Dissection of Semliki Forest virus glycoprotein delivery from the trans-Golgi network to the cell surface in permeabilized BHK cells

The furin protease cleavage recognition sequence of Sindbis virus PE2 can mediate virion attachment to cell surface heparan sulfate

Cell culture-adapted Sindbis virus strains attach to heparan sulfate (HS) receptors during infection of cultured cells (W. B. Klimstra, K. D. Ryman, and R. E. Johnston, J. Virol. 72:7357-7366, 1998). At least three E2 glycoprotein mutations (E2 Arg 1, E2 Lys 70, and E2 Arg 114) can independently confer HS attachment in the background of the consensus sequence Sindbis virus (TR339). In the studies reported here, we have investigated the mechanism by which the E2 Arg 1 mutation confers HS-dependent binding. Substitution of Arg for Ser at E2 1 resulted in a significant reduction in the efficiency of PE2 cleavage, yielding virus particles containing a mixture of PE2 and mature E2. Presence of PE2 was associated with an increase in HS-dependent attachment to cells and efficient attachment to heparin-agarose beads, presumably because the furin recognition site for PE2 cleavage also represents a candidate HS binding sequence. A comparison of mutants with partially or completely inhibited PE2 cleavage demonstrated that efficiency of cell binding was correlated with the amount of PE2 in virus particles. Viruses rendered cleavage defective due to deletions of portions or all of the furin cleavage sequence attached very poorly to cells, indicating that an intact furin cleavage sequence was specifically required for PE2-mediated attachment to cells. In contrast, a virus containing a partial deletion was capable of efficient binding to heparin-agarose beads, suggesting different requirements for heparin bead and cell surface HS binding. Furthermore, virus produced in C6/36 mosquito cells, which cleave PE2 more efficiently than BHK cells, exhibited a reduction in cell attachment efficiency correlated with reduced content of PE2 in particles. Taken together, these results strongly argue that the XBXBBX (B, basic; X, hydrophobic) furin protease recognition sequence of PE2 can mediate the binding of PE2-containing Sindbis viruses to HS. This sequence is very similar to an XBBXBX heparin-HS interaction consensus sequence. The attachment of furin protease cleavage sequences to HS may have relevance to other viruses whose attachment proteins are cleaved during maturation at positively charged recognition sequences.

Intracellular degradation of the HIV-1 envelope glycoprotein. Evidence for, and some characteristics of, an endoplasmic reticulum degradation pathway

Analysis of the fate of HIV-1 envelope protein gp160 (Env) has shown that newly synthesized proteins may be degraded within the biosynthetic pathway and that this degradation may take place in compartments other than the lysosomes. The fate of newly synthesized Env was studied in living BHK-21 cells with the recombinant vaccinia virus expression system. We found that gp160 not only undergoes physiological endoproteolytic cleavage, producing gp120, but is also degraded, producing proteolytic fragments of 120 kDa to 26 kDa in size, as determined by SDS/PAGE in non reducing conditions. Analysis of the 120-kDa proteolytic fragment, and comparison with gp120, showed that it is composed of peptides linked by disulfides bonds and lacks the V3-loop epitope and the C-terminal domain of gp120 (amino acids 506-516). A permeabilized cell system, with impaired transport of labeled Env from the endoplasmic reticulum (ER) to Golgi compartments, was developed to determine the site of degradation and to define some biochemical characteristics of the intracellular degradation process. In the semipermeable BHK-21 cells, there was: (a) no gp120 production (b), a progressive decrease in the amount of newly synthesized gp160 and a concomitant increase in the amount of a 120-kDa proteolytic fragment. This fragment had the same biochemical characteristics as the 120-kDa proteolytic fragment found in living nonpermeabilized cells, and (c) susceptibility of the V3 loop. This degradation process occurred in the ER, as shown by both biochemical and indirect immunofluorescence analysis. Furthermore, there was evidence that changes in redox state are involved in the ER-dependent envelope degradation pathway because adding reducing agents to permeabilized cells caused dose-dependent degradation of the 120-kDa proteolytic fragment and of the remaining gp160 glycoprotein. Thus our results provide direct evidence that regulated degradation of the HIV-1 envelope glycoprotein may take place in the ER of infected cells.

Two-helper RNA system for production of recombinant Semliki forest virus particles

Alphavirus expression systems based on suicidal virus particles carrying recombinant replicons have proven to be a very efficient way to deliver genes for heterologous protein expression. However, present strategies for production of such particles have biosafety limitations due to the generation, by RNA recombination, of replication-proficient viruses (RPVs). Here we describe a new packaging system for Semliki Forest virus (SFV) based on a the use of a two-helper system in which the capsid and spike proteins of the C-p62-6K-E1 polyprotein are expressed from two independent RNA molecules. The capsid gene contains a translational enhancer and therefore that sequence was also engineered in front of the spike sequence p62-6K-E1. A sequence coding for the foot-and-mouth disease virus 2A autoprotease was inserted in frame between the capsid translational enhancer and the spike genes. This allows production of the spike proteins at high levels with cotranslational removal of the enhancer sequence and normal biosynthesis of the spike complex. The autoprotease activity of the capsid protein was abolished by mutation, further increasing the biosafety of the system. Cotransfection of cells with both helper RNAs and an SFV vector replicon carrying the LacZ gene led to production of recombinant particles with titers of up to 8 x 10(8) particles per 10(6) cells. Extensive analysis failed to demonstrate the presence of any RPVs, emphasizing the high biosafety of the system based on two-helper RNAs.

Endoplasmic reticulum and trans-Golgi network generate distinct populations of Alzheimer beta-amyloid peptides

The excessive generation and accumulation of 40- and 42-aa beta-amyloid peptides (Abeta40/Abeta42) in selectively vulnerable brain regions is a major neuropathological feature of Alzheimer's disease. Abeta, derived by proteolytic cleavage from the beta-amyloid precursor protein (betaAPP), is normally secreted. However, recent evidence suggests that significant levels of Abeta also may remain inside cells. Here, we have investigated the subcellular compartments within which distinct amyloid species are generated and the compartments from which they are secreted. Three experimental approaches were used: (i) immunofluorescence performed in intact cortical neurons; (ii) sucrose gradient fractionation performed with mouse neuroblastoma cells stably expressing wild-type betaAPP695 (N2a695); and (iii) cell-free reconstitution of Abeta generation and trafficking from N2a695 cells. These studies demonstrate that: (i) Abeta40 (Abeta1-40 plus Abetax-40, where x is an NH2-terminal truncation) is generated exclusively within the trans-Golgi Network (TGN) and packaged into post-TGN secretory vesicles; (ii) Abetax-42 is made and retained within the endoplasmic reticulum in an insoluble state; (iii) Abeta42 (Abeta1-42 plus Abetax-42) is made in the TGN and packaged into secretory vesicles; and (iv) the amyloid peptides formed in the TGN consist of two pools (a soluble population extractable with detergents and a detergent-insoluble form). The identification of the organelles in which distinct forms of Abeta are generated and from which they are secreted should facilitate the identification of the proteolytic enzymes responsible for their formation.

