The amino-terminal nine amino acid sequence of poliovirus capsid VP4 protein is sufficient to confer N-myristoylation and targeting to detergent-insoluble membranes

Self-organization of apical membrane protein sorting in epithelial cells

Polarized epithelial cells are characterized by the asymmetric distribution of proteins between apical and basolateral domains of the plasma membrane. This asymmetry is highly conserved and is fundamental to epithelial cell physiology, development, and homeostasis. How proteins are segregated for apical or basolateral delivery, a process known as sorting, has been the subject of considerable investigation for decades. Despite these efforts, the rules guiding apical sorting are poorly understood and remain controversial. Here, we consider mechanisms of apical membrane protein sorting and argue that they are largely driven by self-organization and biophysical principles. The preponderance of data to date is consistent with the idea that apical sorting is not ruled by a dedicated protein-based sorting machinery and relies instead on the concerted effects of oligomerization, phase separation of lipids and proteins in membranes, and pH-dependent glycan interactions.

Myristoylation of EV71 VP4 is Essential for Infectivity and Interaction with Membrane Structure

The Enterovirus 71 (EV71) VP4 is co-translationally linked to myristic acid at its amino-terminal glycine residue. However, the role of this myristoylation in the EV71 life cycle remains largely unknown. To investigate this issue, we developed a myristoylation-deficient virus and reporter (luciferase) pseudovirus with a Gly-to-Ala mutation (G2A) on EV71 VP4. When transfecting the EV71-G2A genome encoding plasmid in cells, the loss of myristoylation on VP4 did not affect the expression of viral proteins and the virus morphology, however, it did significantly influence viral infectivity. Further, in myristoylation-deficient reporter pseudovirus-infected cells, the luciferase activity and viral genome RNA decreased significantly as compared to that of wild type virus; however, cytopathic effect and viral capsid proteins were not detected in myristoylation-deficient virus-infected cells. Also, although myristoylation-deficient viral RNA and proteins were detected in the second blind passage of infection, they were much fewer in number compared to that of the wild type virus. The replication of genomic RNA and negative-strand viral RNA were both blocked in myristoylation-deficient viruses, suggesting that myristoylation affects viral genome RNA release from capsid to cytoplasm. Besides, loss of myristoylation on VP4 altered the distribution of VP4-green fluorescent protein protein, which disappeared from the membrane structure fraction. Finally, a liposome leakage assay showed that EV71 myristoylation mediates the permeability of the model membrane. Hence, the amino-terminal myristoylation of VP4 is pivotal to EV71 infection and capsid-membrane structure interaction. This study provides novel molecular mechanisms regarding EV71 infection and potential molecular targets for antiviral drug design.

Myristoylated rhinovirus VP4 protein activates TLR2-dependent proinflammatory gene expression

Asthma exacerbations are often caused by rhinovirus (RV). We and others have shown that Toll-like receptor 2 (TLR2), a membrane surface receptor that recognizes bacterial lipopeptides and lipoteichoic acid, is required and sufficient for RV-induced proinflammatory responses in vitro and in vivo. We hypothesized that viral protein-4 (VP4), an internal capsid protein that is myristoylated upon viral replication and externalized upon viral binding, is a ligand for TLR2. Recombinant VP4 and myristoylated VP4 (MyrVP4) were purified by Ni-affinity chromatography. MyrVP4 was also purified from RV-A1B-infected HeLa cells by urea solubilization and anti-VP4 affinity chromatography. Finally, synthetic MyrVP4 was produced by chemical peptide synthesis. MyrVP4-TLR2 interactions were assessed by confocal fluorescence microscopy, fluorescence resonance energy transfer (FRET), and monitoring VP4-induced cytokine mRNA expression in the presence of anti-TLR2 and anti-VP4. MyrVP4 and TLR2 colocalized in TLR2-expressing HEK-293 cells, mouse bone marrow-derived macrophages, human bronchoalveolar macrophages, and human airway epithelial cells. Colocalization was absent in TLR2-null HEK-293 cells and blocked by anti-TLR2 and anti-VP4. Cy3-labeled MyrVP4 and Cy5-labeled anti-TLR2 showed an average fractional FRET efficiency of 0.24 ± 0.05, and Cy5-labeled anti-TLR2 increased and unlabeled MyrVP4 decreased FRET efficiency. MyrVP4-induced chemokine mRNA expression was higher than that elicited by VP4 alone and was attenuated by anti-TLR2 and anti-VP4. Cytokine expression was similarly increased by MyrVP4 purified from RV-infected HeLa cells and synthetic MyrVP4. We conclude that, during RV infection, MyrVP4 and TLR2 interact to generate a proinflammatory response.

