muscovy duck (Cairina moschata) in China

The complete sequence of an avian paramyxovirus type 1 (APMV-1) strain, FP1/02, isolated from Muscovy duck in China, was determined. Sequence analysis indicated that the complete genome of strain FP1/02 contained 15,192 nucleotides (nt), following the rule of six. The genome contained an extra 6-nt insertion in the non-coding region of the NP gene when compared with other APMV-1 strains, such as strains La Sota and Beaudette C. The cleavage site of the F protein was (112)R-R-Q-K-R↓F(117), indicating that the FP1/02 strain was virulent, but the morbidity and mortality varied with the species of duck. Genotypic analysis based on the F gene revealed that APMV-1 FP1/02 was a member of genotype VII. Phylogenetic analysis showed that the FP1/02 strain shared high identity with other APMV-1 strains such as ZJ1, SF02 and NA-1 isolated from geese.

Full-length genome sequence of avain paramyxovirus type 4 isolated from a mallard duck

Avian paramyxovirus (APMV) consists of nine serotypes, APMV-1 through -9, of which only APMV-1 and APMV-6 have been fully sequenced. Here, we present the complete 15,054 nt RNA genome of APMV-4 isolated from a mallard duck, which conformed to the "rule of six." The APMV-4 genome had six transcriptional units in the order 3'-NP-P/V-M-F-HN-L-5', which coded for the nucleocapsid (N), phospho- (P), matrix (M), fusion (F), hemagglutinin-neuraminidase (HN), and large (L) proteins. Similar to APMV-1 but unlike APMV-6, APMV-4 lacked a small hydrophobic protein. The leader and trailer sequences were 55 and 17 nt in length, respectively, and the 12 nt-terminal regions of both ends of the APMV-4 genome were complementary. Using phylogenetic analysis, APMV-4 was classified as a member of the genus Avulavirus, and was more closely related to APMV-1 than to APMV-2 or APMV-6. These results may help establish the taxonomic position of Paramyxoviridae, Avulavirus members.

Genomic analysis reveals Mycoplasma pneumoniae repetitive element 1-mediated recombination in a clinical isolate

Mycoplasmas are cell wall-less bacteria that evolved by drastic reduction of the genome size. Complete genome analysis of Mycoplasma pneumoniae revealed the presence of numerous copies of four distinct large M. pneumoniae repetitive elements (RepMPs). One copy each of RepMP2/3, RepMP4, and RepMP5 are localized within the P1 operon (MPN140 to MPN142 loci), and their involvement in sequence variation in adhesin P1 and adherence-related protein B/C has been documented. Here we analyzed a clinical strain of M. pneumoniae designated S1 isolated from a 1993 outbreak of respiratory infections in San Antonio, TX. Based on the type of RepMPs within the P1 operon, we classified clinical isolate S1 as type 2 with unique minor sequence variations. Hybridization with oligonucleotide arrays revealed sequence divergence in two previously unsuspected hypothetical genes (MPN137 and MPN138 loci). Closer inspection of this region revealed that the MPN137 and MPN138 loci harbored previously unrecognized unique RepMP1 sequences found only in M. pneumoniae. PCR and sequence analyses revealed a recombination event involving three RepMP1-containing genes that resulted in fusion of MPN137 and MPN138 reading frames and loss of all but a short fragment of another RepMP1-containing locus, MPN130. The multiple copies of unique RepMP1 elements spread throughout the chromosome could allow vast numbers of sequence variations in clinical strains. Comparisons of amino acid sequences showed the presence of leucine zipper motifs in MPN130 and MPN138 proteins in reference strain M129 and the absence of these motifs in the fused protein of S1. The presence of tandem leucine and other repeats points to possible regulatory functions of proteins encoded by RepMP1-containing genes.

Effect of the alterations in the fusion protein of measles virus isolated from brains of patients with subacute sclerosing panencephalitis on syncytium formation

Measles virus (MV) is the causative agent of subacute sclerosing panencephalitis (SSPE) and viruses isolated from brains of the patients contain numerous mutations. We have previously demonstrated that the hemagglutinin (H) protein of MV SSPE strains can interact with the signaling lymphocyte activation molecule (SLAM) and an unidentified molecule on Vero cells, but not with CD46, as a receptor. The mechanism by which MV SSPE strains can induce cell-cell fusion in SLAM-negative Vero cells is not understood. We report here on the effect of mutations in the fusion (F) proteins of three MV SSPE strains on syncytium formation. The F proteins of the three SSPE strains were functional and co-expression with H protein from the MV wild-type or SSPE strains in this study induced formation of large syncytia in Vero cells as well as in cell lines expressing SLAM or CD46. Expression of chimeric F proteins of SSPE strains showed that amino acid substitutions in the F protein extracellular as well as cytoplasmic domain contributed to enhanced cell-cell fusion in Vero cells. These findings suggest a common molecular mechanism and a key role of the F protein for syncytium formation in cells expressing an unidentified third receptor for MV.

Signaling by the arc two-component system provides a link between the redox state of the quinone pool and gene expression

The Arc two-component system is a complex signal transduction system that plays a key role in regulating energy metabolism at the level of transcription in bacteria. This system comprises the ArcB protein, a tripartite membrane-associated sensor kinase, and the ArcA protein, a typical response regulator. Under anoxic growth conditions, ArcB autophosphorylates and transphosphorylates ArcA, which in turn represses or activates the expression of its target operons. Under aerobic conditions, ArcB acts as a phosphatase that catalyzes the dephosphorylation of ArcA-P and thereby releasing its transcriptional regulation. The events for Arc signaling, including signal reception and kinase regulation, signal transmission, amplification, as well as signal output and decay are discussed.

Sequence motifs and prokaryotic expression of the reptilian paramyxovirus fusion protein

Fourteen reptilian paramyxovirus isolates were chosen to represent the known extent of genetic diversity among this novel group of viruses. Selected regions of the fusion (F) gene were sequenced, analyzed and compared. The F gene of all isolates contained conserved motifs homologous to those described for other members of the family Paramyxoviridae including: signal peptide, transmembrane domain, furin cleavage site, fusion peptide, N-linked glycosylation sites, and two heptad repeats, the second of which (HRB-LZ) had the characteristics of a leucine zipper. Selected regions of the fusion gene of isolate Gono-GER85 were inserted into a prokaryotic expression system to generate three recombinant protein fragments of various sizes. The longest recombinant protein was cleaved by furin into two fragments of predicted length. Western blot analysis with virus-neutralizing rabbit-antiserum against this isolate demonstrated that only the longest construct reacted with the antiserum. This construct was unique in containing 30 additional C-terminal amino acids that included most of the HRB-LZ. These results indicate that the F genes of reptilian paramyxoviruses contain highly conserved motifs typical of other members of the family and suggest that the HRB-LZ domain of the reptilian paramyxovirus F protein contains a linear antigenic epitope.