The first step: activation of the Semliki Forest virus spike protein precursor causes a localized conformational change in the trimeric spike

The structure of the particle formed by the SFVmSQL mutant of Semliki Forest virus (SFV) has been defined by cryo-electron microscopy and image reconstruction to a resolution of 21 A. The SQL mutation blocks the cleavage of p62, the precursor of the spike proteins E2 and E3, which normally occurs in the trans-Golgi. The uncleaved spike protein is insensitive to the low pH treatment that triggers membrane fusion during entry of the wild-type virus. The conformation of the spike in the SFVmSQL particle should correspond to that of the inactive precursor found in the early stages of the secretory pathway. Comparison of this "precursor" structure with that of the mature, wild-type, virus allows visualization of the changes that lead to activation, the first step in the pathway toward fusion. We find that the conformational change in the spike is dramatic but localized. The projecting domains of the spikes are completely separated in the precursor and close to generate a cavity in the mature spike. E1, the fusion peptide-bearing protein, interacts only with the p62 in its own third of the trimer before cleavage and then collapses to form a trimer of heterotrimers (E1E2E3)3 surrounding the cavity, poised for the pH-induced conformational change that leads to fusion. The capsid, transmembrane regions and the spike skirts (thin layers of protein that link spikes above the membrane) remain unchanged by cleavage. Similarly, the interactions of the spikes with the nucleocapsid through the transmembrane domains remain constant. Hence, the interactions that lead to virus assembly are unaffected by the SFVmSQL mutation.

Mechanisms of enveloped virus entry into animal cells

The ability of viruses to transfer macromolecules between cells makes them attractive starting points for the design of biological delivery vehicles. Virus-based vectors and sub-viral systems are already finding biotechnological and medical applications for gene, peptide, vaccine and drug delivery. Progress has been made in understanding the cellular and molecular mechanisms underlying virus entry, particularly in identifying virus receptors. However, receptor binding is only a first step and we now have to understand how these molecules facilitate entry, how enveloped viruses fuse with cells or non-enveloped viruses penetrate the cell membrane, and what happens following penetration. Only through these detailed analyses will the full potential of viruses as vectors and delivery vehicles be realised. Here we discuss aspects of the entry mechanisms for several well-characterised viral systems. We do not attempt to provide a fully comprehensive review of virus entry but focus primarily on enveloped viruses.

Adaptation of Sindbis virus to BHK cells selects for use of heparan sulfate as an attachment receptor

Attachment of Sindbis virus to the cell surface glycosaminoglycan heparan sulfate (HS) and the selection of this phenotype by cell culture adaptation were investigated. Virus (TR339) was derived from a cDNA clone representing the consensus sequence of strain AR339 (K. L. McKnight, D. A. Simpson, S. C. Lin, T. A. Knott, J. M. Polo, D. F. Pence, D. B. Johannsen, H. W. Heidner, N. L. Davis, and R. E. Johnston, J. Virol. 70:1981-1989, 1996) and from mutant clones containing either one or two dominant cell culture adaptations in the E2 structural glycoprotein (Arg instead of Ser at E2 position 1 [designated TRSB]) or this mutation plus Arg for Ser at E2 114 [designated TRSB-R114]). The consensus virus, TR339, bound to baby hamster kidney (BHK) cells very poorly. The mutation in TRSB increased binding 10- to 50-fold, and the additional mutation in TRSB-R114 increased binding 3- to 5-fold over TRSB. The magnitude of binding was positively correlated with the degree of cell culture adaptation and with attenuation of these viruses in neonatal mice. HS was identified as the attachment receptor for the mutant viruses by the following experimental results. (i) Low concentrations of soluble heparin inhibited plaque formation on and binding of mutant viruses to BHK cells by >95%. In contrast, TR339 showed minimal inhibition at high concentrations. (ii) Binding and infectivity of TRSB-R114 was sensitive to digestion of cell surface HS with heparinase III, and TRSB was sensitive to both heparinase I and heparinase III. TR339 infectivity was only slightly affected by either digestion. (iii) Radiolabeled TRSB and TRSB-R114 attached efficiently to heparin-agarose beads in binding assays, while TR339 showed virtually no binding. (iv) Binding and infectivity of TRSB and TRSB-R114, but not TR339, were greatly reduced on Chinese hamster ovary cells deficient in HS specifically or all glycosaminoglycans. (v) High-multiplicity-of-infection passage of TR339 on BHK cell cultures resulted in rapid coselection of high-affinity binding to BHK cells and attachment to heparin-agarose beads. Sequencing of the passaged virus population revealed a mutation from Glu to Lys at E2 70, a mutation common to many laboratory strains of Sindbis virus. These results suggest that TR339, the most virulent virus tested, attaches to cells through a low-affinity, primarily HS-independent mechanism. Adaptive mutations, selected during cell culture growth of Sindbis virus, enhance binding and infectivity by allowing the virus to attach by an alternative mechanism that is dependent on the presence of cell surface HS.

fus-1, a pH shift mutant of Semliki Forest virus, acts by altering spike subunit interactions via a mutation in the E2 subunit

Semliki Forest virus (SFV), an enveloped alphavirus, is a well-characterized paradigm for viruses that infect cells via endocytic uptake and low-pH-triggered fusion. The SFV spike protein is composed of a dimer of E1 and E2 transmembrane subunits, which dissociate upon exposure to low pH, liberating E2 and the fusogenic E1 subunit to undergo independent conformational changes. SFV fusion and infection are blocked by agents such as ammonium chloride, which act by raising the pH in the endosome and inhibiting the low-pH-induced conformational changes in the SFV spike protein. We have previously isolated an SFV mutant, fus-1, that requires more acidic pH to trigger its fusion activity and is therefore more sensitive to inhibition by ammonium chloride. The acid shift in the fusion activity of fus-1 was here shown to be due to a more acidic pH threshold for the initial dissociation of the fus-1 spike dimer, thereby resulting in a more acidic pH requirement for the subsequent conformational changes in both fus-1 E1 and fus-1 E2. Sequence analysis demonstrated that the fus-1 phenotype was due to a mutation in the E2 spike subunit, threonine 12 to isoleucine. fus-1 revertants that have regained the parental fusion phenotype and ammonium chloride sensitivity were shown to have also regained E2 threonine 12. Our results identify a region of the SFV E2 spike protein subunit that regulates the pH dependence of E1-catalyzed fusion by controlling the dissociation of the E1/E2 dimer.

Suppressors of cleavage-site mutations in the p62 envelope protein of Semliki Forest virus reveal dynamics in spike structure and function

The E2 spike glycoprotein of Semliki Forest virus is produced as a p62 precursor protein, which is cleaved by host proteases to its mature form, E2. Cleavage is not necessary for particle formation or release but is necessary for infectivity. Previous results had shown that phenotypic revertants of cleavage-deficient p62 mutants are generated, and here we show that these may contain second-site suppressor mutations in the vicinity of the cleavage site. These hot-spot sites were mutated to abolish the generation of such suppressor mutations; however, secondary mutations in another distant domain of the E2 protein appeared instead, all of which still caused cleavage-deficient mutations. Such mutants grew very poorly and were inefficient in virus entry and release. The mutated sites define domains of the spike protein which probably interact to regulate its structure and function. Because of their highly attenuated phenotype and the lower probability of reversion, the new mutations close to the cleavage site were used to make new helper vectors for packaging of recombinant RNA into infectious particles, thus increasing further the biosafety of the vector system based on the Semliki Forest virus replicon.

Development of a chimeric sindbis virus with enhanced per Os infection of Aedes aegypti

The TE/3'2J double subgenomic Sindbis (dsSIN) viruses have been used to stably express genes in Aedes aegypti nerve and salivary gland tissues. However, because these viruses inefficiently infect Ae. aegypti when administered by the per os route, TE/3'2J viruses must be intrathoracically inoculated into the mosquitoes to infect these tissues. A Malaysian Sindbis (SIN) virus isolate (MRE16) does efficiently infect Ae. aegypti midgut tissues after ingestion, and approximately 95% of these mosquitoes also develop disseminated infections within 14 days. We have sequenced the entire 26S RNA of MRE16 virus and have developed a chimeric SIN cDNA infectious clone, designated MRE1001, which contains sequence elements of TE/3'2J and MRE16 virus. MRE1001 virus efficiently infects midgut cells, and greater than 90% of infected mosquitoes develop disseminated infections after 14 days extrinsic incubation. The chimeric MRE1001 cDNA clone should allow identification of viral determinants of midgut infection and dissemination and lead to the development of new SIN virus expression systems.