Cellular N-myristoyltransferases play a crucial picornavirus genus-specific role in viral assembly, virion maturation, and infectivity

In nearly all picornaviruses the precursor of the smallest capsid protein VP4 undergoes co-translational N-terminal myristoylation by host cell N-myristoyltransferases (NMTs). Curtailing this modification by mutation of the myristoylation signal in poliovirus has been shown to result in severe assembly defects and very little, if any, progeny virus production. Avoiding possible pleiotropic effects of such mutations, we here used pharmacological abrogation of myristoylation with the NMT inhibitor DDD85646, a pyrazole sulfonamide originally developed against trypanosomal NMT. Infection of HeLa cells with coxsackievirus B3 in the presence of this drug decreased VP0 acylation at least 100-fold, resulting in a defect both early and late in virus morphogenesis, which diminishes the yield of viral progeny by about 90%. Virus particles still produced consisted mainly of provirions containing RNA and uncleaved VP0 and, to a substantially lesser extent, of mature virions with cleaved VP0. This indicates an important role of myristoylation in the viral maturation cleavage. By electron microscopy, these RNA-filled particles were indistinguishable from virus produced under control conditions. Nevertheless, their specific infectivity decreased by about five hundred fold. Since host cell-attachment was not markedly impaired, their defect must lie in the inability to transfer their genomic RNA into the cytosol, likely at the level of endosomal pore formation. Strikingly, neither parechoviruses nor kobuviruses are affected by DDD85646, which appears to correlate with their native capsid containing only unprocessed VP0. Individual knockout of the genes encoding the two human NMT isozymes in haploid HAP1 cells further demonstrated the pivotal role for HsNMT1, with little contribution by HsNMT2, in the virus replication cycle. Our results also indicate that inhibition of NMT can possibly be exploited for controlling the infection by a wide spectrum of picornaviruses.

Picornaviruses are important human and animal pathogens. Protective vaccines are only available against very few representatives. Furthermore, antiviral drugs have not made it to the market because of serious side effects and viral mutational escape. We here show that pharmacological inhibition of cellular myristoyltransferases severely decreased myristoylation of enteroviral structural proteins as exemplified by coxsackievirus B3, a prominent pathogen causing virus-induced acute and chronic heart disease. The drug DDD85646 substantially diminished virus yield and almost abolished the infectivity of the residual progeny virus. It is highly effective against several other picornaviruses, except those two included in our study that naturally do not process VP0. Our work provides new insight into the role of myristoylation in the life cycle of picornaviruses and identifies the responsible cellular enzyme as a promising candidate for antiviral therapy.

Picornavirus morphogenesis

The Picornaviridae represent a large family of small plus-strand RNA viruses that cause a bewildering array of important human and animal diseases. Morphogenesis is the least-understood step in the life cycle of these viruses, and this process is difficult to study because encapsidation is tightly coupled to genome translation and RNA replication. Although the basic steps of assembly have been known for some time, very few details are available about the mechanism and factors that regulate this process. Most of the information available has been derived from studies of enteroviruses, in particular poliovirus, where recent evidence has shown that, surprisingly, the specificity of encapsidation is governed by a viral protein-protein interaction that does not involve an RNA packaging signal. In this review, we make an attempt to summarize what is currently known about the following topics: (i) encapsidation intermediates, (ii) the specificity of encapsidation (iii), viral and cellular factors that are required for encapsidation, (iv) inhibitors of encapsidation, and (v) a model of enterovirus encapsidation. Finally, we compare some features of picornavirus morphogenesis with those of other plus-strand RNA viruses.