Sperm associated antigen 9 (SPAG9): a new member of c-Jun NH2 -terminal kinase (JNK) interacting protein exclusively expressed in testis

Previously, we cloned and sequenced a novel human sperm associated antigen 9 (SPAG9). Northern blot analysis and RNA in situ hybridization experiments revealed testis- and stage-specific expression of SPAG9 mRNA, mainly confined to round spermatid suggesting haploid germ cell expression Studies on the human and non-human primates (macaque and baboon) have shown a homology of 84.9% and 90.6% at amino acid level and 94% and 96.8% at DNA level, respectively. The presence of high level of homology at amino acid and DNA level indicates that SPAG9 is conserved in human, baboon and macaque sharing common function and common origin in the biological past. In addition, SPAG9 protein revealed structural homology with c-Jun NH2-terminal kinase (JNK) interacting protein (JIP). The amino acid sequence analysis of SPAG9 predicted coiled coil, leucine zipper and transmembrane domain, speculating the involvement of SPAG9 mediated signal transduction pathways in reproductive processes.

Immunogenicity and contraceptive potential of recombinant human sperm associated antigen (SPAG9)

Human sperm-associated antigen 9 (hSPAG9) is a potential target for sperm-based contraceptive vaccine in lieu of its location on the sperm acrosomal compartment and its implication in sperm-egg interaction. SPAG9 is an acrosomal molecule which is not only restricted to a specific region (domain) of the acrosome but also undergoes relocation to the equatorial region in a stage-specific manner during acrosome reaction, demonstrating its potential role in sperm-egg binding. Human SPAG9 nucleotide sequence revealed 94% identity with macaque SPAG9 and 96.8% with baboon SPAG9 over the entire sequence. The amino acid sequence comparison of human SPAG9 with macaque and baboon revealed an overall homology of 84.9% and 90.6%, respectively. The presence of a high level of homology at the amino acid and nucleotide levels indicates that SPAG9 is conserved in macaque, baboon and human sharing common function and common origin in the biological past. Immunogenicity studies were carried in rats, which demonstrated that recombinant hSPAG9 protein adsorbed on alum is highly immunogenic. Antibodies thus generated after immunization reacted with recombinant human SPAG9 (rhSPAG9) and native SPAG9 protein from human sperm in Western blot analysis. In an in vitro assay, anti-rhSPAG9 antibodies inhibited sperm adherence to or penetration in zona-free hamster egg penetration test. Further, anti-SPAG9 antibodies inhibited the binding of human sperm to intact human oocyte as well as to matched hemi-zonae, indicating that the recombinant protein is a suitable contraceptive vaccinogen. Together these results demonstrate that the rhSPAG9 adsorbed on alum is immunogenic in nature, which is a permissible adjuvant for immunogenicity and fertility trials in non-human primates.

Conformational changes of Newcastle disease virus envelope glycoproteins triggered by gangliosides

We have investigated the conformational changes of Newcastle disease virus (NDV) glycoproteins in response to receptor binding, using 1,1-bis(4-anilino)naphthalene-5,5-disulfonic acid (bis-ANS) as a hydrophobicity-sensitive probe. Temperature- and pH-dependent conformational changes were detected in the presence of free bovine gangliosides. The fluorescence of bis-ANS was maximal at pH 5. The binding of bis-ANS to NDV was not affected by chemicals that denature the fusion glycoprotein, such as reducing agents, nor by the presence of neuraminidase inhibitors such as N-acetyl neuramicic acid. Gangliosides partially inhibited fusion and hemadsorption, but not neuraminidase hemagglutinin-neuraminidase glycoprotein (HN) activity. A conformational intermediate of HN, triggered by the presence of gangliosides acting as receptor mimics, was detected. Our results indicate that, upon binding to free gangliosides, HN undergoes a certain conformational change that does not affect the fusion glycoprotein.

Structure and function of a paramyxovirus fusion protein

Paramyxoviruses initiate infection by attaching to cell surface receptors and fusing viral and cell membranes. Viral attachment proteins, hemagglutinin-neuraminidase (HN), hemagglutinin (HA), or glycoprotein (G), bind receptors while fusion (F) proteins direct membrane fusion. Because paramyxovirus fusion is pH independent, virus entry occurs at host cell plasma membranes. Paramyxovirus fusion also usually requires co-expression of both the attachment protein and the fusion (F) protein. Newcastle disease virus (NDV) has assumed increased importance as a prototype paramyxovirus because crystal structures of both the NDV F protein and the attachment protein (HN) have been determined. Furthermore, analysis of structure and function of both viral glycoproteins by mutation, reactivity of antibody, and peptides have defined domains of the NDV F protein important for virus fusion. These domains include the fusion peptide, the cytoplasmic domain, as well as heptad repeat (HR) domains. Peptides with sequences from HR domains inhibit fusion, and characterization of the mechanism of this inhibition provides evidence for conformational changes in the F protein upon activation of fusion. Both proteolytic cleavage of the F protein and interactions with the attachment protein are required for fusion activation in most systems. Subsequent steps in membrane merger directed by F protein are poorly understood.

Sequence analysis of the N, P, M and F genes of Canadian human metapneumovirus strains

The complete nucleotide sequences of the nucleoprotein (N), phosphoprotein (P), matrix protein (M), and fusion protein (F) genes of 15 Canadian human metapneumovirus (hMPV) isolates were determined. Phylogenetic analysis revealed two distinct genetic clusters, or groups for each gene with additional sequence variability within the individual groups. Comparison of the deduced amino acid sequences for the N, M and F genes of the different isolates revealed that all three genes were well conserved with 94.1-97.6% identity between the two distinct clusters The P gene showed more diversity with 81.6-85.7% amino acid identity for isolates between the two clusters, and 94.6-100% for isolates within the same cluster.

The structural biology of type I viral membrane fusion

The fusion of viral membranes with target-cell membranes is an essential step in the entry of enveloped viruses into cells, and recent X-ray structures of paramyxoviral envelope proteins have provided new insights into protein-mediated plasma-membrane fusion. Here, we review our understanding of the structural transitions that are involved in this fusion pathway, compare it to our understanding of influenza virus membrane fusion, and discuss the implications for retroviral membrane fusion.

Fusogenic activity of reconstituted newcastle disease virus envelopes: a role for the hemagglutinin-neuraminidase protein in the fusion process

Enveloped viruses, such as newcastle disease virus (NDV), make their entry into the host cell by membrane fusion. In the case of NDV, the fusion step requires both transmembrane hemagglutinin-neuraminidase (HN) and fusion (F) viral envelope glycoproteins. The HN protein should show fusion promotion activity. To date, the nature of HN-F interactions is a controversial issue. In this work, we aim to clarify the role of the HN glycoprotein in the membrane fusion step. Four types of reconstituted detergent-free NDV envelopes were used, on differing in their envelope protein contents. Fusion of the different virosomes and erythrocyte ghosts was monitored using the octadecyl rhodamine B chloride assay. Only the reconstituted envelopes having the F protein, even in the absence of HN protein, displayed residual fusion activity. Treatment of such virosomes with denaturing agents affecting the F protein abolished fusion, indicating that the fusion detected was viral protein-dependent. Interestingly, the rate of fusion in the reconstituted systems was similar to that of intact viruses in the presence of the inhibitor of HN sialidase activity 2,3-dehydro-2-deoxy-N-acetylneuraminic acid. The results show that the residual fusion activity detected in the reconstituted systems was exclusively due to F protein activity, with no contribution from the fusion promotion activity of HN protein.

