Specific inhibition of ribonucleotide reductases by peptides corresponding to the C-terminal of their second subunit

Previous studies have shown that herpes virus ribonucleotide reductase can be inhibited by a synthetic nonapeptide whose sequence is identical to the C-terminal of the small subunit of the enzyme. This peptide is able to interfere with normal subunit association that takes place through the C-terminal of the small subunit. In this report, we illustrate that inhibition of ribonucleotide reductases by peptides corresponding to the C-terminal of subunit R2 is also observed for the enzyme isolated from Escherichia coli, hamster, and human cells. The nonapeptide corresponding to the bacterial C-terminal sequence was found to inhibit E. coli enzyme with an IC50 of 400 microM, while this peptide had no effect on mammalian ribonucleotide reductase. A corresponding synthetic peptide derived from the C-terminal of the small subunit of the human enzyme inhibited both human and hamster ribonucleotide reductases with IC50 values of 160 and 120 microM, respectively. However, this peptide had no inhibitory activity against the bacterial enzyme. Equivalent peptides derived from herpes virus ribonucleotide reductase had no effect on either the bacterial or mammalian enzymes. Thus, subunit association at the C-terminal of the small subunit appears to be a common feature of ribonucleotide reductases. In addition, the inhibitory phenomenon observed with peptides corresponding to the C-terminal appears not only to be universal, but also specific to the primary sequence of the enzyme.     

The amphiphilic membrane glycoproteins of Semliki forest virus are attached to the lipid bilayer by their COOH-terminal ends

The membrane location of the Semliki Forest virus glycoproteins E1, E2 and E3 was studied by protease treatment of (1) virus particles and (2) rough micro somes from cells infected with SF virus2. Protease treatment of virus particles removes all but the membrane-associated segments of the glycoproteins. Analyses of protease-treated SF virus membranes in 15% to 22.5% gradient acrylamide gels demonstrate the presence of three distinct peptide species with apparent molecular weights of 9000, 6000 and 5500. The 9000 and the 5500 molecular weight peptides have been aligned to the COOH-terminal end of E2 and the 6000 molecular weight peptide to the COOH-terminal end of El. The mapping of the peptides was done in a “Dintzis”-type of experiment (Dintzis, 1961) where we labelled the proteins of the virus with a gradient of [³⁵S]methionine increasing towards their COOH-terminal end.Protease treatment of microsomes from cells infected with SF virus removes only those parts of the viral glycoproteins that are transversing the lipid bilayer. Analyses of such treated membranes in sodium dodecyl sulphate-containing gels show that a 3000 molecular weight piece is digested from the COOH-terminal end of p62, the cellular precursor of E2 and E3. The COOH-terminus of p62 is shown to be equivalent to that of E2. These results thus demonstrate that the two amphiphilic membrane proteins of SF virus, E1 and E2 (p62) are attached to the lipid bilayer by their COOH-terminal ends. The COOH-terminal end of p62 (E2) spans the microsomal membrane. The third membrane protein, E3, probably does not interact with membrane lipids but is bound to the virus on E1 and (or) E2.     

Identification and characterization of an actin-binding site of CapZ

A mAb (1E5) that binds the COOH-terminal region of the beta subunit of chicken CapZ inhibits the ability of CapZ to bind the barbed ends of actin filaments and nucleate actin polymerization. CapZ prepared as fusion proteins in bacteria or nonfusion proteins by in vitro translation has activity similar to that of CapZ purified from muscle. Deletion of the COOH-terminus of the beta subunit of CapZ leads to a loss of CapZ's ability to bind the barbed ends of actin filaments. A peptide corresponding to the COOH-terminal region of CapZ beta, expressed as a fusion protein, binds actin monomers. The mAb 1E5 also inhibits the binding of this peptide to actin. These results suggest that the COOH-terminal region of the beta subunit of CapZ is an actin-binding site. The primary structure of this region is not similar to that of potential actin-binding sites identified in other proteins. In addition, the primary structure of this region is not conserved across species.     