Capsid protein VP4 of human rhinovirus induces membrane permeability by the formation of a size-selective multimeric pore

Non-enveloped viruses must deliver their viral genome across a cell membrane without the advantage of membrane fusion. The mechanisms used to achieve this remain poorly understood. Human rhinovirus, a frequent cause of the common cold, is a non-enveloped virus of the picornavirus family, which includes other significant pathogens such as poliovirus and foot-and-mouth disease virus. During picornavirus cell entry, the small myristoylated capsid protein VP4 is released from the virus, interacts with the cell membrane and is implicated in the delivery of the viral RNA genome into the cytoplasm to initiate replication. In this study, we have produced recombinant C-terminal histidine-tagged human rhinovirus VP4 and shown it can induce membrane permeability in liposome model membranes. Dextran size-exclusion studies, chemical crosslinking and electron microscopy demonstrated that VP4 forms a multimeric membrane pore, with a channel size consistent with transfer of the single-stranded RNA genome. The membrane permeability induced by recombinant VP4 was influenced by pH and was comparable to permeability induced by infectious virions. These findings present a molecular mechanism for the involvement of VP4 in cell entry and provide a model system which will facilitate exploration of VP4 as a novel antiviral target for the picornavirus family.

Human rhinovirus (HRV) is a non-enveloped virus of the picornavirus family and is responsible for respiratory infections (common colds) costing billions of dollars ($) annually. There remains no vaccine or licensed drug to prevent or reduce infection. Related members of the picornavirus family include significant pathogens such as poliovirus, enterovirus 71 and foot-and-mouth disease virus, for which improved control measures are also required. A fundamental step in virus infection is the delivery of the viral genetic material through the barrier of the cellular membrane. Viruses such as HIV and influenza are enveloped in an outer membrane which can fuse with the host cell membrane to allow the viral genome to penetrate into the cytoplasm. However, non-enveloped viruses such as picornaviruses lack a membrane and the mechanisms for penetration of the membrane by these viruses remain poorly understood. The capsid protein VP4 has previously been implicated in the delivery of the picornavirus genome. In this study we demonstrate that HRV VP4 interacts with membranes to make them permeable by the formation of multimeric, size-selective membrane pores with properties consistent with the transport of viral genome through the membrane. This function of VP4 provides a novel antiviral target for this family of viruses.

Binding of glutathione to enterovirus capsids is essential for virion morphogenesis

Enteroviruses (family of the Picornaviridae) cover a large group of medically important human pathogens for which no antiviral treatment is approved. Although these viruses have been extensively studied, some aspects of the viral life cycle, in particular morphogenesis, are yet poorly understood. We report the discovery of TP219 as a novel inhibitor of the replication of several enteroviruses, including coxsackievirus and poliovirus. We show that TP219 binds directly glutathione (GSH), thereby rapidly depleting intracellular GSH levels and that this interferes with virus morphogenesis without affecting viral RNA replication. The inhibitory effect on assembly was shown not to depend on an altered reducing environment. Using TP219, we show that GSH is an essential stabilizing cofactor during the transition of protomeric particles into pentameric particles. Sequential passaging of coxsackievirus B3 in the presence of low GSH-levels selected for GSH-independent mutants that harbored a surface-exposed methionine in VP1 at the interface between two protomers. In line with this observation, enteroviruses that already contained this surface-exposed methionine, such as EV71, did not rely on GSH for virus morphogenesis. Biochemical and microscopical analysis provided strong evidence for a direct interaction between GSH and wildtype VP1 and a role for this interaction in localizing assembly intermediates to replication sites. Consistently, the interaction between GSH and mutant VP1 was abolished resulting in a relocalization of the assembly intermediates to replication sites independent from GSH. This study thus reveals GSH as a novel stabilizing host factor essential for the production of infectious enterovirus progeny and provides new insights into the poorly understood process of morphogenesis.