Analysis of the genomic sequence of a human metapneumovirus

We recently described the isolation of a novel paramyxovirus from children with respiratory tract disease in The Netherlands. Based on biological properties and limited sequence information the virus was provisionally classified as the first nonavian member of the Metapneumovirus genus and named human metapneumovirus (hMPV). This report describes the analysis of the sequences of all hMPV open reading frames (ORFs) and intergenic sequences as well as partial sequences of the genomic termini. The overall percentage of amino acid sequence identity between APV and hMPV N, P, M, F, M2-1, M2-2, and L ORFs was 56 to 88%. Some nucleotide sequence identity was also found between the noncoding regions of the APV and hMPV genomes. Although no discernible amino acid sequence identity was found between two of the ORFs of hMPV and ORFs of other paramyxoviruses, the amino acid content, hydrophilicity profiles, and location of these ORFs in the viral genome suggest that they represent SH and G proteins. The high percentage of sequence identity between APV and hMPV, their similar genomic organization (3'-N-P-M-F-M2-SH-G-L-5'), and phylogenetic analyses provide evidence for the proposed classification of hMPV as the first mammalian metapneumovirus.

Cloning, expression, and crystallization of the fusion protein of Newcastle disease virus

We have recently reported the X-ray crystal structure of a fragment of the fusion protein (F) of Newcastle disease virus (NDV). This work describes the methodology involved in the production and crystallization of that protein in recombinant form. The full-length cDNA of NDV-F was cloned and the ectodomain expressed in both CHO-K1 and Lec-3.2.8.1 cells. The recombinant protein, secreted as a single-chain polypeptide F0', was purified using a c-myc antibody affinity column followed by gel filtration chromatography. Electron microscopic imaging showed the F0' product to consist of unaggregated club-shaped particles. Trypsin treatment of F0' could be used to produce disulfide-linked F2 and F1' chains. However, imaging revealed extensive rosette-like aggregation of the trypsin-treated material, indicative of a conformational change. Only the non-trypsin-treated product was thus suitable for crystallization and two crystal forms were obtained, diffracting to ca. 3.5 and 4.0 A, respectively. Both crystal forms were used in the structure determination.

Carbohydrate modifications of the NDV fusion protein heptad repeat domains influence maturation and fusion activity

The amino acid sequence of the fusion protein (F) of Newcastle disease virus (NDV) has six potential N-linked glycosylation addition sites, five in the ectodomain (at amino acids 85, 191, 366, 447, and 471) and one in the cytoplasmic domain at amino acid 542. Two of these sites, at positions 191 and 471, are within heptad repeat (HR) domains implicated in fusion activity of the protein. To determine glycosylation site usage as well as the function of added carbohydrate, each site was mutated by substituting alanine for the serine or threonine in the addition signal. The sizes of the resulting mutant proteins, expressed in Cos cells, showed that sites at amino acids 85, 191, 366, and 471 are used. This conclusion was verified by comparing sizes of mutant proteins missing all four used sites with that of unglycosylated F protein. The role of each added oligosaccharide in the structure and function of the F protein was determined by characterizing stability, proteolytic cleavage, surface expression, and fusion activity of the mutant proteins. Elimination of the site in F(2) at amino acid 85 had the most detrimental effect, decreasing cleavage, stability, and surface expression as well as fusion activity. The protein missing the site at 191, at the carboxyl terminus of the HR1 domain, also showed modestly reduced surface expression and negligible fusion activity. Proteins missing sites at 366 and 471 (within HR2) were expressed at nearly wild-type levels but had decreased fusion activity. These results suggest that all carbohydrate side chains, individually, influence the folding or activity of the NDV F protein. Importantly, carbohydrate modifications of the HR domains impact fusion activity of the protein.

Structural characterization of the human respiratory syncytial virus fusion protein core

Human respiratory syncytial virus (HRSV) is a major cause of a number of severe respiratory diseases, including bronchiolitis and pneumonia, in infants and young children. The HRSV F protein, a glycoprotein essential for viral entry, is a primary target for vaccine and drug development. Two heptad-repeat regions within the HRSV F sequence were predicted by the computer program learncoil-vmf. These regions are thought to form trimer-of-hairpins-like structures, similar to those found in the fusion proteins of several enveloped viruses. The hairpin structure likely brings the viral and cellular membranes into close apposition, thereby facilitating membrane fusion and subsequent viral entry. Here, we show that peptides, denoted HR-N and HR-C, corresponding to the heptad-repeat regions from the N-terminal and C-terminal segments of the HRSV F protein, respectively, form a stable alpha-helical trimer of heterodimers. The HRSV N/C complex was crystallized and its x-ray structure was determined at 2.3-A resolution. As anticipated, the complex is a six-helix bundle in which the HR-N peptides form a three-stranded, central coiled coil, and the HR-C peptides pack in an antiparallel manner into hydrophobic grooves on the coiled-coil surface. There is remarkable structural similarity between the HRSV N/C complex and the fusion protein core of other viruses, including HIV-1 gp41. In addition, earlier work has shown that HRSV HR-C peptides, like the HIV-1 gp41 C peptides, inhibit viral infection. Thus, drug discovery and vaccine development strategies aimed at inhibiting viral entry by blocking hairpin formation may be applied to the inhibition of HRSV.

Expression of a truncated retroviral envelope gene enhances expression of normal cellular phenotypes

The envelope gene of Moloney murine leukemia virus (Mo-MLV) and its various functional domains have been studied extensively but not as much in terms of their biological effects on cell growth. In this study, we report the biological characterization of a truncated Mo-MLV envelope gene, LN11, which is devoid of a signal peptide. Its expression in various cell types, as compared to the control, enabled the transduced cells to assume a more normal phenotype, which is defined by an increase in contact inhibition and factor dependence, as well as reduced tumorigenicity. LN11-transduced fibroblasts exhibited a higher degree of contact inhibition, assumed a more flattened morphology and were more adherent compared to the control. In v-abl transformed hematopoietic cells, expression of LN11 resulted in slower cell growth, which was due to an enhanced dependence on exogenous growth factors. Enforced expression of LN11 also resulted in a slower rate of tumor development and a reduced tumor load. Thus, modification of a retroviral genome could have a significant impact on cell growth and development. This is one example where we need to consider the safety issue carefully when constructing retrovirus vectors for gene therapy.