Structural localization of the E3 glycoprotein in attenuated Sindbis virus mutants

We have determined the three-dimensional structures of the wild-type Sindbis virus and two of its mutants that retain the E3 sequence within PE2. Using difference imaging between these mutants and the wild-type virus, we have assigned a location for the 64-amino-acid sequence corresponding to E3 in the mutant spike complex. In the wild-type virus, the spike is composed of an E1-E2 heterotrimer. The E3 protein was found to protrude midway between the center of the spike complex and the tips. Based on these results and the work of others, we propose a distribution for the functional domains of the spike proteins within the structure of wild-type Sindbis virus. Within the structure of the virus, the E1 domains form the central portion of the spike complex, while the tips are formed by the E2 domains that flare out from the center of the complex. The structural similarity between these Sindbis virus mutants and Ross River virus suggests that E3 may also be present in the latter, which is also a member of the Alphavirus genus.

Oligomerization-dependent folding of the membrane fusion protein of Semliki Forest virus

The spikes of alphaviruses are composed of three copies of an E2-E1 heterodimer. The E1 protein possesses membrane fusion activity, and the E2 protein, or its precursor form, p62 (sometimes called PE2), controls this function. Both proteins are, together with the viral capsid protein, translated from a common C-p62-E1 coding unit. In an earlier study, we showed that the p62 protein of Semliki Forest virus (SFV) dimerizes rapidly and efficiently in the endoplasmic reticulum (ER) with the E1 protein originating from the same translation product (so-called heterodimerization in cis) (B.-U. Barth, J. M. Wahlberg, and H. Garoff, J. Cell Biol. 128:283-291, 1995). In the present work, we analyzed the ER translocation and folding efficiencies of the p62 and E1 proteins of SFV expressed from separate coding units versus a common one. We found that the separately expressed p62 protein translocated and folded almost as efficiently as when it was expressed from a common coding unit, whereas the independently expressed E1 protein was inefficient in both processes. In particular, we found that the majority of the translocated E1 chains were engaged in disulfide-linked aggregates. This result suggests that the E1 protein needs to form a complex with p62 to avoid aggregation. Further analyses of the E1 aggregation showed that it occurred very rapidly after E1 synthesis and could not be avoided significantly by the coexpression of an excess of p62 from a separate coding unit. These latter results suggest that the p62-E1 heterodimerization has to occur very soon after E1 synthesis and that this is possible only in a cis-directed reaction which follows the synthesis of p62 and E1 from a common coding unit. We propose that the p62 protein, whose synthesis precedes that of the E1 protein, remains in the translocon of the ER and awaits the completion of E1. This strategy enables the p62 protein to complex with the E1 protein immediately after the latter has been made and thereby to control (suppress) its fusion activity.

The nucleocapsid-binding spike subunit E2 of Semliki Forest virus requires complex formation with the E1 subunit for activity

Alphaviruses, such as Semliki Forest virus (SFV), mature by budding at the plasma membrane (PM) of infected cells and enter uninfected ones by a membrane fusion process in the endosomes. Both processes are directed by the p62/E2-E1 membrane protein heterodimer of the virus. The p62 protein, or its mature form E2, provides a cytoplasmic protein domain for interaction with the nucleocapsid (NC) of the virus, and the E1 protein functions as a membrane fusogen. We have previously shown that the p62/E2 protein of SFV controls the membrane fusion activity of E1 through its complex formation with the latter (A. Salminen, J. M. Wahlberg, M. Lobigs, P. Liljeström, and H. Garoff, J. Cell Biol. 116:349-357, 1992). In the present work, we show that the E1 protein controls the NC-binding activity of p62/E2. We have studied E1 expression-deficient SFV variants and shown that although the p62/E2 proteins can be transported to the PM they cannot establish stable NC associations.

The formation of intramolecular disulfide bridges is required for induction of the Sindbis virus mutant ts23 phenotype

The Sindbis virus envelope protein spike is a hetero-oligomeric complex composed of a trimer of glycoprotein E1-E2 heterodimers. Spike assembly is a multistep process which occurs in the endoplasmic reticulum (ER) and is required for the export of E1 from the ER. PE2 (precursor to E2), however, can transit through the secretory pathway and be expressed at the cell surface in the absence of E1. Although oligomer formation does not appear to be required for the export of PE2, there is evidence that defects in E1 folding can affect PE2 transit from the ER. Temperature-sensitive mutant ts23 of Sindbis virus contains two amino acid substitutions in E1, while PE2 and capsid protein have the wild-type sequence; however, at the nonpermissive temperature, both E1 and PE2 are retained within the ER and can be isolated in protein aggregates with the molecular chaperone GRP78-BiP. We previously demonstrated that the temperature sensitivity for ts23 was lost as oligomer formation took place at the permissive temperature, suggesting that temperature sensitivity is initiated early in the process of viral spike assembly (M. Carleton and D. T. Brown, J. Virol. 70:952-959, 1996). Experiments described herein investigated the defects in envelope protein maturation that occur in ts23-infected cells and which result in retention of both envelope proteins in the ER. The data demonstrate that in ts23-infected cells incubated at the nonpermissive temperature, E1 folding is disrupted early after synthesis, resulting in the rapid incorporation of both E1 and PE2 into disulfide-stabilized aggregates. Furthermore, the aberrant E1 conformation which is responsible for induction of the ts phenotype requires the formation of intramolecular disulfide bridges formed prior to E1 association with PE2 and the completion of E1 folding.

Botulinum neurotoxin F, a VAMP-specific endopeptidase, inhibits Ca(2+)-stimulated GH secretion from rat pituitary cells

Botulinum neurotoxin F (BoNTx F) is a zinc-dependent endopeptidase that causes proteolytic cleavage of the vesicle protein VAMP (vesicle-associated membrane protein). VAMP is an important component of the molecular machinery regulating docking and fusion of secretory vesicles with the target membrane. We have investigated presence of VAMP protein in cultured rat anterior pituitary cells. Confocal laser microscopy revealed presence of VAMP-like immunoreactivity in secretory granules of GH-containing cultured rat anterior pituitary cells. Using BoNTx F, we have investigated whether VAMP is involved in growth hormone (GH) secretion. Treatment of streptolysin-O permeabilized GH-secreting cells with BoNTx F (2.0 and 20 nM) significantly inhibited Ca(2+)-induced GH release. The results show that the secretory granules of rat anterior pituitary cell contain VAMP protein and suggest that VAMP is of importance in regulating Ca(2+)-mediated GH secretion.

Role of glycoprotein PE2 in formation and maturation of the Sindbis virus spike

Sindbis virus envelope assembly is a multistep process resulting in the maturation of a rigid, highly ordered T=4 icosahedral protein lattice containing 80 spikes composed of trimers of E1-E2 heterodimers. Intramolecular disulfide bonds within E1 stabilize E1-E1 associations required for envelope formation and maintenance of the envelope's structural integrity. The structural integrity of the envelope protein lattice is resistant to reduction by dithiothreitol (DTT), indicating that E1 disulfides which stabilize structural domains become inaccessible to DTT at some point during virus maturation. The development of E1 resistance to DTT occurs prior to the completion of E1 folding and is temporally correlated with spike assembly in the endoplasmic reticulum. From these data we have predicted that in the final stages of spike assembly, E1 intramolecular disulfides, which stabilize the structural integrity of the envelope protein lattice, are buried within the spike and become inaccessible to the reductive activity of DTT. The spike is formed prior to the completion of E1 folding, and we have suggested that PE2 (the precursor to E2) may play a critical role in E1 folding after PE2-E1 oligomer formation has occurred. In this study we have investigated the role of PE2 in E1 folding, oligomer formation, and development of E1 resistance to both protease digestion and reduction by DTT by using a Sindbis virus replicon (SINrep/E1) which allows for the expression of E1 in the presence of truncated PE2. Through pulse-chase analysis of both Sindbis virus- and SINrep/E1-infected cells, we have determined that the folding of E1 into a trypsin-resistant conformation and into its most compact and stable form is not dependent upon association of E1 with PE2. However, E1 association with PE2 is required for oligomer formation, the export of E1 from the endoplasmic reticulum, and E1 acquisition of resistance to DTT.

