Nuclear import and export of Venezuelan equine encephalitis virus nonstructural protein 2

Viral Subversion of the Chromosome Region Maintenance 1 Export Pathway and Its Consequences for the Cell Host

In eukaryotic cells, the spatial distribution between cytoplasm and nucleus is essential for cell homeostasis. This dynamic distribution is selectively regulated by the nuclear pore complex (NPC), which allows the passive or energy-dependent transport of proteins between these two compartments. Viruses possess many strategies to hijack nucleocytoplasmic shuttling for the benefit of their viral replication. Here, we review how viruses interfere with the karyopherin CRM1 that controls the nuclear export of protein cargoes. We analyze the fact that the viral hijacking of CRM1 provokes are-localization of numerous cellular factors in a suitable place for specific steps of viral replication. While CRM1 emerges as a critical partner for viruses, it also takes part in antiviral and inflammatory response regulation. This review also addresses how CRM1 hijacking affects it and the benefits of CRM1 inhibitors as antiviral treatments.

3Cpro of FMDV inhibits type II interferon-stimulated JAK-STAT signaling pathway by blocking STAT1 nuclear translocation

Foot-and-mouth disease virus (FMDV) has developed various strategies to antagonize the host innate immunity. FMDV Lpro and 3Cpro interfere with type I IFNs through different mechanisms. The structural protein VP3 of FMDV degrades Janus kinase 1 to suppress IFN-γ signaling transduction. Whether non-structural proteins of FMDV are involved in restraining type II IFN signaling pathways is unknown. In this study, it was shown that FMDV replication was resistant to IFN-γ treatment after the infection was established and FMDV inhibited type II IFN induced expression of IFN-γ-stimulated genes (ISGs). We also showed for the first time that FMDV non-structural protein 3C antagonized IFN-γ-stimulated JAK-STAT signaling pathway by blocking STAT1 nuclear translocation. 3Cpro expression significantly reduced the ISGs transcript levels and palindromic gamma-activated sequences (GAS) promoter activity, without affecting the protein level, tyrosine phosphorylation, and homodimerization of STAT1. Finally, we provided evidence that 3C protease activity played an essential role in degrading KPNA1 and thus inhibited ISGs mRNA and GAS promoter activities. Our results reveal a novel mechanism by which an FMDV non-structural protein antagonizes host type II IFN signaling.

The SARS-CoV-2 SSHHPS Recognized by the Papain-like Protease

Viral proteases are highly specific and recognize conserved cleavage site sequences of ∼6-8 amino acids. Short stretches of homologous host-pathogen sequences (SSHHPS) can be found spanning the viral protease cleavage sites. We hypothesized that these sequences corresponded to specific host protein targets since >40 host proteins have been shown to be cleaved by Group IV viral proteases and one Group VI viral protease. Using PHI-BLAST and the viral protease cleavage site sequences, we searched the human proteome for host targets and analyzed the hit results. Although the polyprotein and host proteins related to the suppression of the innate immune responses may be the primary targets of these viral proteases, we identified other cleavable host proteins. These proteins appear to be related to the virus-induced phenotype associated with Group IV viruses, suggesting that information about viral pathogenesis may be extractable directly from the viral genome sequence. Here we identify sequences cleaved by the SARS-CoV-2 papain-like protease (PLpro) in vitro within human MYH7 and MYH6 (two cardiac myosins linked to several cardiomyopathies), FOXP3 (an X-linked Treg cell transcription factor), ErbB4 (HER4), and vitamin-K-dependent plasma protein S (PROS1), an anticoagulation protein that prevents blood clots. Zinc inhibited the cleavage of these host sequences in vitro. Other patterns emerged from multispecies sequence alignments of the cleavage sites, which may have implications for the selection of animal models and zoonosis. SSHHPS/nsP is an example of a sequence-specific post-translational silencing mechanism.

Current Understanding of the Molecular Basis of Venezuelan Equine Encephalitis Virus Pathogenesis and Vaccine Development

Dedication

This review is dedicated in the memory of Dr Radha K. Maheshwari, a great mentor and colleague, whose passion for research and student training has left a lasting effect on this manuscript and many other works.