Heptad-repeat regions of respiratory syncytial virus F1 protein form a six-membered coiled-coil complex

The Respiratory Syncytial Virus (RSV) fusogenic glycoprotein F(1) was characterized using biochemical and biophysical techniques. Two heptad-repeat (HR) regions within F(1) were shown to interact. Proteinase-K digestion experiments highlight the HR1 region (located proximal to the fusion peptide sequence) of the F(1) protein to which an HR2-derived (located proximal to the membrane-spanning domain) peptide binds, thus protecting both the protein and peptide from digestion. Solution-phase analysis of HR1-derived peptides shows that these peptides adopt helical secondary structure as measured by circular dichroism. Sedimentation equilibrium studies indicate that these HR1 peptides self-associate in a monomer/trimer equilibrium with an association constant of 5.2 x 10(8) M(-2). In contrast, HR2-derived peptides form random monomers in solution. CD analysis of mixtures containing peptides from the two regions demonstrate their propensity to interact and form a very stable (T(m) = 87 degrees C), helical (86% helicity) complex comprised of three HR1 and three HR2 members.

A single amino acid change in the Newcastle disease virus fusion protein alters the requirement for HN protein in fusion

The role of a leucine heptad repeat motif between amino acids 268 and 289 in the structure and function of the Newcastle disease virus (NDV) F protein was explored by introducing single point mutations into the F gene cDNA. The mutations affected either folding of the protein or the fusion activity of the protein. Two mutations, L275A and L282A, likely interfered with folding of the molecule since these proteins were not proteolytically cleaved, were minimally expressed at the cell surface, and formed aggregates. L268A mutant protein was cleaved and expressed at the cell surface although the protein migrated slightly slower than wild type on polyacrylamide gels, suggesting an alteration in conformation or processing. L268A protein was fusion inactive in the presence or absence of HN protein expression. Mutant L289A protein was expressed at the cell surface and proteolytically cleaved at better than wild-type levels. Most importantly, this protein mediated syncytium formation in the absence of HN protein expression although HN protein enhanced fusion activity. These results show that a single amino acid change in the F(1) portion of the NDV F protein can alter the stringent requirement for HN protein expression in syncytium formation.

A discrete stage of baculovirus GP64-mediated membrane fusion

Viral fusion protein trimers can play a critical role in limiting lipids in membrane fusion. Because the trimeric oligomer of many viral fusion proteins is often stabilized by hydrophobic 4-3 heptad repeats, higher-order oligomers might be stabilized by similar sequences. There is a hydrophobic 4-3 heptad repeat contiguous to a putative oligomerization domain of Autographa californica multicapsid nucleopolyhedrovirus envelope glycoprotein GP64. We performed mutagenesis and peptide inhibition studies to determine if this sequence might play a role in catalysis of membrane fusion. First, leucine-to-alanine mutants within and flanking the amino terminus of the hydrophobic 4-3 heptad repeat motif that oligomerize into trimers and traffic to insect Sf9 cell surfaces were identified. These mutants retained their wild-type conformation at neutral pH and changed conformation in acidic conditions, as judged by the reactivity of a conformationally sensitive mAb. These mutants, however, were defective for membrane fusion. Second, a peptide encoding the portion flanking the GP64 hydrophobic 4-3 heptad repeat was synthesized. Adding peptide led to inhibition of membrane fusion, which occurred only when the peptide was present during low pH application. The presence of peptide during low pH application did not prevent low pH-induced conformational changes, as determined by the loss of a conformationally sensitive epitope. In control experiments, a peptide of identical composition but different sequence, or a peptide encoding a portion of the Ebola GP heptad motif, had no effect on GP64-mediated fusion. Furthermore, when the hemagglutinin (X31 strain) fusion protein of influenza was functionally expressed in Sf9 cells, no effect on hemagglutinin-mediated fusion was observed, suggesting that the peptide does not exert nonspecific effects on other fusion proteins or cell membranes. Collectively, these studies suggest that the specific peptide sequences of GP64 that are adjacent to and include portions of the hydrophobic 4-3 heptad repeat play a dynamic role in membrane fusion at a stage that is downstream of the initiation of protein conformational changes but upstream of lipid mixing.

Amino acid substitutions within the leucine zipper domain of the murine coronavirus spike protein cause defects in oligomerization and the ability to induce cell-to-cell fusion

The murine coronavirus spike (S) protein contains a leucine zipper domain which is highly conserved among coronaviruses. To assess the role of this leucine zipper domain in S-induced cell-to-cell fusion, the six heptadic leucine and isoleucine residues were replaced with alanine by site-directed mutagenesis. The mutant S proteins were analyzed for cell-to-cell membrane fusion activity as well as for progress through the glycoprotein maturation process, including intracellular glycosylation, oligomerization, and cell surface expression. Single-alanine-substitution mutations had minimal, if any, effects on S-induced cell-to-cell fusion. Significant reduction in fusion activity was observed, however, when two of the four middle heptadic leucine or isoleucine residues were replaced with alanine. Double alanine substitutions that involved either of the two end heptadic leucine residues did not significantly affect fusion. All double-substitution mutant S proteins displayed levels of endoglycosidase H resistance and cell surface expression similar to those of the wild-type S. However, fusion-defective double-alanine-substitution mutants exhibited defects in S oligomerization. These results indicate that the leucine zipper domain plays a role in S-induced cell-to-cell fusion and that the ability of S to induce fusion may be dependent on the oligomeric structure of S.

Interaction of peptides with sequences from the Newcastle disease virus fusion protein heptad repeat regions

Typical of many viral fusion proteins, the sequence of the Newcastle disease virus (NDV) fusion protein has several heptad repeat regions. One, HR1, is located just carboxyl terminal to the fusion peptide, while the other, HR2, is located adjacent to the transmembrane domain. The structure and function of a synthetic peptide with a sequence from the region of the NDV HR1 region (amino acids 150 to 173) were characterized. The peptide inhibited fusion with a half-maximal concentration of approximately 2 microM; however, inhibition was observed only if the peptide was added prior to protease activation of the fusion protein. This inhibition was virus specific since the peptide had minimal effect on fusion directed by the Sendai virus glycoproteins. To explore the mechanism of action, the potential HR1 peptide interaction with a previously characterized fusion inhibitory peptide with a sequence from the HR2 domain (J. K. Young, R. P. Hicks, G. E. Wright, and T. G. Morrison, Virology 238:291-304, 1997) was characterized. The results demonstrated an interaction between the two peptides both functionally and directly. First, while the individual peptides each inhibit fusion, equimolar mixtures of the two peptides had minimal effect on fusion, suggesting that the two peptides form a complex preventing their interaction with a target protein. Second, an HR2 peptide covalently linked with biotin was found to bind specifically to HR1 peptide in a Western blot. The structure of the HR1 peptide was analyzed by nuclear magnetic resonance spectroscopy and found to be an alpha helix.