Efficient multiplication of a Semliki Forest virus chimera containing Sindbis virus spikes

Using the Semliki Forest virus (SFV) and Sindbis virus (SIN) cDNAs we have constructed recombinants in which the spike genes were exchanged. Analyses of expression showed that the SFV/SIN(spike) RNA directed efficient assembly of infectious virus, whereas the reciprocal SIN/SFV(spike) RNA was completely unable to assemble virus. This was apparently due to a defective capsid-spike interaction.

Targeting of Moloney murine leukemia virus gag precursor to the site of virus budding

Retrovirus Moloney murine leukemia virus (M-MuLV) matures by budding at the cell surface. Central to the budding process is the myristoylated viral core protein precursor Gag which, even in the absence of all other viral components, is capable of associating with the cytoplasmic leaflet of the plasma membrane and assembling into extracellular virus-like particles. In this paper we have used heterologous, Semliki Forest virus-driven, expression of M-MuLV Gag to study the mechanism by which this protein is targeted to the cell surface. In pulse-chase experiments, BFA, monensin, and 20 degrees C block did not affect incorporation of Gag into extracellular particles thereby indicating that the secretory pathway is not involved in targeting of Gag to the cell surface. Subcellular fractionation studies demonstrated that newly synthesized Gag became rapidly and efficiently associated with membranes which had a density similar to that of plasma membrane-derived vesicles. Protease-protection studies confirmed that the Gag-containing membranes were of plasma membrane origin, since in crude cell homogenates, the bulk of newly synthesized Gag was protease-resistant as expected of a protein that binds to the cytoplasmic leaflet of the plasma membrane. Taken together these data indicate that targeting of M-MuLV Gag to the cell surface proceeds via direct insertion of the protein to the cytoplasmic side of the plasma membrane. Furthermore, since the membrane insertion reaction is highly efficient and specific, this suggests that the reaction is dependent on as-yet-unidentified cellular factors.

Disulfide bridge-mediated folding of Sindbis virus glycoproteins

The Sindbis virus envelope is composed of 80 E1-E2 (envelope glycoprotein) heterotrimers organized into an icosahedral protein lattice with T=4 symmetry. The structural integrity of the envelope protein lattice is maintained by E1-E1 interactions which are stabilized by intramolecular disulfide bonds. Structural domains of the envelope proteins sustain the envelope's icosahedral lattice, while functional domains are responsible for virus attachment and membrane fusion. We have previously shown that within the mature Sindbis virus particle, the structural domains of the envelope proteins are significantly more resistant to the membrane-permeative, sulfhydryl-reducing agent dithiothreitol (DTT) than are the functional domains (R. P. Anthony, A. M. Paredes, and D. T. Brown, Virology 190:330-336, 1992). We have used DTT to probe the accessibility of intramolecular disulfides within PE2 (the precursor to E2) and E1, as these proteins fold and are assembled into the spike heterotrimer. We have determined through pulse-chase analysis that intramolecular disulfide bonds within PE2 are always sensitive to DTT when the glycoproteins are in the endoplasmic reticulum. The reduction of these disulfides results in the disruption of PE2-E1 associations. E1 acquires increased resistance to DTT as it folds through a series of disulfide intermediates (E1alpha, -beta, and -gamma) prior to assuming its native and most compact conformation (E1epsilon). The transition from a DTT-sensitive form into a form which exhibits increased resistance to DTT occurs after E1 has folded into its E1beta conformation and correlates temporally with the dissociation of BiP-E1 complexes and the formation of PE2-E1 heterotrimers. We propose that the disulfide bonds within E1 which stabilize the protein domains required for maintaining the structural integrity of the envelope protein lattice form early within the folding pathway of E1 and become inaccessible to DTT once the heterotrimer has formed.

Wortmannin-sensitive trafficking pathways in Chinese hamster ovary cells. Differential effects on endocytosis and lysosomal sorting

Phosphatidylinositol (PI) 3'-kinases are a family of lipid kinases implicated in the regulation of cell growth by oncogene products and tyrosine kinase growth factor receptors. The catalytic subunit of the p85/p110 PI 3'-kinase is homologous to VPS-34, a phosphatidylinositol-specific lipid kinase involved in the sorting of newly synthesized hydrolases to the yeast vacuole. This suggests that PI 3'-kinases may play analogous roles in mammalian cells. We have measured a number of secretory and endocytic trafficking events in Chinese hamster ovary cells in the presence of wortmannin, a potent inhibitor of PI 3'-kinase. Wortmannin caused a 40-50% down-regulation of surface transferrin receptors, with a dose dependence identical to that required for maximal inhibition of the p85/p110 PI 3'-kinase in intact cells. The redistribution of transferrin receptors reflected a 60% increase in the internalization rate and a 35% decrease in the recycling rate. Experiments with fluorescent transferrin showed that entry of transferrin receptors into the recycling compartment and efflux of receptors out of the compartment were slowed by wortmannin. Wortmannin altered the morphology of the recycling compartment, which was more vesiculated than in untreated cells. Using Semliki Forest virus as a probe, we also found that delivery of the endocytosed virus to its lysosomal site of degradation was slowed by wortmannin, whereas endosomal acidification was unaffected. In contrast to these effects on endocytosis and recycling, wortmannin did not affect intracellular processing of newly synthesized viral spike proteins. Wortmannin did induce missorting of the lysosomal enzyme cathepsin D to the secretory pathway, but only at a dose 20-fold greater than that required to inhibit p85/p110 PI 3'-kinase activity or to redistribute transferrin receptors. Our data demonstrate the presence of wortmannin-sensitive enzymes at three distinct steps of the endocytic cycle in Chinese hamster ovary cells: internalization, transit from early endosomes to the recycling and degradative compartments, and transit from the recycling compartment back to the cell surface. The wortmannin-sensitive enzymes critical for endocytosis and recycling are distinct from those involved in sorting newly synthesized lysosomal enzymes.

Metabolism of Alzheimer beta-amyloid precursor protein: regulation by protein kinase A in intact cells and in a cell-free system

Various compounds that affect signal transduction regulate the relative utilization of alternative processing pathways for the beta-amyloid precursor protein (beta APP) in intact cells, increasing the production of nonamyloidogenic soluble beta APP (s beta APP) and decreasing that of amyloidogenic beta-amyloid peptide. In a recent study directed toward elucidating the mechanisms underlying phorbol ester-stimulated s beta APP secretion from cells, it was demonstrated that protein kinase C increases the formation from the trans-Golgi network (TGN) of beta APP-containing secretory vesicles. Here we present evidence that forskolin increases s beta APP production from intact PC12 cells, and protein kinase A stimulates formation from the TGN of beta APP-containing vesicles. Although protein kinase A and protein kinase C converge at the level of formation from the TGN of beta APP-containing vesicles, additional evidence indicates that the regulatory mechanisms involved are distinct.