Abstract

Venezuelan equine encephalitis virus (VEEV) is an alphavirus in the family Togaviridae. VEEV is highly infectious in aerosol form and a known bio-warfare agent that can cause severe encephalitis in humans. Periodic outbreaks of VEEV occur predominantly in Central and South America. Increased interest in VEEV has resulted in a more thorough understanding of the pathogenesis of this disease. Inflammation plays a paradoxical role of antiviral response as well as development of lethal encephalitis through an interplay between the host and viral factors that dictate virus replication. VEEV has efficient replication machinery that adapts to overcome deleterious mutations in the viral genome or improve interactions with host factors. In the last few decades there has been ongoing development of various VEEV vaccine candidates addressing the shortcomings of the current investigational new drugs or approved vaccines. We review the current understanding of the molecular basis of VEEV pathogenesis and discuss various types of vaccine candidates.

The Methyltransferase-Like Domain of Chikungunya Virus nsP2 Inhibits the Interferon Response by Promoting the Nuclear Export of STAT1

Chikungunya virus (CHIKV) is a mosquito-borne alphavirus that has evolved effective mechanisms to counteract the type I interferon (IFN) response. Upon recognition of the virus, cells secrete IFNs, which signal through transmembrane receptors (IFNAR) to phosphorylate STAT proteins (pSTAT). pSTAT dimers are transported into the nucleus by importin-α5 and activate the transcription of IFN-stimulated genes (ISGs), increasing cellular resistance to infection. Subsequently, STAT proteins are shuttled back into the cytoplasm by the exportin CRM1. CHIKV nonstructural protein 2 (nsP2) reduces ISG expression by inhibiting general host cell transcription and by specifically reducing the levels of nuclear pSTAT1 via an unknown mechanism. To systematically examine where nsP2 acts within the JAK/STAT signaling cascade, we used two well-characterized mutants of nsP2, P718S and KR649AA. Both mutations abrogate nsP2's ability to shut off host transcription, but only the KR649AA mutant localizes exclusively to the cytoplasm and no longer specifically inhibits JAK/STAT signaling. These mutant nsP2 proteins did not differentially affect IFNAR expression levels or STAT1 phosphorylation in response to IFNs. Coimmunoprecipitation experiments showed that in the presence of nsP2, STAT1 still effectively bound importin-α5. Chemically blocking CRM1-mediated nuclear export in the presence of nsP2 additionally showed that nuclear translocation of STAT1 is not affected by nsP2. nsP2 putatively has five domains. Redirecting the nsP2 KR649AA mutant or just nsP2's C-terminal methyltransferase-like domain into the nucleus strongly reduced nuclear pSTAT in response to IFN stimulation. This demonstrates that the C-terminal domain of nuclear nsP2 specifically inhibits the IFN response by promoting the nuclear export of STAT1.IMPORTANCE Chikungunya virus is an emerging pathogen associated with large outbreaks on the African, Asian, European, and both American continents. In most patients, infection results in high fever, rash, and incapacitating (chronic) arthralgia. CHIKV effectively inhibits the first line of defense, the innate immune response. As a result, stimulation of the innate immune response with interferons (IFNs) is ineffective as a treatment for CHIKV disease. The IFN response requires an intact downstream signaling cascade called the JAK/STAT signaling pathway, which is effectively inhibited by CHIKV nonstructural protein 2 (nsP2) via an unknown mechanism. The research described here specifies where in the JAK/STAT signaling cascade the IFN response is inhibited and which protein domain of nsP2 is responsible for IFN inhibition. The results illuminate new aspects of antiviral defense and CHIKV counterdefense strategies and will direct the search for novel antiviral compounds.

Mutation of Asn-475 in the Venezuelan Equine Encephalitis Virus nsP2 Cysteine Protease Leads to a Self-Inhibited State

The alphaviral nsP2 cysteine protease of the Venezuelan equine encephalitis virus (VEEV) is a validated antiviral drug target. Clan CN proteases contain a cysteine protease domain that is intimately packed with an S-adenosyl-l-methionine-dependent RNA methyltransferase (SAM MTase) domain. Within a cleft formed at the interface of these two domains, the peptide substrate is thought to bind. The nucleophilic cysteine can be found within a conserved motif, ⁴⁷⁵NVCWAK⁴⁸⁰, which differs from that of papain (²²CGSCWAFS²⁹). Mutation of the motif residue, N475, to alanine unexpectedly produced a self-inhibited state in which the N-terminal residues flipped into the substrate-binding cleft. Notably, the N-terminal segment was not hydrolyzed-consistent with a catalytically incompetent state. The N475A mutation resulted in a 70-fold decrease in k_cat/K_m. A side chain-substrate interaction was predicted by the structure; the S701A mutation led to a 17-fold increase in K_m. An Asn at the n-2 position relative to the Cys was also found in the coronaviral papain-like proteases/deubiquitinases (PLpro) of the SARS and MERS viruses, and in several papain-like human ubiquitin specific proteases (USP). The large conformational change in the N475A variant suggests that Asn-475 plays an important role in stabilizing the N-terminal residues and in orienting the carbonyl during nucleophilic attack but does not directly hydrogen bond the oxyanion. The state trapped in crystallo is an unusual result of site-directed mutagenesis but reveals the role of this highly conserved Asn and identifies key substrate-binding contacts that may be exploited by peptide-like inhibitors.