Identification and expression study of a Xenopus homologue of prenylated SNARE gene

We have utilized the differential display PCR method to isolate transcripts expressed during early embryogenesis of Xenopus laevis. Among many transcripts that have been found to be expressed differentially during the development, one transcript which was expressed predominantly in the unfertilized egg, was isolated as a full-length cDNA and the sequence was determined. This cDNA contained a predicted size of 198 amino acids. A search of the GenBank database revealed that the predicted amino acid sequence of the cDNA is highly homologous-87.8% identical-to the recently identified human protein, HsYKT6, a prenylated vesicle associated-SNARE ((soluble NSF (N-ethylmaleimide-sensitive fusion protein) attachment protein receptor)). Thus we have named the gene as Xsnare1. RT-PCR analysis showed that the Xsnare1 mRNA expressed throughout the oogenesis, in egg and in the early phase of embryogenesis and the level of expression declined after gastrulation. These results suggest that the Xsnare1, a maternally active, putative Xenopus homologue of prenylated v-SNARE, is a developmentally regulatory gene and may be play a role in the process of the early development of Xenopus laevis.

Molecular cloning, functional expression and tissue distribution of rat acyl-coenzyme A:cholesterol acyltransferase

Acyl-coenzyme A:cholesterol acyltransferase (ACAT) is an enzyme catalyzing the intracellular formation of cholesteryl esters from free cholesterol and fatty acyl-CoA. In the present study, we cloned rat ACAT cDNA and determined its tissue distribution. Rat ACAT cDNA, having a coding region of 1635 bp with its deduced protein sequence of 545 amino acids and two typical motifs such as signature sequences and leucine heptad motif, showed 83, 92 and 90% identity with human, mouse, and hamster ACAT, respectively. Expression of rat ACAT cDNA in A293 cells and CHO cells resulted in a 3.0 to 3.5-fold increase in the enzyme activity. Among twelve tissues examined, ACAT activity was highest in adrenal followed by liver and intestine while that of aorta was extremely low. The mRNA level was also the highest in adrenal among four tissues examined. However, in contrast to its high ACAT activity, the liver mRNA level was extremely low (adrenal >> intestine > aorta >> liver). Consistent with mRNA levels, immunohistochemical analyses with a specific ACAT antibody detected significant ACAT signals in adrenal and intestine but a negligible signal in liver. These results indicate that adrenal most abundantly expresses ACAT in rat. Furthermore, rat liver showed a high ACAT activity but an extremely low ACAT mRNA and negligible immunohistochemical reactivity, suggesting the presence of a structurally different ACAT protein(s) in rat liver.

The role of leucine residues in the structure and function of a leucine zipper peptide inhibitor of paramyxovirus (NDV) fusion

To investigate the molecular mechanisms involved in paramyxovirus-induced cell fusion, the function and structure of synthetic peptide analogs of the sequence from the leucine zipper region (heptad repeat region 2) of the Newcastle disease virus fusion protein (F) were characterized. As previously reported (Young et al., Virology, 238, 291), a peptide with the sequence ALDKLEESNSKLDKVNVKLT (amino acids 478-497 of the F protein) inhibited syncytia formation after transfection of Cos cells with the hemagglutinin-neuraminidase and F protein cDNAs. A peptide analog which had an alanine residue in place of the first leucine residue in the zipper motif (ALDKAEESNSKLDKVNVKLT) retained inhibitory activity but less than the original peptide. Further loss in activity was observed in a peptide in which two of the leucine residues were replaced with alanine (ALDKAEESNSKADKVNVKLT), and a peptide which had all leucine residues in the zipper motif replaced with alanine (ALDKAEESNSKADKVNVKLT) had no inhibitory activity. The three-dimensional conformations of these peptides in aqueous solution were determined through the use of nuclear magnetic spectroscopy and molecular modeling. Results showed that while the wild-type peptide formed a helix with properties between an alpha-helix and a 3(10) helix with leucine residues aligned along one face of the helix, progressive substitution of leucine residues with alanine resulted in the progressive loss of helical structure. The results suggest that alterations of leucine residues in the zipper motif disrupt secondary structure of the peptide and that this structure is critical to the inhibitory activity of the peptide.

Cloning of a novel human testis mRNA specifically expressed in testicular haploid germ cells, having unique palindromic sequences and encoding a leucine zipper dimerization motif

A novel gene coding for a human sperm surface protein (HSS) has been cloned from a human testis cDNA library. The 2.5 kb cDNA (hss) contains five direct repeats, three mirror repeats, three possible stem loop structures and three unique palindromic motifs located at functionally relevant positions. Open reading frame encodes a protein of 766 amino acids. The deduced protein analysis revealed a NH2 terminus extracellular domain having a leucine zipper (LZ) motif, a transmembrane helical domain and a cytoplasmic domain at the COOH terminus. Tissue specific expression studies demonstrated the presence of hss transcript only in testis. Cell type expression of hss mRNA was observed only on the round spermatids suggesting haploid germ cell expression.

Analysis of a peptide inhibitor of paramyxovirus (NDV) fusion using biological assays, NMR, and molecular modeling

To investigate the molecular mechanisms involved in paramyxovirus-induced cell fusion, the function and structure of a peptide with a 20-amino-acid sequence from the leucine zipper region (heptad repeat region 2) of the Newcastle disease virus fusion protein (F) were characterized. A peptide with the sequence ALDKLEESNSKLDKVNVKLT (amino acids 478-497 of the F protein) was found to inhibit syncytia formation after virus infection and after transfection of Cos cells with the HN (hemagglutinin-neuraminidase) and F protein cDNAs. Using an F protein gene that requires addition of exogenous trypsin for cleavage, it was shown that the peptide exerted its inhibitory effect prior to cleavage. The three-dimensional conformation of the peptide in aqueous solution was determined through the use of NMR and molecular modeling. Results showed that the peptide formed a helix with properties between an alpha-helix and a 3(10)-helix and that leucine residues aligned along one face of the helix. Side chain salt bridges and hydrogen bonds likely contributed to the stability of the peptide secondary structure. Analysis of the aqueous solution conformation of the peptide suggested mechanisms for specificity of interaction with the intact F protein.

The membrane-located osmosensory kinase, EnvZ, that contains a leucine zipper-like motif functions as a dimer in Escherichia coli

The Escherichia coli EnvZ protein is a membrane-located osmosensor, which is a typical member of histidine kinases involved in His-Asp phosphotransfer signaling. We found that EnvZ has a leucine zipper-like motif in its presumed periplasmic domain. The functional importance of this leucine zipper-like sequence was assessed by introducing a number of appropriate amino acid substitutions. The results collectively suggest that certain leucine residues in the leucine zipper-like structure play an important role in the osmotic signal transduction mediated by EnvZ. When cysteine was substituted for the crucial leucine residues, the EnvZ dimer with disulfide bridge was detected in the cytoplasmic membrane. It was thus demonstrated that the EnvZ osmosensor exists and exerts its signaling ability as a dimer.