Structure-activity studies on synthetic peptides inhibiting herpes simplex virus ribonucleotide reductase

Herpes simplex virus type 1 and type 2 (HSV-1 and HSV-2) ribonucleotide reductase is formed by the association of two nonidentical subunits. A peptide corresponding to the COOH terminus of the subunit 2, Tyr-Ala-Gly-Ala-Val-Val-Asn-Asp-Leu (H2-(7-15)), has been shown to completely inhibit the reductase activity (IC50 = 36 microM) without affecting the host isoenzyme. In order to study the relationship between chemical requirements and inhibitory potencies, a series of peptides, including fragments and analogs of H2-(7-15), were synthesized. The minimum active core can be assigned to the Val-Val-Asn-Asp-Leu sequence (IC50 = 760 microM). N alpha-Extended peptides, such as Ser-Thr-Ser-Tyr-Ala-Gly-Ala-Val-Val-Asn-Asp-Leu (H2-(4-15)) and Glu-Cys-Arg-Ser-Thr-Ser-Tyr-Ala-Gly-Ala-Val-Val-Asn-Asp-Leu (H2-(1-15) ), respectively, have inhibitory potencies 2.1- and 1.4-fold greater than the nonapeptide H2-(7-15). N alpha-Deamination or acetylation of H2-(7-15) increases its potency by 1.8- and 3.0-fold, respectively, whereas amidation of the alpha-carboxylic function diminishes its activity by 3.2-fold. These results indicate that the alpha-amino group is not essential for maximum potency but suggest that a free carboxylic function is required. Substitution of Tyr7 or Ala8 by their respective D-isomer leads to a decrease of potency, suggesting that a specific conformation of the NH2-terminal portion is required to have a maximum activity. Monosubstitution in positions 11, 13, 14, and 15, by L-alanine completely abolishes activity stressing the importance of each amino acid residue contained in the minimum active core. Finally, nonapeptides corresponding to the COOH-terminal portion of the subunit 2 of Epstein-Barr and varicella-zoster virus ribonucleotide reductases also inhibit the HSV-1 reductase activity. The varicella-zoster virus nonapeptide is 4.0 times more potent than H2-(7-15), whereas the Epstein-Barr virus nonapeptide is 3.1 times less potent. These results should help us to design a new generation of potent inhibitors of herpes virus ribonucleotide reductases.     

Characterization of procollagen synthesized by matrix-free cells isolated from chick embryo tendons.

The genetic type and molecular structure of the precursor forms of collagen synthesized by matrix-free tendon cells isolated from 17-day old chick embryos were examined by chromatographic and electrophoretic techniques. The [14C]proline-labeled collagenous proteins secreted by the cells resolved on diethylaminoethylcellulose into two peaks, A and B. Both peaks contained type I collagenous proteins since on chromatography on carboxymethylcellulose, after limited pepsin proteolysis, both peaks contained alpha1 and alpha2 chains of collagen in a 2:1 ratio, and cyanogen bromide peptide maps of the 14C-labeled protein in both peaks were similar to cyanogen bromide peptide maps derived from authentic type I collagen. Enzymatic digestion with purified mammalian collagenase demonstrated that the collagen precursor in peak B contained noncollagenous peptide extensions at both the amino- and carboxy-terminal ends of the molecule, while peak A had only carboxy-terminal extension peptides. Although both the amino- and carboxy-terminal extensions incorporated radioactive cystine, only the carboxy-terminal extensions contained interchain disulfide bonds. The carboxy-terminal extensions were also shown to incorporate radioactive tryptophan. Since most of the precursor forms of collagen recovered in the incubation medium chromatographed in peak B, it is concluded that matrix-free tendon cells secrete only type I procollagen with extension peptides at both the amino- and carboxy-terminal ends of the molecule.     

Generation of MHC class I peptide antigens by protein processing in the secretory route by furin

Cytosolic degradation of endogenously synthesized proteins by the proteasome and translocation of processed peptides to the endoplasmic reticulum by the transporters associated with antigen presentation constitutes the classical route for antigen presentation by MHC class I proteins. We have previously defined an alternative pathway in the secretory route involving proteolytic maturation of precursor proproteins for chimeric hepatitis B virus secretory core protein HBe containing a class I epitope at its carboxy-terminus. We extend those results by demonstrating that intracellular delivery of the trans -Golgi network protease furin increases both proteolytic maturation and antigen presentation of the chimeric HBe proteins. An additional class I epitope from the HIV envelope gp160 protein was inserted into this COOH-terminal region of two different chimeric HBe proteins. This epitope was also presented to CTL in a transporter-independent manner involving furin, and protein maturation and antigen presentation were also enhanced by furin over-expression. Presentation of this second epitope was restricted by a different class I allele, thus suggesting that antigen presentation by this new pathway may apply to any antigenic epitope and class I molecule. These results define the furin proteolytic maturation pathway of HBe in the secretory route as a general antigen processing route for MHC class I presentation.     