Venezuelan Equine Encephalitis Virus Capsid-The Clever Caper

Venezuelan equine encephalitis virus (VEEV) is a New World alphavirus that is vectored by mosquitos and cycled in rodents. It can cause disease in equines and humans characterized by a febrile illness that may progress into encephalitis. Like the capsid protein of other viruses, VEEV capsid is an abundant structural protein that binds to the viral RNA and interacts with the membrane-bound glycoproteins. It also has protease activity, allowing cleavage of itself from the growing structural polypeptide during translation. However, VEEV capsid protein has additional nonstructural roles within the host cell functioning as the primary virulence factor for VEEV. VEEV capsid inhibits host transcription and blocks nuclear import in mammalian cells, at least partially due to its complexing with the host CRM1 and importin α/β1 nuclear transport proteins. VEEV capsid also shuttles between the nucleus and cytoplasm and is susceptible to inhibitors of nuclear trafficking, making it a promising antiviral target. Herein, the role of VEEV capsid in viral replication and pathogenesis will be discussed including a comparison to proteins of other alphaviruses.

Glutathionylation of chikungunya nsP2 protein affects protease activity

BACKGROUND

Chikungunya fever is an emerging disease caused by the chikungunya virus and is now being spread worldwide by the mosquito Aedes albopictus. The infection can cause a persistent severe joint pain and recent reports link high levels of viremia to neuropathologies and fatalities. The viral protein nsP2 is a multifunctional enzyme that plays several critical roles in virus replication. Virus infection induces oxidative stress in host cells which the virus utilizes to aid viral propagation. Cellular oxidative stress also triggers glutathionylation which is a post-translational protein modification that can modulate physiological roles of affected proteins.

METHODS

The nsP2 protease is necessary for processing of the virus nonstructural polyprotein generated during replication. We use the recombinant nsP2 protein to measure protease activity before and after glutathionylation. Mass spectrometry allowed the identification of the glutathione-modified cysteines. Using immunoblots, we show that the glutathionylation of nsP2 occurs in virus-infected cells.

RESULTS

We show that in virus-infected cells, the chikungunya nsP2 can be glutathionylated and we show this modification can impact on the protease activity. We also identify 6 cysteine residues that are glutathionylated of the 20 cysteines in the protein.

CONCLUSIONS

The virus-induced oxidative stress causes modification of viral proteins which appears to modulate virus protein function.

GENERAL SIGNIFICANCE

Viruses generate oxidative stress to regulate and hijack host cell systems and this environment also appears to modulate virus protein function. This may be a general target for intervention in viral pathogenesis.

Kinetic, Mutational, and Structural Studies of the Venezuelan Equine Encephalitis Virus Nonstructural Protein 2 Cysteine Protease