The ectodomain of the human T-cell leukemia virus type 1 TM glycoprotein is involved in postfusion events

To examine the contribution of the transmembrane envelope glycoprotein (TM) to the infectivity of the human T-cell leukemia virus type 1 (HTLV-1), single amino acid substitutions were introduced throughout its ectodomain. The mutated envelopes were tested for intracellular maturation and for functions, including ability to elicit syncytium formation and ability to mediate cell-to-cell transmission of the virus. Three major phenotypes, defining three functionally distinct regions, were identified. (i) Mutations causing defects in intracellular maturation of the envelope precursor are mostly distributed in the central portion of the TM ectodomain, containing the immunosuppressive peptide. This region, which includes vicinal cysteines thought to form an intramolecular disulfide bridge, is probably essential for correct folding of the protein. (ii) Mutations resulting in reduced syncytium-forming ability despite correct intracellular maturation are clustered in the amino-terminal part of the TM ectodomain, within the leucine zipper-like motif. Similar motifs with a propensity to form coiled-coil structures have been implicated in the fusion process driven by other viral envelope proteins, and HTLV-1 may thus conform to this general rule for viral fusion. (iii) Mutants with increased syncytium-forming ability define a region immediately amino-terminal to the membrane-spanning domain. Surprisingly, these mutants exhibited severe defects in infectivity, despite competence for fusion. Existence of this phenotype indicates that capacity for cell-to-cell fusion is not sufficient to ensure viral entry, even in cell-to-cell transmission. The ectodomain of the TM glycoprotein thus may be involved in postfusion events required for full infectivity of HTLV-1, which perhaps represents a unique feature of this poorly infectious retrovirus.

The Drosophila bunched gene is a homologue of the growth factor stimulated mammalian TSC-22 sequence and is required during oogenesis

A Drosophila melanogaster sequence homologous to the mammalian growth factor-stimulated TSC-22 gene was isolated in an enhancer trap screen for genes expressed in anterodorsal follicle cells during oogenesis. This sequence includes a 225 aa residue open reading frame that encompasses a leucine zipper motif immediately preceded by a highly conserved region (TSC box), similarly located but distinct from the basic domain of bZIP proteins. The gene encoding this sequence, bunched (bun), has been independently isolated and characterized with respect to its role in peripheral nervous system development and eye development (Treisman, J.E., Lai, Z.-C. and Rubin, G.M. (1995) Shortsighted acts in the decapentaplegic pathway in the Drosophila eye development and has homology to a mouse TGF-beta-responsive gene. Development 121, 2835-2845). In agreement with the expression of the enhancer detector insertion, in situ hybridization reveals that bun transcripts localize to the anterior dorsal follicle cells at stages 10-12 of oogenesis. Changes in bun enhancer trap expression in genetic backgrounds that disrupt the grk/Egfr signaling pathway suggest that bun is regulated by growth factor patterning of dorsal anterior follicle cell fates. Clonal analysis shows that bun is required for the proper elaboration of dorsal cell fates leading to the formation of the dorsal appendages.

Virus-like particle-induced fusion from without in tissue culture cells: role of outer-layer proteins VP4 and VP7

We recently described an assay that measures fusion from without induced in tissue culture cells by rotavirus, a nonenveloped, triple-protein-layered member of the Reoviridae family (M. M. Falconer, J. M. Gilbert, A. M. Roper, H. B. Greenberg, and J. S. Gavora, J. Virol. 69:5582-5591, 1995). The conditions required for syncytium formation are similar to those for viral penetration of the plasma membrane during the course of viral infection of host cells, as the presence of the outer-layer proteins VP4 and VP7 and the cleavage of VP4 are required. Here we present evidence that virus-like particles (VLPs) produced in Spodoptera frugiperda Sf-9 cells from recombinant baculoviruses expressing the four structural proteins of rotavirus can induce cell-cell fusion to the same extent as native rotavirus. This VLP-mediated fusion activity was dependent on trypsinization of VP4, and the strain-specific phenotype of individual VP4 molecules was retained in the syncytium assay similar to what has been seen with reassortant rotaviruses. We show that intact rotavirus and VLPs induce syncytia with cells that are permissive to rotavirus infection whereas nonpermissive cells are refractory to syncytium formation. This finding further supports our hypothesis that the syncytium assay accurately reflects very early events involved in viral infection and specifically the events related to viral entry into the cell. Our results also demonstrate that neither viral replication nor rotavirus proteins other than VP2, VP6, VP4, and VP7 are required for fusion and that both VP4 and VP7 are essential. The combination of a cell-cell fusion assay and the availability of recombinant VLPs will permit us to dissect the mechanisms of rotavirus penetration into host cells.

The Schizosaccharomyces pombe gms1+ gene encodes an UDP-galactose transporter homologue required for protein galactosylation

In a previous study, we isolated a Schizosaccharomyces pombe mutant defective in protein galactosylation (Takegawa, K., Tanaka, N., Tabuchi, M. and Iwahara, S. (1996) Biosci. Biochem. Biotech. 60, 1156-1159). From an S. pombe genomic library, we cloned the gms1+ gene which restored the galactosylation of cell wall glycoproteins. Gms1 protein shares significant sequence similarity with human UDP-galactose and murine CMP-sialic acid transporters. The fission yeast strains deleted for the gms1+ gene lacked galactose residues in sell surface glycoproteins and were significantly decreased in UDP-galactose transport activity. These results showed that the gms1+ encodes an UDP-galactose transporter, and this protein appears to be an essential role for the incorporation of UDP-galactose into the lumen of Golgi in s. pombe.

Structural requirements for low-pH-induced rearrangements in the envelope glycoprotein of tick-borne encephalitis virus

The exposure of the flavivirus tick-borne encephalitis (TBE) virus to an acidic pH is necessary for virus-induced membrane fusion and leads to a quantitative and irreversible conversion of the envelope protein E dimers to trimers. To study the structural requirements for this oligomeric rearrangement, the effect of low-pH treatment on the oligomeric state of different isolated forms of protein E was investigated. Full-length E dimers obtained by solubilization of virus with the detergent Triton X-100 formed trimers at low pH, whereas truncated E dimers lacking the stem-anchor region underwent a reversible dissociation into monomers without forming trimers. These data suggest that the low-pH-induced rearrangement in virions is a two-step process involving a reversible dissociation of the E dimers followed by an irreversible formation of trimers, a process which requires the stem-anchor portion of the protein. This region contains potential amphipathic alpha-helical and conserved structural elements whose interactions may contribute to the rearrangements which initiate the fusion process.

Repetitive elements of the Mycoplasma hominis adhesin p50 can be differentiated by monoclonal antibodies

The gene encoding p50, an adhesin of Mycoplasma hominis, was identified, cloned, and sequenced. Comparison of the derived amino acid sequence with the N-terminal amino acids sequenced by the Edman reaction of the native protein revealed that p50 is expressed as a 467-amino-acid precursor. Posttranslational modification leads to a 441-amino-acid lipoprotein with an extended, predominantly helical structure and a leucine zipper. Computer analysis of the amino acid sequence identified a threefold-repetitive sequence motif comprising approximately three-quarters of the total protein. Different regions of the p50 polypeptide chain were expressed in Escherichia coli. Western blot (immunoblot) analysis of the E. coli lysates revealed that the epitopes of four p50-specific monoclonal antibodies were localized in the middle and C-terminal part of the protein. Epitope mapping by exonuclease III digestion showed that all of the four monoclonal antibodies bound within the same region of the threefold-repetitive amino acid sequence motif. The repeats, which were highly homologous but not identical in structure, could be differentiated by the monoclonal antibodies.