The cytoplasmic and transmembrane domains of the vaccinia virus B5R protein target a chimeric human immunodeficiency virus type 1 glycoprotein to the outer envelope of nascent vaccinia virions.

The outer envelope of the extracellular form of vaccinia virus (EEV) is derived from the Golgi membrane and contains at least six viral proteins. Transfection studies indicated that the EEV protein encoded by the B5R gene associates with Golgi membranes when synthesized in the absence of other viral products. A domain swapping strategy was then used to investigate the possibility that the B5R protein contains an EEV targeting signal. We constructed chimeric genes encoding the human immunodeficiency virus (HIV) type 1 glycoprotein with the cytoplasmic and transmembrane domains replaced by the corresponding 42-amino-acid C-terminal segment of the B5R protein. Recombinant vaccinia viruses that stably express a chimeric B5R-HIV protein or a control HIV envelope protein with the original cytoplasmic and transmembrane domains were isolated. Cells infected with recombinant vaccinia viruses that expressed either the unmodified or the chimeric HIV envelope protein formed syncytia with cells expressing the CD4 receptor for HIV. However, biochemical and microscopic studies demonstrated that the HIV envelope proteins with the B5R cytoplasmic and transmembrane domains were preferentially targeted to the EEV. These data are consistent with the presence of EEV localization signals in the cytoplasmic and transmembrane domains of the B5R protein.     

Activation and maturation mechanisms of boar acrosin zymogen based on the deduced primary structure

We have isolated cDNA clones encoding boar acrosin, a serine protease participating in the initial stage of fertilization, from boar testis lambda gt11 cDNA libraries. Nucleotide sequencing of the overlapping clones indicates that the composite cDNA inserts contain 1,391 base pairs coding for a 5'-untranslated region, an open reading frame, a stop codon, a 3'-untranslated region, and a poly(A)+ tail. A polyadenylation signal, AATAAA, is located 33 bases upstream from the start of the poly(A)+ tail. The amino acid sequence deduced from the cDNAs shows that boar acrosin is initially synthesized as a prepro-protein with a 16-residue signal peptide at the NH2 terminus. This signal sequence is followed by a 399-residue sequence corresponding to the acrosin zymogen. COOH-terminal sequence analysis of boar sperm 55-kDa proacrosin and its processed forms indicates that the mature acrosin molecule contains 322 amino acid residues in two polypeptide chains, a 23-residue light chain and a 299-residue heavy chain, with a combined molecular mass of 35,735 Da, and that the 55-kDa proacrosin molecule has 14-, 18-, and 43-residue segments as COOH-terminal extensions that are removed during proacrosin maturation. The COOH-terminal 43-residue segment is rich in proline residues, including an unusual repeat of 23 consecutive prolines. The deduced amino acid sequence of boar acrosin shows a high degree of identity with major portions of other serine proteases, including the active site region and the location of cysteine residues. We conclude that boar acrosin is synthesized as a single-chain polypeptide with the regions corresponding to the light and heavy chains covalently connected by two disulfide bonds, and that the single-chain molecule is autoactivated by cleavage of the Arg23-Val24 bond after removal of the COOH-terminal 14-residue segment, resulting in the formation of the light and heavy chains. This two-chain molecule is then converted to the mature enzyme by removal of the COOH-terminal 18- and 43-residue segments.     

Intermediates in the limited proteolytic conversion of procollagen to collagen.

The conversion of chick bone procollagen to collagen proceeds in a stepwise fashion to produce a limited number of intermediates. Initial proteolytic cleavages remove NH2-terminal nonhelical extensions and yield an intermediate which remains disulfide-bonded via COOH-terminal extensions. Subsequent stepwise scission of one or two chains of the triple-stranded molecule in its COOH-terminal domain produces intermediates which can only be distinguished after dissociation of the noncovalently bonded alpha chains. A final cleavage in this region produces the collagen molecule and a disulfide-bonded triple-stranded fragment which represents the COOH-terminal domain. In all likelihood the endopeptidases which effect cleavage in the NH2- and COOH-terminal regions differ. More than two enzymes may be required for conversion of procollagen to collagen if the nonhelical domains are not released in an en bloc fashion.     