The Venezuelan equine encephalitis virus (VEEV) nonstructural protein 2 (nsP2) cysteine protease (EC 3.4.22.-) is essential for viral replication and is involved in the cytopathic effects (CPE) of the virus. The VEEV nsP2 protease is a member of MEROPS Clan CN and characteristically contains a papain-like protease linked to an S-adenosyl-l-methionine-dependent RNA methyltransferase (SAM MTase) domain. The protease contains an alternative active site motif, (475)NVCWAK(480), which differs from papain's (CGS(25)CWAFS), and the enzyme lacks a transition state-stabilizing residue homologous to Gln-19 in papain. To understand the roles of conserved residues in catalysis, we determined the structure of the free enzyme and the first structure of an inhibitor-bound alphaviral protease. The peptide-like E64d inhibitor was found to bind beneath a β-hairpin at the interface of the SAM MTase and protease domains. His-546 adopted a conformation that differed from that found in the free enzyme; one or both of the conformers may assist in leaving group departure of either the amine or Cys thiolate during the catalytic cycle. Interestingly, E64c (200 μM), the carboxylic acid form of the E64d ester, did not inhibit the nsP2 protease. To identify key residues involved in substrate binding, a number of mutants were analyzed. Mutation of the motif residue, N475A, led to a 24-fold reduction in kcat/Km, and the conformation of this residue did not change after inhibition. N475 forms a hydrogen bond with R662 in the SAM MTase domain, and the R662A and R662K mutations both led to 16-fold decreases in kcat/Km. N475 forms the base of the P1 binding site and likely orients the substrate for nucleophilic attack or plays a role in product release. An Asn homologous to N475 is similarly found in coronaviral papain-like proteases (PLpro) of the Severe Acute Respiratory Syndrome (SARS) virus and Middle East Respiratory Syndrome (MERS) virus. Mutation of another motif residue, K480A, led to a 9-fold decrease in kcat and kcat/Km. K480 likely enhances the nucleophilicity of the Cys. Consistent with our substrate-bound models, the SAM MTase domain K706A mutation increased Km 4.5-fold to 500 μM. Within the β-hairpin, the N545A mutation slightly but not significantly increased kcat and Km. The structures and identified active site residues may facilitate the discovery of protease inhibitors with antiviral activity.

Diversification of importin-α isoforms in cellular trafficking and disease states

The human genome encodes seven isoforms of importin α which are grouped into three subfamilies known as α1, α2 and α3. All isoforms share a fundamentally conserved architecture that consists of an N-terminal, autoinhibitory, importin-β-binding (IBB) domain and a C-terminal Arm (Armadillo)-core that associates with nuclear localization signal (NLS) cargoes. Despite striking similarity in amino acid sequence and 3D structure, importin-α isoforms display remarkable substrate specificity in vivo. In the present review, we look at key differences among importin-α isoforms and provide a comprehensive inventory of known viral and cellular cargoes that have been shown to associate preferentially with specific isoforms. We illustrate how the diversification of the adaptor importin α into seven isoforms expands the dynamic range and regulatory control of nucleocytoplasmic transport, offering unexpected opportunities for pharmacological intervention. The emerging view of importin α is that of a key signalling molecule, with isoforms that confer preferential nuclear entry and spatiotemporal specificity on viral and cellular cargoes directly linked to human diseases.

3Cpro of foot-and-mouth disease virus antagonizes the interferon signaling pathway by blocking STAT1/STAT2 nuclear translocation

Foot-and-mouth disease virus (FMDV) causes a highly contagious, debilitating disease in cloven-hoofed animals with devastating economic consequences. To survive in the host, FMDV has evolved to antagonize the host type I interferon (IFN) response. Previous studies have reported that the leader proteinase (L(pro)) and 3C(pro) of FMDV are involved in the inhibition of type I IFN production. However, whether the proteins of FMDV can inhibit type I IFN signaling is less well understood. In this study, we first found that 3C(pro) of FMDV functioned to interfere with the JAK-STAT signaling pathway. Expression of 3C(pro) significantly reduced the transcript levels of IFN-stimulated genes (ISGs) and IFN-stimulated response element (ISRE) promoter activity. The protein level, tyrosine phosphorylation of STAT1 and STAT2, and their heterodimerization were not affected. However, the nuclear translocation of STAT1/STAT2 was blocked by the 3C(pro) protein. Further mechanistic studies demonstrated that 3C(pro) induced proteasome- and caspase-independent protein degradation of karyopherin α1 (KPNA1), the nuclear localization signal receptor for tyrosine-phosphorylated STAT1, but not karyopherin α2, α3, or α4. Finally, we showed that the protease activity of 3C(pro) contributed to the degradation of KPNA1 and thus blocked STAT1/STAT2 nuclear translocation. Taken together, results of our experiments describe for the first time a novel mechanism by which FMDV evolves to inhibit IFN signaling and counteract host innate antiviral responses.

IMPORTANCE

We show that 3C(pro) of FMDV antagonizes the JAK-STAT signaling pathway by blocking STAT1/STAT2 nuclear translocation. Furthermore, 3C(pro) induces KPNA1 degradation, which is independent of proteasome and caspase pathways. The protease activity of 3C(pro) contributes to the degradation of KPNA1 and governs the ability of 3C(pro) to inhibit the JAK-STAT signaling pathway. This study uncovers a novel mechanism evolved by FMDV to antagonize host innate immune responses.