Enteroviruses contain many significant human pathogens, including poliovirus, enterovirus 71, coxsackieviruses and rhinoviruses. Most enterovirus infections subside mild or asymptomatically, but may also result in severe morbidity and mortality. Here, we report on the mechanism of antiviral action of a small molecule, TP219, as an inhibitor of enterovirus morphogenesis. Morphogenesis represents an important stage at the end of the virus replication cycle and requires multiple steps, of which some are only poorly understood. Better understanding of this process holds much potential to facilitate the development of new therapies to combat enterovirus infections. We demonstrate that TP219 rapidly depletes intracellular glutathione (GSH) by covalently binding free GSH resulting in the inhibition of virus morphogenesis without affecting viral RNA replication. We discovered that GSH directly interacts with viral capsid precursors and mature virions and that this interaction is required for the formation of an assembly intermediate (pentameric particles) and consequently infectious progeny. Remarkably, enteroviruses that were capable of replicating in the absence of GSH contained a surface-exposed methionine at the protomeric interface. We propose that GSH is an essential and stabilizing host factor during morphogenesis and that this stabilization is a prerequisite for a functional encapsidation of progeny viral RNA.

Ser/Thr kinase-like protein of Nicotiana benthamiana is involved in the cell-to-cell movement of Bamboo mosaic virus

To investigate the plant genes affected by Bamboo mosaic virus (BaMV) infection, we applied a cDNA-amplified fragment length polymorphism technique to screen genes with differential expression. A serine/threonine kinase-like (NbSTKL) gene of Nicotiana benthamiana is upregulated after BaMV infection. NbSTKL contains the homologous domain of Ser/Thr kinase. Knocking down the expression of NbSTKL by virus-induced gene silencing reduced the accumulation of BaMV in the inoculated leaves but not in the protoplasts. The spread of GFP-expressing BaMV in the inoculated leaves is also impeded by a reduced expression of NbSTKL. These data imply that NbSTKL facilitates the cell-to-cell movement of BaMV. The subcellular localization of NbSTKL is mainly on the cell membrane, which has been confirmed by mutagenesis and fractionation experiments. Combined with the results showing that active site mutation of NbSTKL does not change its subcellular localization but significantly affects BaMV accumulation, we conclude that NbSTKL may regulate BaMV movement on the cell membrane by its kinase-like activity. Moreover, the transient expression of NbSTKL does not significantly affect the accumulation of Cucumber mosaic virus (CMV) and Potato virus X (PVX); thus, NbSTKL might be a specific protein facilitating BaMV movement.

Role of lipids on entry and exit of bluetongue virus, a complex non-enveloped virus

Non-enveloped viruses such as members of Picornaviridae and Reoviridae are assembled in the cytoplasm and are generally released by cell lysis. However, recent evidence suggests that some non-enveloped viruses exit from infected cells without lysis, indicating that these viruses may also utilize alternate means for egress. Moreover, it appears that complex, non-enveloped viruses such as bluetongue virus (BTV) and rotavirus interact with lipids during their entry process as well as with lipid rafts during the trafficking of newly synthesized progeny viruses. This review will discuss the role of lipids in the entry, maturation and release of non-enveloped viruses, focusing mainly on BTV.

