The herpes simplex virus 1 U(L)34 protein interacts with a cytoplasmic dynein intermediate chain and targets nuclear membrane

Viral strategies for intracellular trafficking: motors and microtubules

To overcome barriers to diffusion, many viruses utilize the microtubule-associated molecular motor cytoplasmic dynein 1 to drive transport towards the nucleus of a target cell. Cytoplasmic dynein 1 generates movement towards the minus end of microtubules located at the microtubule organizing centre (MTOC), a structure that is typically in close proximity to the nucleus. Physiological cargoes for cytoplasmic dynein include membranous organelles, protein complexes and aggregates of misfolded protein. In this review, we discuss the study of microtubule-based translocation of viruses and raise questions about the mechanisms for association with and then dissociation from cytoplasmic dynein with a goal of understanding whether viruses are seen by the intracellular trafficking machinery as functional protein complexes or misfolded protein aggregates.

Emerin is hyperphosphorylated and redistributed in herpes simplex virus type 1-infected cells in a manner dependent on both UL34 and US3

Cells infected with wild-type herpes simplex virus type 1 (HSV-1) show disruption of the organization of the nuclear lamina that underlies the nuclear envelope. This disruption is reflected in changes in the localization and phosphorylation of lamin proteins. Here, we show that HSV-1 infection causes relocalization of the LEM domain protein emerin. In cells infected with wild-type virus, emerin becomes more mobile in the nuclear membrane, and in cells infected with viruses that fail to express UL34 protein (pUL34) and US3 protein (pUS3), emerin no longer colocalizes with lamins, suggesting that infection causes a loss of connection between emerin and the lamina. Infection causes hyperphosphorylation of emerin in a manner dependent upon both pUL34 and pUS3. Some emerin hyperphosphorylation can be inhibited by the protein kinase Cdelta (PKCdelta) inhibitor rottlerin. Emerin and pUL34 interact physically, as shown by pull-down and coimmunoprecipitation assays. Emerin expression is not, however, necessary for infection, since virus growth is not impaired in cells derived from emerin-null transgenic mice. The results suggest a model in which pUS3 and PKCdelta that has been recruited by pUL34 hyperphosphorylate emerin, leading to disruption of its connections with lamin proteins and contributing to the disruption of the nuclear lamina. Changes in emerin localization, nuclear shape, and lamin organization characteristic of cells infected with wild-type HSV-1 also occur in cells infected with recombinant virus that does not make viral capsids, suggesting that these changes occur independently of capsid envelopment.

Autographa californica multiple nucleopolyhedrovirus EXON0 (ORF141) is required for efficient egress of nucleocapsids from the nucleus

Autographa californica multiple nucleopolyhedrovirus (AcMNPV) exon0 (orf141) has been shown to be required for the efficient production of budded virus (BV). The deletion of exon0 reduces the level of BV production by up to 99% (X. Dai, T. M. Stewart, J. A. Pathakamuri, Q. Li, and D. A. Theilmann, J. Virol. 78:9633-9644, 2004); however, the function or mechanism by which EXON0 affects BV production is unknown. In this study, we further elucidated the function of EXON0 by investigating the localization of EXON0 in infected Sf9 cells and in virions and by identifying interactions between EXON0 and other viral proteins. In addition, electron microscopy was used to study the cellular localization of nucleocapsids in cells transfected with an exon0 knockout (KO) virus. The results showed that EXON0 was localized to both the cytoplasm and the nuclei of infected Sf9 cells throughout the infection. Western blotting results also showed that EXON0 was purified along with BV and occlusion-derived virus (ODV). The fractionation of BV into the nucleocapsid and envelope components showed that EXON0 localized to the BV nucleocapsid. Yeast two-hybrid screening, coimmunoprecipitation, and confocal microscopy revealed that it interacted with nucleocapsid proteins FP25 and BV/ODV-C42. Cells transfected with the exon0 KO virus exhibited normally appearing nucleocapsids in the nuclei in numbers equal to those in the nuclei of cells transfected with the EXON0 repaired virus. In contrast, the numbers of nucleocapsids in the cytoplasm of cells transfected with the exon0 KO virus were significantly lower than those in the cytoplasm of cells transfected with the repaired virus. These results support the conclusion that EXON0 is required in the BV pathway for the efficient egress of nucleocapsids from the nucleus to the cytoplasm.