Molecular cloning and characterization of two isoforms of Saccharomyces cerevisiae acyl-CoA:sterol acyltransferase

Esterification of cholesterol by acyl-CoA:cholesterol acyltransferase (ACAT) is a key element in maintaining cholesterol homeostasis in cells of higher animals. In the budding yeast, Saccharomyces cerevisiae, accumulation of ergosteryl esters accompanies entry into stationary phase and sporulation. We have determined that two genes in yeast, SAT1 and SAT2, encode isozymes of acyl-CoA:sterol acyltransferase (ASAT) which are functionally related to ACAT. The SAT1 isozyme is the major catalytic isoform, accounting for at least 65-75% of total ASAT activity. Targeted deletions of one or both genes do not compromise mitotic cell growth or spore germination. However, diploids that are homozygous for a SAT1 null mutation exhibit significantly reduced sporulation efficiency. Furthermore, a larger fraction of the sporulating diploids arrest after the first meiotic division. Human ACAT expressed in sat1 sat2 mutant cells can catalyze esterification of cholesterol and, to a lesser extent, ergosterol in vitro, but restores ergosteryl oleate formation in vivo to only approximately 8% of that catalyzed by yeast ASAT in wild-type cells.

Molecular cloning of the Golgi apparatus uridine diphosphate-N-acetylglucosamine transporter from Kluyveromyces lactis

The mannan chains of Kluyveromyces lactis mannoproteins are similar to those of Saccharomyces cerevisiae except that they lack mannose phosphate and have terminal alpha1-->2-linked N-acetylglucosamine. The biosynthesis of these chains probably occurs in the lumen of the Golgi apparatus, by analogy to S. cerevisiae. The sugar donors, GDP-mannose and UDP-GlcNAc, must first be transported from the cytosol, their site of synthesis, via specific Golgi membrane transporters into the lumen where they are substrates in the biosynthesis of these mannoproteins. A mutant of K. lactis, mnn2-2, that lacks terminal N-acetylglucosamine in its mannan chains in vivo, has recently been characterized and shown to have a specific defect in transport of UDP-GlcNAc into the lumen of Golgi vesicles in vitro. We have now cloned the gene encoding the K. lactis Golgi membrane UDP-GlcNAc transporter by complementation of the mnn2-2 mutation. The mnn2-2 mutant was transformed with a genomic library from wild-type K. lactis in a pKD1-derived vector; transformants were isolated and phenotypic correction was monitored following cell surface labeling with fluorescein isothiocyanate conjugated to Griffonia simplicifolia II lectin, which binds terminal N-acetylglucosamine, and a fluorescent activated cell sorter. A 2.4-kb DNA fragment was found to restore the wild-type lectin binding phenotype. Upon loss of the plasmid containing this fragment, reversion to the mutant phenotype occurred. The above fragment contained an open reading frame for a multitransmembrane spanning protein of 328 amino acids. The protein contains a leucine zipper motif and has high homology to predicted proteins from S. cerevisiae and C. elegans. In an assay in vitro, Golgi vesicles isolated from the transformant had regained their ability to transport UDP-GlcNAc. Taken together, the above results strongly suggest that the cloned gene encodes the Golgi UDP-GlcNAc transporter of K. lactis.

Biophysical characterization of recombinant proteins expressing the leucine zipper-like domain of the human immunodeficiency virus type 1 transmembrane protein gp41

Envelope oligomerization is thought to serve several crucial functions during the life cycle of human immunodeficiency virus type 1 (HIV-1). We recently reported that virus entry requires coiled-coil formation of the leucine zipper-like domain of the HIV-1 transmembrane envelope glycoprotein gp41 (C. Wild, T. Oas, C. McDanal, D. Bolognesi, and T. Matthews, Proc. Natl. Acad. Sci. USA 89:10537-10541, 1992; C. Wild, J. W. Dubay, T. Greenwell, T. Baird, Jr., T. G. Oas, C. McDanal, E. Hunter, and T. Matthews, Proc. Natl. Acad. Sci. USA 91:12676-12680, 1994). To determine the oligomeric state mediated by this region of the envelope, we have expressed the zipper motif as a fusion partner with the monomeric maltose-binding protein of Escherichia coli. The biophysical properties of this protein were characterized by velocity and equilibrium sedimentation, size exclusion chromatography, light scattering, and chemical cross-linking analyses. Results indicate that the leucine zipper sequence from HIV-1 is capable of multimerizing much larger and otherwise monomeric proteins into extremely stable tetramers. Recombinant proteins containing an alanine or a serine substitution at a critical isoleucine residue within the zipper region were also generated and similarly analyzed. The alanine- and serine-substituted proteins behaved as tetrameric and monomeric species, respectively, consistent with the influence of these same substitutions on the helical coiled-coil structure of synthetic peptide models. On the basis of these findings, we propose that the fusogenic gp4l structure involves tetramerization of the leucine zipper domain which is situated approximately 30 residues from the N-terminal fusion peptide sequence.

Peptides from conserved regions of paramyxovirus fusion (F) proteins are potent inhibitors of viral fusion

The synthetic peptides DP-107 and DP-178 (T-20), derived from separate domains within the human immunodeficiency virus type 1 (HIV-1) transmembrane (TM) protein, gp4l, are stable and potent inhibitors of HIV-1 infection and fusion. Using a computer searching strategy (computerized antiviral searching technology, C.A.S.T.) based on the predicted secondary structure of DP-107 and DP-178 (T-20), we have identified conserved heptad repeat domains analogous to the DP-107 and DP-178 regions of HIV-1 gp41 within the glycoproteins of other fusogenic viruses. Here we report on antiviral peptides derived from three representative paramyxoviruses, respiratory syncytial virus (RSV), human parainfluenza virus type 3 (HPIV-3), and measles virus (MV). We screened crude preparations of synthetic 35-residue peptides, scanning the DP-178-like domains, in antiviral assays. Peptide preparations demonstrating antiviral activity were purified and tested for their ability to block syncytium formation. Representative DP-178-like peptides from each paramyxovirus blocked homologous virus-mediated syncytium formation and exhibited EC50 values in the range 0.015-0.250 microM. Moreover, these peptides were highly selective for the virus of origin. Identification of biologically active peptides derived from domains within paramyxovirus F1 proteins analogous to the DP-178 domain of HIV-1 gp4l is compelling evidence for equivalent structural and functional features between retroviral and paramyxoviral fusion proteins. These antiviral peptides provide a novel approach to the development of targeted therapies for paramyxovirus infections.