Bluetongue virus outer capsid protein VP5 interacts with membrane lipid rafts via a SNARE domain

Bluetongue virus (BTV) is a nonenveloped double-stranded RNA virus belonging to the family Reoviridae. The two outer capsid proteins, VP2 and VP5, are responsible for virus entry. However, little is known about the roles of these two proteins, particularly VP5, in virus trafficking and assembly. In this study, we used density gradient fractionation and methyl beta cyclodextrin, a cholesterol-sequestering drug, to demonstrate not only that VP5 copurifies with lipid raft domains in both transfected and infected cells, but also that raft domain integrity is required for BTV assembly. Previously, we showed that BTV nonstructural protein 3 (NS3) interacts with VP2 and also with cellular exocytosis and ESCRT pathway proteins, indicating its involvement in virus egress (A. R. Beaton, J. Rodriguez, Y. K. Reddy, and P. Roy, Proc. Natl. Acad. Sci. USA 99:13154-13159, 2002; C. Wirblich, B. Bhattacharya, and P. Roy J. Virol. 80:460-473, 2006). Here, we show by pull-down and confocal analysis that NS3 also interacts with VP5. Further, a conserved membrane-docking domain similar to the motif in synaptotagmin, a protein belonging to the SNARE (soluble N-ethylmaleimide-sensitive fusion attachment protein receptor) family was identified in the VP5 sequence. By site-directed mutagenesis, followed by flotation and confocal analyses, we demonstrated that raft association of VP5 depends on this domain. Together, these results indicate that VP5 possesses an autonomous signal for its membrane targeting and that the interaction of VP5 with membrane-associated NS3 might play an important role in virus assembly.

Recombinant VP4 of human rhinovirus induces permeability in model membranes

In common with all nonenveloped viruses, the mechanism of picornavirus membrane penetration during cell entry is poorly understood. The small, myristylated capsid protein VP4 has been implicated in this process. Here we show that recombinant VP4 of human rhinovirus 16 has the ability to associate with and induce membrane permeability in otherwise intact liposomes. This provides further evidence that VP4 plays a key role in picornavirus cell entry.

Lysosome membrane lipid microdomains: novel regulators of chaperone-mediated autophagy

Chaperone-mediated autophagy (CMA) is a selective mechanism for the degradation of soluble cytosolic proteins in lysosomes. The limiting step of this type of autophagy is the binding of substrates to the lysosome-associated membrane protein type 2A (LAMP-2A). In this work, we identify a dynamic subcompartmentalization of LAMP-2A in the lysosomal membrane, which underlies the molecular basis for the regulation of LAMP-2A function in CMA. A percentage of LAMP-2A localizes in discrete lysosomal membrane regions during resting conditions, but it exits these regions during CMA activation. Disruption of these regions by cholesterol-depleting agents or expression of a mutant LAMP-2A excluded from these regions enhances CMA activity, whereas loading of lysosomes with cholesterol significantly reduces CMA. Organization of LAMP-2A into multimeric complexes, required for translocation of substrates into lysosomes via CMA, only occurs outside the lipid-enriched membrane microdomains, whereas the LAMP-2A located within these regions is susceptible to proteolytic cleavage and degradation. Our results support that changes in the dynamic distribution of LAMP-2A into and out of discrete microdomains of the lysosomal membrane contribute to regulate CMA.

Unusual topological arrangement of structural motifs in the baboon reovirus fusion-associated small transmembrane protein

Select members of the Reoviridae are the only nonenveloped viruses known to induce syncytium formation. The fusogenic orthoreoviruses accomplish cell-cell fusion through a distinct class of membrane fusion-inducing proteins referred to as the fusion-associated small transmembrane (FAST) proteins. The p15 membrane fusion protein of baboon reovirus is unique among the FAST proteins in that it contains two hydrophobic regions (H1 and H2) recognized as potential transmembrane (TM) domains, suggesting a polytopic topology. However, detailed topological analysis of p15 indicated only the H1 domain is membrane spanning. In the absence of an N-terminal signal peptide, the H1 TM domain serves as a reverse signal-anchor to direct p15 membrane insertion and a bitopic N(exoplasmic)/C(cytoplasmic) topology. This topology results in the translocation of the smallest ectodomain ( approximately 20 residues) of any known viral fusion protein, with the majority of p15 positioned on the cytosolic side of the membrane. Mutagenic analysis indicated the unusual presence of an N-terminal myristic acid on the small p15 ectodomain is essential to the fusion process. Furthermore, the only other hydrophobic region (H2) present in p15, aside from the TM domain, is located within the endodomain. Consequently, the p15 ectodomain is devoid of a fusion peptide motif, a hallmark feature of membrane fusion proteins. The exceedingly small, myristoylated ectodomain and the unusual topological distribution of structural motifs in this nonenveloped virus membrane fusion protein necessitate alternate models of protein-mediated membrane fusion.