Investigation of an octapeptide inhibitor of Escherichia coli ribonucleotide reductase by transferred nuclear Overhauser effect spectroscopy

Several peptides contained within the C-terminal sequence of the B2 subunit of Escherichia coli ribonucleotide reductase (RNR) were investigated for their ability to inhibit the enzyme, presumably by interfering with association of the B1 and B2 subunits. AcYLVGQIDSE, corresponding by sequence homology to a nonapeptide that inhibits herpes simplex RNR [Gaudreau et al. (1987) J. Biol. Chem. 262, 12413] shows no inhibition of the E. coli enzyme (IC50 greater than 3 mM), whereas AcDDLSNFQL, the C-terminal octapeptide of the E. coli B2 subunit, is a noncompetitive inhibitor (Ki = 160 microM). Neither bradykinin (RPPGFSPFR) nor the pentapeptide AcSNFQL inhibits the E. coli enzyme. Transferred nuclear Overhauser enhancement spectroscopy was used to probe the conformation of AcDDLSNFQL when it is bound to the B1 subunit. These experiments suggest that the peptide adopts a turn in the region of Asn5 and Phe6 and that a hydrophobic cluster of the phenylalanine and leucine side chains is involved in the interaction surface.     

Thermal stability and intersubunit interactions of cholera toxin in solution and in association with its cell-surface receptor ganglioside GM1

The thermal stability of cholera toxin free in solution and in association with its cell-surface receptor ganglioside GM1 has been studied by using high-sensitivity differential scanning calorimetry and differential solubility thermal gel analysis. In the absence of ganglioside GM1, cholera toxin undergoes two distinct thermally induced transitions centered at 51 and 74 degrees C, respectively. The low-temperature transition has been assigned to the irreversible thermal denaturation of the active A subunit. The second transition has been assigned to the reversible unfolding of the B subunit pentamer. The isolated B subunit pentamer exhibits a single transition also centered at 74 degrees C, suggesting that the attachment of the A subunit does not contribute to the stability of the pentamer. In the intact toxin, the A subunit dissociates from the B subunit pentamer at a temperature that coincides with the onset of the B subunit thermal unfolding. In aqueous solution, the denatured A subunit precipitates after dissociation from the B subunit pentamer. This phenomenon can be detected calorimetrically by the appearance of an exothermic heat effect. In the presence of ganglioside GM1, the B subunit is greatly stabilized as indicated by an increase of 20 degrees C in the transition temperature. In addition, ganglioside GM1 greatly enhances the cooperative interactions between B subunits. In the absence of ganglioside, each monomer within the B pentamer unfolds in an independent fashion whereas the fully ganglioside-bound pentamer behaves as a single cooperative unit. On the contrary, the thermotropic behavior of the A subunit is only slightly affected by the presence of increasing concentrations of ganglioside GM1.(ABSTRACT TRUNCATED AT 250 WORDS)     

Chimeric animal and plant viruses expressing epitopes of outer membrane protein F as a combined vaccine against Pseudomonas aeruginosa lung infection