A novel Ca2+-binding protein, p22, is required for constitutive membrane traffic

We have identified a novel protein, p22, required for "constitutive" exocytic membrane traffic. p22 belongs to the EF-hand superfamily of Ca2+-binding proteins and shows extensive similarity to the regulatory subunit of protein phosphatase 2B, calcineurin B. p22 is a cytosolic N-myristoylated protein that undergoes conformational changes upon binding of Ca2+. Antibodies against a p22 peptide block the targeting/fusion of transcytotic vesicles with the apical plasma membrane, but recombinant wild-type p22 overcomes that inhibition. Nonmyristoylated p22, or p22 incapable of undergoing Ca2+-induced conformational changes, cannot reverse the antibody-mediated inhibition. The data suggest that p22 may act by transducing cellular Ca2+ signals to downstream effectors. p22 is ubiquitously expressed, and we propose that its function is required for membrane trafficking events common to many cells.

Different biosynthetic transport routes to the plasma membrane in BHK and CHO cells

The question of how membrane proteins are delivered from the TGN to the cell surface in fibroblasts has received little attention. In this paper we have studied how their post-Golgi delivery routes compare with those in epithelia] cells. We have analyzed the transport of the vesicular stomatitis virus G protein, the Semliki Forest virus spike glycoprotein, both basolateral in MDCK cells, and the influenza virus hemagglutinin, apical in MDCK cells. In addition, we also have studied the transport of a hemagglutinin mutant (Cys543Tyr) which is basolateral in MDCK cells. Aluminum fluoride, a general activator of heterotrimeric G proteins, inhibited the transport of the basolateral cognate proteins, as well as of the hemagglutinin mutant, from the TGN to the cell surface in BHK and CHO cells, while having no effect on the surface delivery of the wild-type hemagglutinin. Only wild-type hemagglutinin became insoluble in the detergent CHAPS during transport through the BHK and CHO Golgi complexes, whereas the basolateral marker proteins remained CHAPS-soluble. We also have developed an in vitro assay using streptolysin O-permeabilized BHK cells, similar to the one we have previously used for analyzing polarized transport in MDCK cells (Pimplikar, S.W., E. Ikonen, and K. Simons. 1994. J. Cell Biol. 125:1025-1035). In this assay anti-NSF and rab-GDI inhibited transport of Semliki Forest virus spike glycoproteins from the TGN to the cell surface while having little effect on transport of the hemagglutinin. Altogether these data suggest that fibroblasts have apical and basolateral cognate routes from the TGN to the plasma membrane.

Differential processing of sindbis virus glycoprotein PE2 in cultured vertebrate and arthropod cells

A step in the maturation of Sindbis virus glycoproteins is the cleavage of the precursor glycoprotein PE2 into E3 and E2 by furin or a furin-like host cell protease. The results presented here suggest that PE2 cleavage is an obligatory event for Sindbis virus maturation in C6/36 cells and demonstrate that certain mutants display a cell-specific PE2 cleavage phenotype. We previously have described Sindbis virus variants which fail to cleave PE2 because of incorporation of a signal for N-linked glycosylation immediately adjacent to the PE2 cleavage site but are viable in BHK-21 cells by virtue of an additional mutation at E2 216 or E2 191 (TRSB-NE2G216 and TRSB-NE2T191, respectively) (H. W. Heidner, K. L. McKnight, N. L. Davis, and R. E. Johnston, J. Virol. 68:2683-2692, 1994). Other viable PE2 cleavage-defective mutants were constructed by substituting the parental residue at E2 position 1 (Arg), with Leu or Val (TRSB-E2L1 and TRSB-E2V1, respectively) (H.W. Heidner and R. E. Johnston, J. Virol. 68:8064-8070, 1994). When grown in BHK-21 cells, all four of these viruses replicated normally and incorporated PE2 in place of E2 in released virions. However, growth of TRSB-NE2G216 and TRSB-NE2T191 was severely restricted in cultured arthropod cells (C6/36 cells). Analysis of infected C6/36 cells by flow cytometry demonstrated that the restricted growth of TRSB-NE2G216 and TRSB-NE2T191 was not due to an impaired ability to initiate infection. In addition, TRSB-NE2G216 and TRSB-NE2T191 remained growth restricted in C6/36 cells following introduction of in vitro transcriptions by electroporation. In contrast, the PE2 cleavage defect of TRSB-E2L1 and TRSB-E2V1 was cell type specific. In C6/36 cells, the majority of PE2 was converted to E2, and these viruses replicated normally in C6/36 cells. These results demonstrated a consistent link between expression of a PE2 cleavage defect and restricted growth in C6/36 cells and suggest that cleavage of PE2 is required for maturation of Sindbis virus late in infection of C6/36 cells.

Events in the endoplasmic reticulum abrogate the temperature sensitivity of Sindbis virus mutant ts23

Temperature-sensitive mutations in proteins produced at or heated to a nonpermissive temperature render the proteins defective in some aspect of their maturation into functional entities. The characterization of temperature-sensitive mutations in model proteins, such as virus membrane proteins, has allowed the elucidation of critical events in the maturation of virus membranes as well as in the intracellular folding, processing, and transport of membrane proteins in general. We have used a transport-defective, temperature-sensitive mutant of Sindbis virus, ts23, which has two amino acid changes in the envelope protein E1, to further examine requirements placed upon the glycoproteins for their export to the plasma membrane. Pulse-chase experiments in which we utilized the transport inhibitors monensin and brefeldin A allowed us to synthesize and assemble the glycoproteins of ts23 into export-competent heterodimers at the permissive temperature while concurrently blocking their export to the cell surface. After removal of the inhibitors and a shift to the nonpermissive temperature, we assayed for protein transport, cell-cell fusion, and infectious-particle production. Taken together, the data show that the irreversible loss of the temperature-sensitive phenotype of ts23 can be correlated with the folding of E1 and the formation of export-competent PE2-E1 heterodimers in the endoplasmic reticulum. Furthermore, we have found that E1 pairs with PE2 to form the heterodimer prior to the completion of E1 folding.

Endoproteolytic cleavage of HIV-1 gp160 envelope precursor occurs after exit from the trans-Golgi network (TGN)

Endoproteolytic processing of human immunodeficiency virus type 1 (HIV-1) gp160 membrane glycoprotein precursor into gp 120 and gp41 is necessary for formation of infectious HIV particles [1]. We have studied the intracellular site of this processing using inhibition of transport at reduced temperature (20 degrees C). That reduced temperature (20 degrees C) inhibits the intracellular transport also in Jurkat-tat cells was demonstrated using the Semliki Forest virus p62 precursor processing as model. In HIV-1 infected Jurkat-tat cells the proteolytic processing of gp 160 precursor did not occur when the protein was accumulated in the TGN at 20 degrees C temperature. When the temperature was shifted to 37 degrees C the HIV-1 gp 160, which had accumulated in the TGN at the reduced temperature, was proteolytically processed. The processing of gp 160 was inhibited when the temperature reversion was carried out in the presence of brefeldin A (BFA) or aluminium fluoride (ALFn) indicating that the exit from the TGN is required for the proteolytic cleavage of HIV-1 gp160 precursor. The results suggest that the processing of gp 160 takes place at a yet unidentified transport step which is distal to the TGN/20 degrees C block site.