High susceptibility of a human oligodendroglial cell line to herpes simplex type 1 infection

More than 20 infectious agents, ranging from retroviruses to mycobacteria, have been associated with multiple sclerosis onset or relapses in which oligodendrocytes, the myelin-forming cells of the central nervous system, are the initial target of the pathogenic status. In this work, the nature of the susceptibility of the human precursor oligodendroglial KG-1C cell line to herpes simplex virus type 1 (HSV-1) was investigated. Infection of KG-1C cells was characterized by a high level of virus production and a notable progression of the cytopathic effect. After infection, there was a significant shut-off of host mRNA translation, which was correlated with evident synthesis of viral proteins. An examination by electron microscopy of the infected cells revealed the presence of large clusters of mitochondria located in the proximity of intracellular HSV-1 particle groups. In addition, transmission electron microscopy and nuclear fluorescence analysis showed neither signs of chromatin condensation nor of apoptotic bodies. Furthermore, procaspase-3 remained uncleaved, suggesting that apoptosis does not take place, at least in this system. Finally, expression and localization of MAL2, a subpopulation of detergent-insoluble lipid raft protein, was studied. Detection of MAL2 significantly increased after infection and it was colocalized with HSV-1 proteins. From these findings the authors conclude that human oligodendrocyte-like cells are highly susceptible to HSV-1 infection. The implications of this for central nervous system viral infection are discussed.

Genome delivery and ion channel properties are altered in VP4 mutants of poliovirus

During entry into host cells, poliovirus undergoes a receptor-mediated conformational transition to form 135S particles with irreversible exposure of VP4 capsid sequences and VP1 N termini. To understand the role of VP4 during virus entry, the fate of VP4 during infection by site-specific mutants at threonine-28 of VP4 (4028T) was compared with that of the parental Mahoney type 1 virus. Three virus mutants were studied: the entry-defective, nonviable mutant 4028T.G and the viable mutants 4028T.S and 4028T.V, in which residue threonine-28 was changed to glycine, serine, and valine, respectively. We show that mutant and wild-type (WT) VP4 proteins are localized to cellular membranes after the 135S conformational transition. Both WT and viable 4028T mutant particles interact with lipid bilayers to form ion channels, whereas the entry-defective 4028T.G particles do not. In addition, the electrical properties of the channels induced by the mutant viruses are different from each other and from those of WT Mahoney and Sabin type 3 viruses. Finally, uncoating and/or cytoplasmic delivery of the viral genome is altered in the 4028T mutants: the 4028T.G lethal mutant does not release its genome into the cytoplasm, and genome delivery is slower during infection by mutant 4028T.V 135S particles than by mutant 4028T.S or WT 135S particles. The distinctive electrical characteristics of the different 4028T mutant channels indicate that VP4 sequences might form part of the channel structure. The different entry phenotypes of these VP4 mutants suggest that the ion channels may be related to VP4's role during genome uncoating and/or delivery.

A novel cell entry pathway for a DAF-using human enterovirus is dependent on lipid rafts

The glycosylphosphatidylinositol (GPI)-anchored complement regulatory protein decay-accelerating factor (DAF) is used by a number of enteroviruses as a receptor during infection. DAF and other GPI-anchored proteins can be found in cholesterol-rich ordered domains within the plasma membrane that are known as "lipid rafts." We have shown, by using drugs to specifically inhibit various endocytosis routes, that infection by a DAF-using strain of echovirus 11 (EV11) is dependent upon cholesterol and an intact cytoskeleton, whereas a non-DAF-using mutant derived from it was unaffected by these drugs. Using RNA transfection and virus-binding assays, we have shown that this requirement for cholesterol, the actin cytoskeleton, and the microtubule network occurs postbinding of the virus but prior to uncoating of the RNA, indicating a role during virus entry. Confocal microscopy of virus infection supported the role of cholesterol and the cytoskeleton during entry. In addition, [(35)S]methionine-labeled DAF-using EV11, but not the non-DAF-using EV11, could be copurified with lipid raft components during infection after Triton X-100 extraction. These data indicate that DAF usage by EV11 enables the virus to associate with lipid rafts and enter cells through this novel route.