Homomeric interactions between transmembrane proteins of Moloney murine leukemia virus

We have studied homomeric interactions between transmembrane proteins (TM) of the Moloney murine leukemia virus envelope using the Saccharomyces cerevisiae two-hybrid system. TM interacts strongly with itself but not with various control proteins. Deletional and mutational analyses indicated that the putative leucine zipper motif in the extracellular domain of TM is essential and sufficient to mediate the binding. The first three repeats of the leucine zipper-like motif are the most important in mediating the interaction. The TM-TM interaction detected in this system may play a role in several stages of viral replication.

Mutational analysis of the leucine zipper motif in the Newcastle disease virus fusion protein

The paramyxovirus fusion proteins have a highly conserved leucine zipper motif immediately upstream from the transmembrane domain of the F1 subunit (R. Buckland and F. Wild, Nature [London] 338:547, 1989). To determine the role of the conserved leucines in the oligomeric structure and biological activity of the Newcastle disease virus (NDV) fusion protein, the heptadic leucines at amino acids 481, 488, and 495 were changed individually and in combination to an alanine residue. While single amino acid changes had little effect on fusion, substitution of two or three leucine residues abolished the fusogenic activity of the protein, although cell surface expression of the mutants was higher than that of the wild-type protein. Substitution of all three leucine residues with alanine did not alter the size of the fusion protein oligomer as determined by sedimentation in sucrose gradients. Furthermore, deletion of the C-terminal 91 amino acids, including the leucine zipper motif and transmembrane domain, resulted in secretion of an oligomeric polypeptide. These results indicate that the conserved leucines are not necessary for oligomer formation but are required for the fusogenic ability of the protein. When the polar face of the potential alpha helix was altered by nonconservative changes of serine to alanine (position 473), glutamic acid to lysine or alanine (position 482), asparagine to lysine (position 485), or aspartic acid to alanine (position 489), the fusogenic ability of the protein was not significantly disrupted. In addition, a double mutant (E482A,D489A) which removed negative charges along one side of the helix had negligible effects on fusion activity.

The nucleotide sequence of the fusion protein gene of the peste des petits ruminants virus: the long untranslated region in the 5'-end of the F-protein gene of morbilliviruses seems to be specific to each virus

cDNA corresponding to the fusion protein (F) gene of the vaccine strain of peste des petits ruminants virus (PPRV) was cloned and sequenced. The gene was 2321 nucleotides long excluding the poly(A) tail. As with other morbilliviruses, it had a long G/C rich stretch of about 525 nucleotides. There was no start codon before position 489 in the nucleotide sequence. From the 489th nucleotide to the 549th nucleotide, there were 4 ATG codons, two of which were in frame. The fourth ATG codon was in the best context to act as a start codon for encoding a protein which will be composed of 546 amino acids with a predicted molecular weight 59,310 Da. The comparison of the nucleic acid sequences of different morbillivirus F-protein genes revealed that the 5'-end sequence of the mRNA is specific to each virus. This sequence contains a long stretch of nucleotides rich in G/C content. When protein sequences were compared, it appears that, during evolution, substitutions in amino acid occurred in the F-protein of morbilliviruses such that the structure required for the fusion activity remains unchanged. These substitutions appear to have only occurred in the leader and the membrane anchor sequences, probably as an adaptation of the protein to the host cell.

Oligomerization of the hydrophobic heptad repeat of gp41

The transmembrane protein of human immunodeficiency virus type 1 (HIV-1) contains a leucine zipper-like (hydrophobic heptad) repeat which has been predicted to form an amphipathic alpha helix. To evaluate the potential of the hydrophobic heptad repeat to induce protein oligomerization, this region of gp41 has been cloned into the bacterial expression vector pRIT2T. The resulting plasmid, pRIT3, expresses a fusion protein consisting of the Fc binding domain of monomeric protein A, a bacterial protein, and amino acids 538 to 593 of HIV-1 gp41. Gel filtration chromatography demonstrated the presence of oligomeric forms of the fusion protein, and analytical centrifugation studies confirmed that the chimeric protein formed a higher-order multimer that was greater than a dimer. Thus, we have identified a region of HIV-1 gp41 which is capable of directing the oligomerization of a fusion protein containing monomeric protein A. Point mutations, previously shown to inhibit the biological activity of the HIV-1 envelope glycoprotein, have been engineered into the segment of gp41 contained in the fusion protein, and expressed mutant proteins were purified and analyzed via fast protein liquid chromatography. A point mutation in the heptad repeat, which changed the central isoleucine to an alanine, caused a significant (> 60%) decrease in oligomerization, whereas changing the central isoleucine to aspartate or proline resulted in almost a complete loss of oligomerization. Deletions of one, two, or three amino acids following the first isoleucine also resulted in a profound decrease in oligomerization. The inhibitory effects of the mutations on oligomer formation correlated with the effects of the same mutations on envelope glycoprotein-mediated fusion. A possible role of the leucine zipper-like region in the fusion process and in an oligomerization event distinct from assembly of the envelope glycoprotein complex is discussed.

Molecular characterization of EAP-300: a high molecular weight, embryonic polypeptide containing an amino acid repeat comprised of multiple leucine-zipper motifs

In this study we report the biochemical and initial molecular characterization of EAP-300, a developmentally regulated embryonal protein that has been shown previously to be expressed by radial glia in various regions of the CNS, including putative glial barriers. In the present study we have shown that the 300 kDa EAP-300 polypeptide is developmentally regulated in all tissues expressing the protein, which include various PNS and CNS tissues and muscle. In neural tissue the protein is readily detected during early embryogenesis, subsequently down-regulated at later stages, and is not detected in the adult. In contrast to neural tissue, small amounts of the protein are expressed in heart, consistent with earlier studies which showed that EAP-300 expression was maintained in the Purkinje cells of the heart conduction system. Metabolic labeling demonstrates that EAP-300 is a phosphoprotein, and is fatty acylated based on incorporation of [3H]palmitate. We also show that the normal developmental down-regulation of EAP-300 by glia does not occur in vitro, and these data therefore suggest that the signal(s) that regulates EAP-300 gene expression during development in vivo is absent in dissociated cell cultures. We have also initiated molecular studies of EAP-300 by screening embryonic brain cDNA expression libraries with a mixture of EAP-300 monoclonal antibodies. Sequence analysis of partial EAP-300 cDNAs indicate that the protein is related, if not identical, to IFAPa-400, a developmentally regulated intermediate filament-associated protein in chick that is proposed to participate in cell differentiation. These studies also indicate that EAP-300 mRNA is developmentally regulated and is expressed by glial cells in putative CNS barrier structures. Our studies also suggest that two pools of EAP-300 may exist in cells, implying that unlike IFAPa-400 the EAP-300 protein may not always be associated with intermediate filaments. Interestingly, our studies demonstrate that EAP-300 contains a novel repeat amino acid domain comprised of multiple leucine-zipper motifs, which may contribute to its function during glial differentiation.