Communication of post-Golgi elements with early endocytic pathway: regulation of endoproteolytic cleavage of Semliki Forest virus p62 precursor

A number of cellular proteins and viral spike proteins are cleaved at a basic recognition sequence. To characterize the membrane traffic step at which this proteolysis occurs we have studied the intracellular processing site of Semliki Forest virus (SFV) spike precursor p62 in BHK21 cells. The p62 is endoproteolytically cleaved at a tetrabasic Arg-His-Arg-Arg recognition sequence. Previously, it has been shown that the SFV p62 remains uncleaved when accumulated to the trans-Golgi network (TGN/20 degrees C block site). We show here that exit from the trans-Golgi is required for the cleavage of p62. Proteolytic processing was inhibited in synchronized assays when the 20 degrees C transport block was released in the presence of brefeldin A, energy inhibitors (azide and deoxyglucose; carbonyl cyanide m-chlorophenylhydrazone, CCCP) or an effector of trimeric G proteins, AlFn. Endocytosed antibodies against the SFV spike glycoproteins or antibodies against a peptide corresponding to the enzymatically active motif of furin inhibited cleavage of p62 at a post-TGN location. The results indicate a post-TGN communication step between exocytic and endocytic elements. Kinetic experiments suggested that this communication may involve an early compartment of the endocytic pathway.

Mutations in the putative fusion peptide of Semliki Forest virus affect spike protein oligomerization and virus assembly

The two transmembrane spike protein subunits of Semliki Forest virus (SFV) form a heterodimeric complex in the rough endoplasmic reticulum. This complex is then transported to the plasma membrane, where spike-nucleocapsid binding and virus budding take place. By using an infectious SFV clone, we have characterized the effects of mutations within the putative fusion peptide of the E1 spike subunit on spike protein dimerization and virus assembly. These mutations were previously demonstrated to block spike protein membrane fusion activity (G91D) or cause an acid shift in the pH threshold of fusion (G91A). During infection of BHK cells at 37 degrees C, virus spike proteins containing either mutation were efficiently produced and transported to the plasma membrane, where they associated with the nucleocapsid. However, the assembly of mutant spike proteins into mature virions was severely impaired and a cleaved soluble fragment of E1 was released into the medium. In contrast, incubation of mutant-infected cells at reduced temperature (28 degrees C) dramatically decreased E1 cleavage and permitted assembly of morphologically normal virus particles. Pulse-labeling studies showed that the critical period for 28 degrees C incubation was during virus assembly, not spike protein synthesis. Thus, mutations in the putative fusion peptide of SFV confer a strong and thermoreversible budding defect. The dimerization of the E1 spike protein subunit with E2 was analyzed by using either cells infected with virus mutants or mutant virus particles assembled at 28 degrees C. The altered-assembly phenotype of the G91D and G91A mutants correlated with decreased stability of the E1-E2 dimer.

Structure-function relation of the NH2-terminal domain of the Semliki Forest virus capsid protein

The capsid (C) protein of alphaviruses consists of two protein domains: a serine protease at the COOH terminus and an NH2-terminal domain which is thought to interact with RNA in the virus nucleocapsid (NC). The latter domain is very rich in positively charged amino acid residues. In this work, we have introduced large deletions into the corresponding region of a full-length cDNA clone of Semliki Forest virus, expressed the transcribed RNA in BHK-21 cells, and monitored the autoprotease activity of C, the formation of intracellular NCs, and the release of infectious virus. Our results show that if the gene region encoding the whole NH2-terminal domain is removed, the expressed C protein fragment cannot assemble into NCs and virus particles but it is still able to function as an autoprotease. Thus, these results underline the general importance of the NH2-terminal domain in the virus assembly process and furthermore show that the serine protease domain can function independently of the NH2 terminus. Surprisingly, analysis of additional C protein deletion variants showed that not all of the NH2-terminal domain is required for virus assembly, but large deletions involving up to one-third of its positively charged residues are still compatible with NC and virus formation. The fact that so much flexibility is allowed in the structure of the NH2-terminal domain of C suggests that most of this region is involved in nonspecific interactions with the encapsidated RNA, probably through its positively charged amino acid residues.

The oligomerization reaction of the Semliki Forest virus membrane protein subunits

The Semliki Forest virus (SFV) spike is composed of three copies of a membrane protein heterodimer. The two subunits of this heterodimer (p62 and E1) are synthesized sequentially from a common mRNA together with the capsid (C) in the order C-p62-E1. In this work heterodimerization of the spike proteins has been studied in BHK 21 cells. The results indicate that: (a) the polyprotein is cotranslationally cleaved into individual chains; (b) the two membrane protein subunits are initially not associated with each other in the endoplasmic reticulum (ER); (c) heterodimerization occurs predominantly between subunits that originate from the same translation product (heterodimerization in cis); (d) the kinetics of subunit association are very fast (t1/2 = 4 min); and (e) this heterodimerization is highly efficient. To explain the cis-directed heterodimerization reaction we suggest that the p62 protein, which is made before E1 during 26S mRNA translation, is retained at its translocation site until also the E1 chain has been synthesized and translocated at this same site. The mechanism for p62 retention could either be that the p62 anchor sequence cannot diffuse out from an "active" translocation site or that the p62 protein is complexed with a protein folding facilitating machinery that is physically linked to the translocation apparatus.

The amino-terminal residue of Sindbis virus glycoprotein E2 influences virus maturation, specific infectivity for BHK cells, and virulence in mice

The E2 glycoprotein of Sindbis virus is synthesized as a precursor, PE2, which is cleaved by furin or a furin-like host cell protease at a late stage of maturation. The four-residue PE2 cleavage signal conforms to the basic amino acid-X-basic-basic motif which is present in many other viral and cellular glycoproteins which are processed by the cellular enzyme(s). In this report, we present evidence that the amino acid which immediately follows the signal, the N-terminal residue of E2, can influence protease recognition, binding, and/or cleavage of PE2. Constructs encoding nine different amino acids at E2 position 1 (E2 1) were produced by site-directed mutagenesis of the full-length cDNA clone of our laboratory strain of Sindbis virus AR339 (pTRSB). Viruses derived from clones encoding Arg (TRSB), Asp, Ser, Phe, His, and Asn in a nonglycosylated form at E2 1 contained predominantly E2. Viruses encoding Ile, Leu, or Val at E2 1 contained the uncleaved form of PE2. The specific infectivity of TRSB (E2 Arg-1) for baby hamster kidney (BHK-21) cells was from 5- to greater than 100-fold higher than those of isogenic constructs with other residues at E2 1, suggesting that E2 Arg-1 represents a BHK-21 cell adaptive mutation in our laboratory strain. In newborn CD-1 mice, TRSB was more virulent than the PE2-containing viruses but less virulent than other PE2-cleaving viruses with alternative amino acids at E2 1. These results indicate that in TRSB, E2 Arg-1 increased the efficiency of virus-cell interactions in cultured BHK-21 cells but simultaneously decreased the ability of virus to mediate in vivo virus-cell interactions critical for the induction of disease. This suggests that the N terminus of E2 may participate in or be associated with virion domains which mediate these viral functions.

A significantly improved Semliki Forest virus expression system based on translation enhancer segments from the viral capsid gene

We recently described a system for heterologous gene expression in a variety of mammalian cell types that is based on an efficiently replicating Semliki Forest virus (SFV) variant in which an RNA encoding a foreign protein replaces the RNA that normally encodes the viruses' structural polyprotein. Although expression levels are sufficiently high for many purposes, in general they are only 10% of the level of the polyprotein in a wild type SFV infection. Here we show that the first 102 bases of the viral capsid gene function as a translational enhancer, and that SFV vectors incorporating this RNA increase heterologous protein synthesis to the level of wild type polyprotein.

Visualization of fusion activation in the Semliki Forest virus spike

BACKGROUND

Viral spike proteins such as those of Semliki Forest virus (SFV) undergo a conformational change triggered by low pH which results in the fusion of the viral envelope with cellular membranes. The viral spike precursor of SFV is insensitive to low pH, and hence is fusion incompetent, until it is proteolytically cleaved to give the fusion competent mature form.