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

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

Distance-dependent cellular palmitoylation of de-novo-designed sequences and their translocation to plasma membrane subdomains

Using recursive PCR, we created an artificial protein sequence that consists of a consensus myristoylation motif (MGCTLS) followed by the triplet AGS repeated nine times and fused to the GFP reporter. This linker-GFP sequence was utilized as a base to produce multiple mutants that were used to transfect COS-7 cells. Constructs where a `palmitoylable' cysteine residue was progressively moved apart from the myristoylation site to positions 3, 9, 15 and 21 of the protein sequence were made, and these mutants were used to investigate the effect of protein myristoylation on subsequent palmitoylation,subcellular localization, membrane association and caveolin-1 colocalization. In all cases, dual acylation of the GFP chimeras correlated with translocation to Triton X-100-insoluble cholesterol/sphingomyelin-enriched subdomains. Whereas a strong Golgi labeling was observed in all the myristoylated chimeras, association with the plasma membrane was only observed in the dually acylated constructs. Taking into account the conflicting data regarding the existence and specificity of cellular palmitoyl-transferases, our results provide evidence that de-novo-designed sequences can be efficiently S-acylated with palmitic acid in vivo, strongly supporting the hypothesis that non-enzymatic protein palmitoylation can occur within mammalian cells. Additionally, this palmitoylation results in the translocation of the recombinant construct to low-fluidity domains in a myristate-palmitate distance-dependent manner.

Rafts promote assembly and atypical targeting of a nonenveloped virus, rotavirus, in Caco-2 cells

Rotavirus follows an atypical pathway to the apical membrane of intestinal cells that bypasses the Golgi. The involvement of rafts in this process was explored here. VP4 is the most peripheral protein of the triple-layered structure of this nonenveloped virus. High proportions of VP4 associated with rafts within the cell as early as 3 h postinfection. In the meantime a significant part of VP4 was targeted to the Triton X-100-resistant microdomains of the apical membrane, suggesting that this protein possesses an autonomous signal for its targeting. At a later stage the other structural rotavirus proteins were also found in rafts within the cells together with NSP4, a nonstructural protein required for the final stage of virus assembly. Rafts purified from infected cells were shown to contain infectious particles. Finally purified VP4 and mature virus were shown to interact with cholesterol- and sphingolipid-enriched model lipid membranes that changed their phase preference from inverted hexagonal to lamellar structures. Together these results indicate that a direct interaction of VP4 with rafts promotes assembly and atypical targeting of rotavirus in intestinal cells.

An Arabidopsis calcium-dependent protein kinase is associated with the endoplasmic reticulum

Arabidopsis contains 34 genes that are predicted to encode calcium-dependent protein kinases (CDPKs). CDPK enzymatic activity previously has been detected in many locations in plant cells, including the cytosol, the cytoskeleton, and the membrane fraction. However, little is known about the subcellular locations of individual CDPKs or the mechanisms involved in targeting them to those locations. We investigated the subcellular location of one Arabidopsis CDPK, AtCPK2, in detail. Membrane-associated AtCPK2 did not partition with the plasma membrane in a two-phase system. Sucrose gradient fractionation of microsomes demonstrated that AtCPK2 was associated with the endoplasmic reticulum (ER). AtCPK2 does not contain transmembrane domains or known ER-targeting signals, but does have predicted amino-terminal acylation sites. AtCPK2 was myristoylated in a cell-free extract and myristoylation was prevented by converting the glycine at the proposed site of myristate attachment to alanine (G2A). In plants, the G2A mutation decreased AtCPK2 membrane association by approximately 50%. A recombinant protein, consisting of the first 10 amino acids of AtCPK2 fused to the amino-terminus of beta-glucuronidase, was also targeted to the ER, indicating that the amino terminus of AtCPK2 can specify ER localization of a soluble protein. These results indicate that AtCPK2 is localized to the ER, that myristoylation is likely to be involved in the membrane association of AtCPK2, and that the amino terminal region of AtCPK2 is sufficient for correct membrane targeting.