The fine line between protection and pathology in neurotropic flavivirus and alphavirus infections

ABSTRACT: 

Flavivirus and alphavirus are two families of medically important arboviruses known to cause devastating neurologic disease. Exciting knowledge regarding epidemiology, disease and host immune responses are constantly unraveling. In this review, we aim to piece existing knowledge of neurotropic flavi- and alpha-viruses into a general, coherent picture of host–pathogen interactions. Special interest lies in the protective and pathologic host immunity to flavi- and alpha-viral infections, with a strong focus on West Nile virus, Japanese Encephalitis virus and Venezuelan equine encephalitis virus as representatives of their family.

New insights into the functions and localization of nuclear transglutaminase 2

Transglutaminase 2 (TG2; EC 2.3.2.13) is the most abundantly expressed member of the transglutaminase family and exerts opposing effects on cell growth, differentiation and apoptosis via multiple activities, including transamidase, GTPase, cell adhesion, protein disulfide isomerase, kinase and scaffold activities. It is distributed in and around various parts of a cell, including the extracellular matrix, plasma membrane, cytosol, mitochondria and nucleus. Generally, nuclear TG2 represents only 5-7% of the total TG2 in a cell, and various stimuli will increase nuclear TG2 via cellular stress and/or an increased intracellular Ca(2+) concentration. There is increasing evidence indicating the importance of nuclear TG2 in regulating gene expression via post-translational modification of (or interaction with) transcriptional factors and related proteins. These include E2F1, hypoxia inducible factor 1, Sp1 and histones. Through this mechanism, TG2 controls cell growth or survival, differentiation and apoptosis, and is involved in the pathogenesis and/or treatment of neurodegenerative diseases, liver diseases and cancers. The balance between import from the cytoplasm to the nucleus, and export from the nucleus to the cytoplasm, determines the level of TG2 in the nucleus. Selective regulation of the expression, activity or localization of nuclear TG2 will be important for basic research, as well as clinical applications, suggesting a new era for this long-studied enzyme.

Role of interferon antagonist activity of rabies virus phosphoprotein in viral pathogenicity

The fixed rabies virus (RV) strain Nishigahara kills adult mice after intracerebral inoculation, whereas the chicken embryo fibroblast cell-adapted strain Ni-CE causes nonlethal infection in adult mice. We previously reported that the chimeric CE(NiP) strain, which has the phosphoprotein (P protein) gene from the Nishigahara strain in the genetic background of the Ni-CE strain, causes lethal infection in adult mice, indicating that the P gene is responsible for the different pathogenicities of the Nishigahara and Ni-CE strains. Previous studies demonstrated that RV P protein binds to the interferon (IFN)-activated transcription factor STAT1 and blocks IFN signaling by preventing its translocation to the nucleus. In this study, we examine the molecular mechanism by which RV P protein determines viral pathogenicity by comparing the IFN antagonist activities of the Nishigahara and Ni-CE P proteins. The results, obtained from both RV-infected cells and cells transfected to express P protein only, show that Ni-CE P protein is significantly impaired for its capacity to block IFN-activated STAT1 nuclear translocation and, consequently, inhibits IFN signaling less efficiently than Nishigahara P protein. Further, it was demonstrated that a defect in the nuclear export of Ni-CE P protein correlates with a defect in its ability to cause the mislocalization of STAT1. These data provide the first evidence that the capacity of the RV P protein to inhibit STAT1 nuclear translocation and IFN signaling correlates with the viral pathogenicity.

Alphavirus antiviral drug development: scientific gap analysis and prospective research areas

The New World alphaviruses Venezuelan equine encephalitis virus (VEEV), eastern equine encephalitis virus (EEEV), and western equine encephalitis virus (WEEV) pose a significant threat to human health as the etiological agents of serious viral encephalitis through natural infection as well as through their potential use as a biological weapon. At present, there is no FDA-approved medical treatment for infection with these viruses. The Defense Threat Reduction Agency, Joint Science and Technology Office for Chemical and Biological Defense (DTRA/JSTO), is currently funding research aimed at developing antiviral drugs and vaccines against VEEV, EEEV, and WEEV. A review of antiviral drug discovery efforts for these viruses revealed significant gaps in the data, assays, and models required for successful drug development. This review provides a description of these gaps and highlights specific critical research areas for the development of a target-based drug discovery program for the VEEV, EEEV, and WEEV nonstructural proteins. These efforts will increase the probability of the successful development of a pharmaceutical intervention against these viral threat agents.