RESULTS

Three-dimensional image reconstructions from cryo-electron micrographs were used to compare the virion structure of wild-type SFV with that of a mutant SFV in which cleavage of the spike precursor had been blocked. Upon maturation to the fusion competent form, the spike undergoes a conformational change in which copies of the polypeptide containing the fusion sequence (E1) move from peripheral to lateral positions bringing them closer together.

CONCLUSIONS

This first visualization of the maturation of a viral spike protein complex suggests a mechanism for the conformational change which controls the fusion process.

The alphaviruses: gene expression, replication, and evolution

The alphaviruses are a genus of 26 enveloped viruses that cause disease in humans and domestic animals. Mosquitoes or other hematophagous arthropods serve as vectors for these viruses. The complete sequences of the +/- 11.7-kb plus-strand RNA genomes of eight alphaviruses have been determined, and partial sequences are known for several others; this has made possible evolutionary comparisons between different alphaviruses as well as comparisons of this group of viruses with other animal and plant viruses. Full-length cDNA clones from which infectious RNA can be recovered have been constructed for four alphaviruses; these clones have facilitated many molecular genetic studies as well as the development of these viruses as expression vectors. From these and studies involving biochemical approaches, many details of the replication cycle of the alphaviruses are known. The interactions of the viruses with host cells and host organisms have been exclusively studied, and the molecular basis of virulence and recovery from viral infection have been addressed in a large number of recent papers. The structure of the viruses has been determined to about 2.5 nm, making them the best-characterized enveloped virus to date. Because of the wealth of data that has appeared, these viruses represent a well-characterized system that tell us much about the evolution of RNA viruses, their replication, and their interactions with their hosts. This review summarizes our current knowledge of this group of viruses.

The alphaviruses: gene expression, replication, and evolution

The alphaviruses are a genus of 26 enveloped viruses that cause disease in humans and domestic animals. Mosquitoes or other hematophagous arthropods serve as vectors for these viruses. The complete sequences of the +/- 11.7-kb plus-strand RNA genomes of eight alphaviruses have been determined, and partial sequences are known for several others; this has made possible evolutionary comparisons between different alphaviruses as well as comparisons of this group of viruses with other animal and plant viruses. Full-length cDNA clones from which infectious RNA can be recovered have been constructed for four alphaviruses; these clones have facilitated many molecular genetic studies as well as the development of these viruses as expression vectors. From these and studies involving biochemical approaches, many details of the replication cycle of the alphaviruses are known. The interactions of the viruses with host cells and host organisms have been exclusively studied, and the molecular basis of virulence and recovery from viral infection have been addressed in a large number of recent papers. The structure of the viruses has been determined to about 2.5 nm, making them the best-characterized enveloped virus to date. Because of the wealth of data that has appeared, these viruses represent a well-characterized system that tell us much about the evolution of RNA viruses, their replication, and their interactions with their hosts. This review summarizes our current knowledge of this group of viruses.

Incorporation of homologous and heterologous proteins into the envelope of Moloney murine leukemia virus

The efficiencies with which homologous and heterologous proteins are incorporated into the envelope of Moloney murine leukemia virus (M-MuLV) have been analyzed by utilizing a heterologous, Semliki Forest virus-driven M-MuLV assembly system and quantitative pulse-chase assays. Homologous M-MuLV spike protein was found to be efficiently incorporated into extracellular virus particles when expressed at a relatively low density at the plasma membrane. In contrast, efficient incorporation of heterologous proteins (the spike complex of Semliki Forest virus and a cytoplasmically truncated mutant of the human transferrin receptor) was observed only when these proteins were expressed at high densities at the cell surface. These results imply that homologous and heterologous proteins are incorporated into the M-MuLV envelope via two distinct pathways.

Lethality of PE2 incorporation into Sindbis virus can be suppressed by second-site mutations in E3 and E2

Sindbis virions contain two glycoproteins, E1 and E2. E2 is produced initially as a precursor, PE2, from which the amino-terminal 64 amino acids are cleaved by a cellular protease at a late stage in virion maturation. A mutation at E2 position 1 (Arg to Asn) was placed into Sindbis virus AR339 by site-directed mutagenesis of a full-length AR339 cDNA clone, pTRSB, to produce pTRSB-N. The mutation created a signal for N-linked glycosylation immediately adjacent to the PE2 cleavage signal. Virions derived from pTRSB-N were glycosylated at E2 position 1, and they quantitatively incorporated PE2 in place of E2. When pTRSB-N transcripts were electroporated into BHK-21 cells, TRSB-N particles were released with nearly normal efficiency; however, the specific infectivity of TRSB-N particles was very low. Analysis of seven infectious revertants of TRSB-N revealed that reversion was linked to (i) mutations that eliminated the signal for N-linked glycosylation and thus restored the PE2 cleavage phenotype or (ii) conservation of the PE2 cleavage defect combined with incorporation of suppressor mutations in E3 or E2. The genotype of each revertant was reconstructed in the genetic background of TRSB-N, and each reverting mutation also was replaced individually into the genetic background of wild-type virus (TRSB). Each PE2-containing revertant was attenuated in newborn CD-1 mice and replicated poorly in cultured mosquito cells (C6/36). Reverting mutations in the genetic background of TRSB did not reduce virulence in mice or growth in mosquito cells, suggesting that the phenotypes of attenuation in mice and reduced growth in mosquito cells were linked to failure of PE2 cleavage and not to the reverting mutations themselves.

Temperature-sensitive steps in the transport of Semliki Forest virus envelope proteins in mosquito C6/36 cells

We have analysed the temperature dependence of the transport of Semliki Forest virus (SFV) envelope proteins in mosquito cells, the natural host cells of alphaviruses. These cells are cultivated at a lower temperature (28 degrees C) and have a different lipid composition as compared to mammalian cells. When the incubation temperature was reduced at early times after infection, the onset of virus shedding was delayed and the maximal titers decreased correspondingly to the temperature. No virus was shed at 12 degrees C. No evidence was observed for a block of virus release due to a shift of the sites of virus maturation. When the incubation temperature was reduced at later times after infection a critical temperature of 12 degrees C was again observed. At this temperature no transport of viral proteins took place, p62 remained uncleaved, the glycan processing of E1 did not occur and the envelope proteins accumulated in a pre-Golgi compartment. We suggest a mathematical formula which allows the extrapolation of transport data to the temperature at which intracellular protein transport becomes blocked.

Formation and rearrangement of disulfide bonds during maturation of the Sindbis virus E1 glycoprotein

The rigidly ordered icosahedral lattice of the Sindbis virus envelope is composed of a host-derived membrane bilayer in which the viral glycoproteins E1 and E2 reside. E1-E1 interactions stabilized by intramolecular disulfide bridges play a significant role in maintaining the envelope's structural integrity (R. P. Anthony and D. T. Brown, J. Virol. 65:1187-1194, 1991; R. P. Anthony, A. M. Paredes, and D. T. Brown, Virology 190:330-336, 1992). We have examined the acquisition of disulfide bridges within E1 during its maturation. Prior to exit from the endoplasmic reticulum, E1 folds via at least three intermediates, differing in the number and/or arrangement of their disulfides, into a single, compact form. This E1 species remains stable with respect to its disulfides until late in the secretory pathway, when E1 attains a metastable conformation. At this point, when appropriately triggered, intramolecular thiol-disulfide exchange reactions within E1 can occur, resulting in the generation of alternative E1 species. This metastable nature of mature E1 may have important implications for the mechanism of virus disassembly during the initial stages of the infection process (B. Abell and D. T. Brown, J. Virol. 67:5496-5501, 1993).