Outer membrane protein F of Pseudomonas aeruginosa has vaccine efficacy against infection by P. aeruginosa as demonstrated in a variety of animal models. Through the use of synthetic peptides, three surface-exposed epitopes have been identified. These are called peptides 9 (aa 261-274 in the mature F protein, TDAYNQKLSERRAN), 10 (aa 305-318, NATAEGRAINRRVE), and 18 (aa 282-295, NEYGVEGGRVNAVG). Both the peptide 9 and 10 epitopes are protective when administered as a vaccine. In order to develop a vaccine that is suitable for use in humans, including infants with cystic fibrosis, the use of viral vector systems to present the protective epitopes has been investigated. An 11-amino acid portion of epitope 10 (AEGRAINRRVE) was successfully inserted into the antigenic B site of the hemagglutinin on the surface of influenza virus. This chimeric influenza virus protects against challenge with P. aeruginosa in the mouse model of chronic pulmonary infection. Attempts to derive a chimeric influenza virus carrying epitope 9 have been unsuccessful. A chimeric plant virus, cowpea mosaic virus (CPMV), with epitopes 18 and 10 expressed in tandem on the large coat protein subunit (CPMV-PAE5) was found to elicit antibodies that reacted exclusively with the 10 epitope and not with epitope 18. Use of this chimeric virus as a vaccine afforded protection against challenge with P. aeruginosa in the mouse model of chronic pulmonary infection. Chimeric CPMVs with a single peptide containing epitopes 9 and 18 expressed on either of the coat proteins are in the process of being evaluated. Epitope 9 was successfully expressed on the coat protein of tobacco mosaic virus (TMV), and this chimeric virus is protective when used as a vaccine in the mouse model of chronic pulmonary infection. However, initial attempts to express epitope 10 on the coat protein of TMV have been unsuccessful. Efforts are continuing to construct chimeric viruses that express both the 9 and 10 epitopes in the same virus vector system. Ideally, the use of a vaccine containing two epitopes of protein F is desirable in order to greatly reduce the likelihood of selecting a variant of P. aeruginosa that escapes protective antibodies in immunized humans via a mutation in a single epitope within protein F. When the chimeric influenza virus containing epitope 10 and the chimeric TMV containing epitope 9 were given together as a combined vaccine, the immunized mice produced antibodies directed toward both epitopes 9 and 10. The combined vaccine afforded protection against challenge with P. aeruginosa in the chronic pulmonary infection model at approximately the same level of efficacy as provided by the individual chimeric virus vaccines. These results prove in principle that a combined chimeric viral vaccine presenting both epitopes 9 and 10 of protein F has vaccine potential warranting continued development into a vaccine for use in humans.     

Structural characterization of pertussis toxin A subunit

The relationship between the structure of the A subunit of pertussis toxin and its function was analyzed. Limited tryptic digestion of the A subunit converted the protein to two stable fragments (Mr = 20,000 and 18,000). Antibodies raised to synthetic peptides homologous to regions in the A subunit were used to map these fragments. Both fragments were shown to contain the NH2-terminal portion but not the COOH-terminal portion of the A subunit. While these fragments exhibited NAD glycohydrolase activity, they were unable to reassociate with the B oligomer of the toxin. Thus the COOH-terminal portion of the A subunit does not contain the residues which are required for the NAD glycohydrolase activity of the toxin. However, this region of the molecule may be important for maintaining the oligomeric structure of the toxin. These results suggest that the A subunit of pertussis toxin is similar in structure to the A subunit of cholera toxin. In addition, antibodies raised to a synthetic peptide identical to residues 6-17 of the A subunit of pertussis toxin will bind to the A subunit of cholera toxin.     

Direct evidence for interaction between COOH-terminal regions of cytochrome b558 subunits and cytosolic 47-kDa protein during activation of an O(2-)-generating system in neutrophils.

Cytochrome b558 in phagocytes is a transmembrane protein composed of large and small subunits and considered to play a key role in O2- generation during the respiratory burst. The COOH-terminal regions of the cytochrome subunits protrude to the cytoplasmic side and are assumed to be the sites for association with cytosolic components to form an active O(2-)-generating complex (Imajoh-Ohmi, S., Tokita, K., Ochiai, H., Nakamura, M., and Kanegasaki, S. (1992) J. Biol. Chem. 267, 180-184). We show here that two synthetic peptides corresponding to the COOH-terminal region of each subunit inhibit NADPH-dependent oxygen uptake induced by sodium dodecyl sulfate (SDS) in a cell-free system consisting of plasma membrane and cytosol. The inhibition was observed when either peptide was added to the system before, but not after, the activation with SDS suggesting that interaction between the COOH-terminal regions of the cytochrome subunits and cytosolic components is important for the assembly and the activity of the O(2-)-generating system. Using the cross-linking reagent dimethyl 3,3'-dithiobis-propionimidate, we found that the cytosolic 47-kDa protein, an essential component of the O(2-)-generating system, interacted with the synthetic peptides in the presence of SDS. In addition to the 47-kDa protein, a 17-kDa protein was found to be associated with the peptide corresponding to the COOH-terminal region of the small subunit. These results indicate that the cytosolic COOH-terminal regions of cytochrome b558 subunits are the binding sites for both the cytosolic 47-kDa protein and the 17-kDa protein and that the binding takes place during activation of the system.     