Dual modes of rabies P-protein association with microtubules: a novel strategy to suppress the antiviral response

Conventional nuclear import is independent of the cytoskeleton, but recent data have shown that the import of specific proteins can be either facilitated or inhibited by microtubules (MTs). Nuclear import of the P-protein from rabies virus involves a MT-facilitated mechanism, but here, we show that P-protein is unique in that it also undergoes MT-inhibited import, with the mode of MT-interaction being regulated by the oligomeric state of the P-protein. This is the first demonstration that a protein can utilise both MT-inhibited and MT-facilitated import mechanisms, and can switch between these different modes of MT interaction to regulate its nuclear trafficking. Importantly, we show that the P-protein exploits MT-dependent mechanisms to manipulate host cell processes by switching the import of the interferon-activated transcription factor STAT1 from a conventional to a MT-inhibited mechanism. This prevents STAT1 nuclear import and signalling in response to interferon, which is vital to the host innate antiviral response. This is the first report of MT involvement in the viral subversion of interferon signalling that is central to virus pathogenicity, and identifies novel targets for the development of antiviral drugs or attenuated viruses for vaccine applications.

Venezuelan equine encephalitis virus disrupts STAT1 signaling by distinct mechanisms independent of host shutoff

Venezuelan equine encephalitis virus (VEEV) is an important human and veterinary pathogen causing sporadic epizootic outbreaks of potentially fatal encephalitis. The type I interferon (IFN) system plays a central role in controlling VEEV and other alphavirus infections, and IFN evasion is likely an important determinant of whether these viruses disseminate and cause disease within their hosts. Alphaviruses are thought to limit the induction of type I IFNs and IFN-stimulated genes by shutting off host cell macromolecular synthesis, which in the case of VEEV is partially mediated by the viral capsid protein. However, more specific strategies by which alphaviruses inhibit type I IFN signaling have not been characterized. Analyses of cells infected with VEEV and VEEV replicon particles (VRP) demonstrate that viral infection rapidly disrupts tyrosine phosphorylation and nuclear translocation of the transcription factor STAT1 in response to both IFN-beta and IFN-gamma. This effect was independent of host shutoff and expression of viral capsid, suggesting that VEEV uses novel mechanisms to interfere with type I and type II IFN signaling. Furthermore, at times when STAT1 activation was efficiently inhibited, VRP infection did not limit tyrosine phosphorylation of Jak1, Tyk2, or STAT2 after IFN-beta treatment but did inhibit Jak1 and Jak2 activation in response to IFN-gamma, suggesting that VEEV interferes with STAT1 activation by the type I and II receptor complexes through distinct mechanisms. Identification of the viral requirements for this novel STAT1 inhibition will further our understanding of alphavirus molecular pathogenesis and may provide insights into effective alphavirus-based vaccine design.

Venezuelan equine encephalitis virus capsid protein inhibits nuclear import in Mammalian but not in mosquito cells

Venezuelan equine encephalitis virus (VEEV) represents a continuous public health threat in the United States. It has the ability to cause fatal disease in humans and in horses and other domestic animals. We recently demonstrated that replicating VEEV interferes with cellular transcription and uses this phenomenon as a means of downregulating a cellular antiviral response. VEEV capsid protein was found to play a critical role in this process, and its approximately 35-amino-acid-long peptide, fused with green fluorescent protein, functioned as efficiently as did the entire capsid. We detected a significant fraction of VEEV capsid associated with nuclear envelope, which suggested that this protein might regulate nucleocytoplasmic trafficking. In this study, we demonstrate that VEEV capsid and its N-terminal sequence efficiently inhibit multiple receptor-mediated nuclear import pathways but have no effect on the passive diffusion of small proteins. The capsid protein of the Old World alphavirus Sindbis virus and the VEEV capsid, with a previously defined frameshift mutation, were found to have no detectable effect on nuclear import. Importantly, the VEEV capsid did not noticeably interfere with nuclear import in mosquito cells, and this might play a critical role in the ability of the virus to develop a persistent, life-long infection in mosquito vectors. These findings demonstrate a new aspect of VEEV-host cell interactions, and the results of this study are likely applicable to other New World alphaviruses, such as eastern and western equine encephalitis viruses.