Assembly and entry mechanisms of Semliki Forest virus

The alphavirus Semliki Forest (SFV) is an enveloped virus with a positive single-stranded RNA genome. The genome is complexed with 240 copies of a capsid protein into a nucleocapsid structure. In the membrane the virus carries an equal number of copies of a membrane protein heterodimer. The latter oligomers are grouped into clusters of three. These structures form the spikes of the virus and carry its entry functions, that is receptor binding and membrane fusion activity. The membrane protein heterodimer is synthesized as a p62E1 precursor protein which upon transport to the cell surface is cleaved into the mature E2E1 form. Recent studies have given much new information on the assembly and entry mechanism of this simple RNA virus. Much of this work has been possible through the construction of a complete cDNA clone of the SFV genome which can be used for in vitro transcription of infectious RNA. One important finding has been to show that a spike deletion variant and a capsid protein deletion variant are budding-negative when expressed separately but can easily complement each other when transfected into the same cell. This shows clearly that enveloped viruses use different budding strategies: one which depends on a nucleocapsid-spike interaction as exemplified by SFV and another one which is based on a direct core-lipid bilayer interaction as shown before to be the case with retroviruses. Another important finding concerns the activation process of the presumed fusion protein of SFV, the E1 subunit. In the original p62E1 heterodimer E1 is completely inactive. Activation proceeds in several steps. First p62 cleavage activates the potential for low pH inducible fusion. Next the low pH which surrounds incoming virus in endosomes induces dissociation of the heterodimeric structure. This is followed by a rearrangement of E1 subunits into homotrimers which are fusion active.

Differences in antigenicity of E2 in Semliki Forest virus particles and in infected cells

Using six monoclonal antibodies to epitopes a-f on the glycoprotein E2 of Semliki Forest virus (SFV) we found antigenic differences between E2 in infected cells and in virus particles, respectively, if glycosylation was impaired by 2-deoxy-D-glucose or inhibited by N-methyl-1-deoxynojirimycin. Furthermore we concluded that a conformational change of E2 takes place on virus budding.

Cholesterol is required in the exit pathway of Semliki Forest virus

The enveloped alphavirus Semliki Forest virus (SFV) infects cells via a membrane fusion reaction triggered by low pH. For fusion to occur cholesterol is required in the target membrane, as demonstrated both in in vitro fusion assays and in vivo for virus infection of a host cell. In this paper we examine the role of cholesterol in postfusion events in the SFV life cycle. Cholesterol-depleted insect cells were transfected with SFV RNA or infected at very high multiplicities to circumvent the fusion block caused by the absence of cholesterol. Under these conditions, the viral spike proteins were synthesized and transported to the site of p62 cleavage with normal kinetics. Surprisingly, the subsequent exit of virus particles was dramatically slowed compared to cholesterol-containing cells. The inhibition of virus production could be reversed by the addition of cholesterol to depleted cells. In contrast to results with SFV, no cholesterol requirement for virus exit was observed for the production of either the unrelated vesicular stomatitis virus or a cholesterol-independent SFV fusion mutant. Thus, cholesterol was only critical in the exit pathway of viruses that also require cholesterol for fusion. These results demonstrate a specific and unexpected lipid requirement in virus exit, and suggest that in addition to its role in fusion, cholesterol is involved in the assembly or budding of SFV.

Prohormone processing in the trans-Golgi network: endoproteolytic cleavage of prosomatostatin and formation of nascent secretory vesicles in permeabilized cells

Many peptide hormones are synthesized as larger precursors which undergo endoproteolytic cleavage at paired basic residues to generate a bioactive molecule. Morphological evidence from several laboratories has implicated either the TGN or immature secretory granules as the site of prohormone cleavage. To identify the site where prohormone cleavage is initiated, we have used retrovirally infected rat anterior pituitary GH3 cells which express high levels of prosomatostatin (proSRIF) (Stoller, T. J., and D. Shields. J. Cell Biol. 1988. 107:2087-2095). By incubating these cells at 20 degrees C, a temperature that prevents exit from the Golgi apparatus, proSRIF accumulated quantitatively in the TGN and no proteolytic processing was evident; processing resumed upon shifting the cells back to 37 degrees C. After the 20 degrees C block, the cells were mechanically permeabilized and pro-SRIF processing determined. Cleavage of proSRIF to the mature hormone was approximately 35-50% efficient, required incubation at 37 degrees C and ATP hydrolysis, but was independent of GTP or cytosol. The in vitro ATP-dependent proSRIF processing was inhibited by inclusion of chloroquine, a weak base, CCCP, a protonophore, or by preincubating the permeabilized cells with low concentrations of N-ethylmaleimide, an inhibitor of vacuolar-type ATP-dependent proton pumps. These data suggest that: (a) proSRIF cleavage is initiated in the TGN, and (b) this reaction requires an acidic pH which is facilitated by a Golgi-associated vacuolar-type ATPase. A characteristic feature of polypeptide hormone-producing cells is their ability to store the mature hormone in dense core secretory granules. To investigate the mechanism of protein sorting to secretory granules, the budding of nascent secretory vesicles from the TGN was determined. No vesicle formation occurred at 20 degrees C; in contrast, at 37 degrees C, the budding of secretory vesicles was approximately 40% efficient and was dependent on ATP, GTP, and cytosolic factors. Vesicle formation was inhibited by GTP gamma S suggesting a role for GTP-binding proteins in this process. Vesicle budding was dependent on cytosolic factors that were tightly membrane associated and could be removed only by treating the permeabilized cells with high salt. After high salt treatment, vesicle formation was dependent on added cytosol or the dialyzed salt extract. The formation of nascent secretory vesicles contrasts with prosomatostatin processing which required only ATP for efficient cleavage. Our results demonstrate that prohormone cleavage which is initiated in the TGN, precedes vesicle formation and that processing can be uncoupled from the generation of nascent secretory vesicles.

Palmitoylation of Semliki Forest virus glycoproteins in insect cells (C6/36) occurs in an early compartment and is coupled to the cleavage of the precursor p62

The acylation of the envelope proteins of Semliki Forest virus by palmitic acid in infected mosquito (C6/36) cells was investigated. It is shown that in these cells palmitic acid was incorporated post-translationally via hydroxylamine-labile linkages onto cysteines in the inner domains of the viral envelope proteins. The kinetics of incorporation, however, differed considerably as compared to higher eukaryotic cells. (i) The precursor of the envelope proteins E2 and E3, p62, was weakly and incompletely palmitoylated irrespective of the duration of labeling. (ii) Under all conditions tested complete acylation of E2 was delayed as compared to E1. (iii) Heavy protein complexes were formed consisting of unacylated p62 and partially unacylated E1. From this data, we conclude that during the maturation of SFV glycoproteins in mosquito cells differently acylated intermediates of p62/E2 exist. Furthermore, acylation of p62/E2 and cleavage of p62 are coupled events, occurring in an early compartment and allowing the release of the envelope oligomers for transport.

Semliki Forest virus expression system: production of conditionally infectious recombinant particles

In the recently developed Semliki Forest virus (SFV) DNA expression system, recombinant RNA encoding the viral replicase, and helper RNA molecules encoding the structural proteins needed for virus assembly are cotransfected into cells. Since the helper RNA lacks the sequence needed for its packaging into nucleocapsids, only recombinant RNAs should be packaged. We have found, however, that small amounts of replication-proficient SFV particles can still be produced. Here we describe the construction of a helper variant with a mutation in the gene encoding the viral spike protein such that its product cannot undergo normal proteolytic processing to activate viral entry functions. Hence, the recombinant stock is noninfectious, but may be activated by cleavage with chymotrypsin. When recombinant virus produced with the new helper was examined in a variety of assays, including sensitive animal tests, we were unable to detect any replication-competent SFV particles. We therefore conclude that this conditional expression system meets extremely stringent biosafety requirements.