Monoclonal antibodies as structural probes of surface residues in the regulatory subunit of cAMP-dependent protein kinase II from porcine heart

Two murine monoclonal antibodies (H5 and B6) generated against bovine heart type II regulatory subunit of cAMP-dependent protein kinase were shown to cross-react equally well with the homologous subunit from porcine heart. The antibodies demonstrated specificity for only the type II regulatory subunit and showed negligible cross-reactivity with the type I regulatory subunit, the catalytic subunit, and cGMP-dependent protein kinase. Following limited proteolysis of type II regulatory subunit with chymotrypsin, the H5 monoclonal antibody was shown to cross-react with the Mr = 37,000 cAMP-binding domain corresponding to the COOH-terminal region of the polypeptide chain. To more specifically localize the antigenic sites, the porcine type II regulatory subunit was carboxymethylated and cleaved with cyanogen bromide. Both monoclonal antibodies cross-reacted with the NH2-terminal CNBr peptide, and this peptide demonstrated affinities similar to native bovine type II regulatory subunit in competitive displacement radioimmunoassays. Tryptic cleavage of this CNBr fragment destroyed all antigenicity for both monoclonal antibodies, whereas antigenicity was retained following chymotryptic digestion. A single major immunoreactive chymotryptic fragment that cross-reacted with H5 was isolated by gel filtration and reverse phase high performance liquid chromatography. this peptide retained the complete antigenic site and had the following sequence: Asn-Pro-Asp-Glu-Glu-Glu-Glu-Asp-Thr-Asp-Pro-Arg-Val-Ile-His-Pro-Lys-Thr-Asp-Gl n. This antigenic site was localized just beyond the major site of autophosphorylation, approximately a third of the distance from the NH2-terminal end of the polypeptide chain.     

Myristoyl CoA:protein N-myristoyltransferase activities from rat liver and yeast possess overlapping yet distinct peptide substrate specificities

A variety of eukaryotic viral and cellular proteins possesses an NH2-terminal N-myristoylglycine residue important for their biological functions. Recent studies of the primary structural requirements for peptide substrates of the enzyme responsible for this modification in yeast demonstrated that residues 1, 2, and 5 play a critical role in enzyme: ligand interactions (Towler, D. A., Adams, S. P., Eubanks, S. R., Towery, D. S., Jackson-Machelski, E., Glaser, L., and Gordon J. I. (1987b) Proc. Natl. Acad. Sci. U. S. A. 84, 2708-2812). This was determined by examining as substrates a series of synthetic peptides whose sequences were systematically altered from a "parental" peptide derived from the known N-myristoylprotein bovine heart cyclic AMP-dependent protein kinase (A kinase) catalytic subunit. We have now extended these studies in order to examine structure/activity relationships in the COOH-terminal regions of octapeptide substrates of yeast N-myristoyltransferase (NMT). The interaction between yeast NMT and the side chain of residue 5 in peptide ligands is apparently sterically constrained, since Thr5 is unable to promote the very high affinity binding observed with a Ser5 substitution. A substrate hexapeptide core has been defined which contains much of the information necessary for recognition by this lower eukaryotic NMT. Addition of COOH-terminal basic residues to this hexapeptide enhances peptide binding, while COOH-terminal acidic residues destabilize NMT: ligand interactions. Based on the results obtained from our in vitro studies of over 80 synthetic peptides and yeast NMT, we have identified a number of potential N-myristoylproteins from searches of available protein databases. These include hepatitis B virus pre-S1, human SYN-kinase, rodent Gi alpha, and bovine transducin-alpha. Peptides corresponding to the NH2-terminal sequences of these proteins and several known N-myristoylproteins were assayed using yeast NMT as well as partially purified rat liver NMT. While a number of the synthetic peptides exhibited similar catalytic properties with the yeast and mammalian enzymes, surprisingly, the SYN-kinase, Gi alpha, and transducin-alpha peptides were N-myristoylated by rat NMT but not by yeast NMT. This suggests that either multiple NMT activities exist in rat liver or the yeast and rodent enzymes have similar but distinct peptide substrate specificities.     