Lipid rafts and HIV pathogenesis: host membrane cholesterol is required for infection by HIV type 1

In a previous study we showed that budding of HIV-1 particles occurs at highly specialized membrane microdomains known as lipid rafts. These microdomains are characterized by a distinct lipid composition that includes high concentrations of cholesterol, sphingolipids, and glycolipids. Since cholesterol is known to play a key role in the entry of some other viruses, our observation of HIV budding from lipid rafts led us to investigate the role in HIV-1 entry of cholesterol and lipid rafts in the plasma membrane of susceptible cells. We have used 2-OH-propyl-beta-cyclodextrin (beta-cyclodextrin) to deplete cellular cholesterol and disperse lipid rafts. Our results show that removal of cellular cholesterol rendered primary cells and cell lines highly resistant to HIV-1-mediated syncytium formation and to infection by both CXCR4- and CCR5-specific viruses. beta-Cyclodextrin treatment of cells partially reduced HIV-1 binding, while rendering chemokine receptors highly sensitive to antibody-mediated internalization. There was no effect on CD4 expression. All of the above-described effects were readily reversed by incubating cholesterol-depleted cells with low concentrations of cholesterol-loaded beta-cyclodextrin to restore cholesterol levels. Cholesterol depletion made cells resistant to SDF-1-induced binding to ICAM-1 through LFA-1. Since LFA-1 contributes significantly to cell binding by HIV-1, this latter effect may have contributed to the observed reduction in HIV-1 binding to cells after treatment with beta-cyclodextrin. Our results indicate that cholesterol may be critical to the HIV-1 coreceptor function of chemokine receptors and is required for infection of cells by HIV-1.

The C-terminal residues of poliovirus proteinase 2A(pro) are critical for viral RNA replication but not for cis- or trans-proteolytic cleavage

Poliovirus proteinase 2A(pro) is an essential enzyme involved in cleavages of viral and cellular proteins during the infectious cycle. Evidence has been obtained that 2A(pro) is also involved in genome replication. All enteroviruses have a negatively charged cluster of amino acids at their C terminus (E(E)/(D)(E)/(D)AMEQ-NH(2)), a common motif suggesting function. When aligned with enterovirus sequences, the 2A(pro) proteinase of human rhinovirus type 2 (HRV2) has a shorter C terminus (EE.Q:-NH(2)) and, indeed, the HRV2 2A(pro) cannot substitute for poliovirus 2A(pro) to yield a viable chimeric virus. Here evidence is provided that the C-terminal cluster of amino acids plays an unknown role in poliovirus genome replication. Deletion of the EEAME sequence from poliovirus 2A(pro) is lethal without significantly influencing proteinase function. On the other hand, addition of EAME to HRV2 2A(pro), yielding a C terminus of this enzyme of EEEAMEQ:, stimulated RNA replication of a poliovirus/HRV2 chimera 100-fold. The novel role of the C-terminal sequence motif is manifested at the level of protein function, since silent mutations in its coding region had no effect on virus proliferation. Poliovirus type 1 Mahoney 2A(pro) could be provided in trans to rescue the lethal deletion EEAME in the poliovirus variant. Encapsidation studies left open the question of whether the C terminus of poliovirus 2A(pro) is involved in particle formation. It is concluded that the C terminus of poliovirus 2A(pro) is an essential domain for viral RNA replication but is not essential for proteolytic processing.