Murine leukemia virus R Peptide inhibits influenza virus hemagglutinin-induced membrane fusion

The cytoplasmic tail of the murine leukemia virus (MuLV) envelope (Env) protein is known to play an important role in regulating viral fusion activity. Upon removal of the C-terminal 16 amino acids, designated as the R peptide, the fusion activity of the Env protein is activated. To extend our understanding of the inhibitory effect of the R peptide and investigate the specificity of inhibition, we constructed chimeric influenza virus-MuLV hemagglutinin (HA) genes. The influenza virus HA protein is the best-studied membrane fusion model, and we investigated the fusion activities of the chimeric HA proteins. We compared constructs in which the coding sequence for the cytoplasmic tail of the influenza virus HA protein was replaced by that of the wild-type or mutant MuLV Env protein or in which the cytoplasmic tail sequence of the MuLV Env protein was added to the HA cytoplasmic domain. Enzyme-linked immunosorbent assays and Western blot analysis showed that all chimeric HA proteins were effectively expressed on the cell surface and cleaved by trypsin. In BHK21 cells, the wild-type HA protein had a significant ability after trypsin cleavage to induce syncytium formation at pH 5.1; however, neither the chimeric HA protein with the full-length cytoplasmic tail of MuLV Env nor the full-length HA protein followed by the R peptide showed any syncytium formation. When the R peptide was truncated or mutated, the fusion activity was partially recovered in the chimeric HA proteins. A low-pH conformational-change assay showed that similar conformational changes occurred for the wild-type and chimeric HA proteins. All chimeric HA proteins were capable of promoting hemifusion and small fusion pore formation, as shown by a dye redistribution assay. These results indicate that the R peptide of the MuLV Env protein has a sequence-dependent inhibitory effect on influenza virus HA protein-induced membrane fusion and that the inhibitory effect occurs at a late stage in fusion pore enlargement.     

Heptad repeat 2-based peptides inhibit avian sarcoma and leukosis virus subgroup a infection and identify a fusion intermediate

Fusion proteins of enveloped viruses categorized as class I are typified by two distinct heptad repeat domains within the transmembrane subunit. These repeats are important structural elements that assemble into the six-helix bundles characteristic of the fusion-activated envelope trimer. Peptides derived from these domains can be potent and specific inhibitors of membrane fusion and virus infection. To facilitate our understanding of retroviral entry, peptides corresponding to the two heptad repeat domains of the avian sarcoma and leukosis virus subgroup A (ASLV-A) TM subunit of the envelope protein were characterized. Two peptides corresponding to the C-terminal heptad repeat (HR2), offset from one another by three residues, were effective inhibitors of infection, while two overlapping peptides derived from the N-terminal heptad repeat (HR1) were not. Analysis of envelope mutants containing substitutions within the HR1 domain revealed that a single amino acid change, L62A, significantly reduced sensitivity to peptide inhibition. Virus bound to cells at 4 degrees C became sensitive to peptide within the first 5 min of elevating the temperature to 37 degrees C and lost sensitivity to peptide after 15 to 30 min, consistent with a transient intermediate in which the peptide binding site is exposed. In cell-cell fusion experiments, peptide inhibitor sensitivity occurred prior to a fusion-enhancing low-pH pulse. Soluble receptor for ASLV-A induces a lipophilic character in the envelope which can be measured by stable liposome binding, and this activation was found to be unaffected by inhibitory HR2 peptide. Finally, receptor-triggered conformational changes in the TM subunit were also found to be unaffected by inhibitory peptide. These changes are marked by a dramatic shift in mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, from a subunit of 37 kDa to a complex of about 80 kDa. Biotinylated HR2 peptide bound specifically to the 80-kDa complex, demonstrating a surprisingly stable envelope conformation in which the HR2 binding site is exposed. These experiments support a model in which receptor interaction promotes formation of an envelope conformation in which the TM subunit is stably associated with its target membrane and is able to bind a C-terminal peptide.     

Heat-labile enterotoxin crystal forms with variable A/B5 orientation. Analysis of conformational flexibility.

A new native crystal form of heat-labile enterotoxin (LT) has two AB5 complexes in the asymmetric unit with different orientations of the A subunit with respect to the B pentamer. Comparison with other crystal forms of LT shows that there is considerable conformational freedom for orientating the A subunit with respect to the B pentamer. The rotations of A in different crystal forms do not follow one specific axis, but most of them share a hinge point, close to the main interaction area between A and B5. Analysis of the two high-resolution structures available shows that these rotations cause very little change in the actual interactions between A and B5.