The Herpesvirus capsid surface protein, VP26, and the majority of the tegument proteins are dispensable for capsid transport toward the nucleus

Upon entering a cell, alphaherpesvirus capsids are transported toward the minus ends of microtubules and ultimately deposit virus DNA within the host nucleus. The virus proteins that mediate this centripetal transport are unknown but are expected to be either viral tegument proteins, which are a group of capsid-associated proteins, or a surface component of the capsid itself. Starting with derivatives of pseudorabies virus that encode a fluorescent protein fused to a structural component of the virus, we have made a collection of 12 mutant viruses that lack either the VP26 capsid protein or an individual tegument protein. Using live-cell fluorescence microscopy, we tracked individual virus particles in axons following infection of primary sensory neurons. Quantitative analysis of the VP26-null virus indicates that this protein plays no observable role in capsid transport. Furthermore, viruses lacking tegument proteins that are nonessential for virus propagation in cell culture were also competent for axonal transport. These results indicate that a protein essential for viral propagation mediates transport of the capsid to the nucleus.

In vitro interaction of poliovirus with cytoplasmic dynein

OBJECTIVE

Poliovirus (PV) enters the host by the oral route and can infect the central nervous system (CNS) by two mechanisms: crossing the blood-brain barrier and traveling along the nerves from the muscle to the spinal cord. In the latter mechanism, the PV receptor, CD155, and the motor protein, dynein, have been implicated in the transport of PV to the CNS. In this work we analyzed the possible interaction of PV with dynein.

METHODS

PV was bound to a Sepharose 4B beads and they were used to analyze the interaction of PV with cytoplasmic proteins from neuroblastoma cells by affinity chromatography and Western blot.

RESULTS

The interaction with cytoplasmic dynein was observed only when the Sepharose beads bound to PV were used and not in the control ones, where proteins from uninfected cells were coupled.

CONCLUSION

These preliminary results open the possibility that PV uses the dynein directly in its retrograde axonal transport.

The pseudorabies virus VP1/2 tegument protein is required for intracellular capsid transport

Transport of capsids in cells is critical to alphaherpesvirus infection and pathogenesis; however, viral factors required for transport have yet to be identified. Here we provide a detailed examination of capsid dynamics during the egress phase of infection in Vero cells infected with pseudorabies virus. We demonstrate that the VP1/2 tegument protein is required for processive microtubule-based transport of capsids in the cytoplasm. A second tegument protein that binds to VP1/2, UL37, was necessary for wild-type transport but was not essential for this process. Both proteins were also required for efficient nuclear egress of capsids to the cytoplasm.

Heteronuclear NMR Identifies a Nascent Helix in Intrinsically Disordered Dynein Intermediate Chain: Implications for Folding and Dimerization

The intermediate chain of dynein forms a tight subcomplex with dimeric light chains LC8 and Tctex-1, and together they constitute the cargo attachment complex. There is considerable interest in identifying the role of these light chains in the assembly of the two copies of the intermediate chain. The N-terminal domain of the intermediate chain, IC1-289, contains the binding sites for the light chains, and is a highly disordered monomer but gains helical structure upon binding to light chains LC8 and Tctex-1. To provide insights into the structural and dynamic changes that occur in the intermediate chain upon light chains binding, we have used NMR spectroscopy to compare the properties of two distinct sub-domains of IC1-289: IC84-143 which is the light chains binding domain, and IC198-237, which contains a predicted coiled coil necessary for the increase in ordered structure upon light chain binding. Neither construct has stable secondary structure when probed by circular dichroism and amide chemical shift dispersion. Specific residues of IC84-143 involved in binding to the light chains were identified by their increase in resonance line broadening and the corresponding large intensity reduction in 1H-15N HSQC spectra. Interestingly, IC84-143 shows no sign of structure formation after binding to either LC8 or Tctex-1 or to both. IC198-237, on the other hand, contains a population of a nascent helix at low temperature as identified by heteronuclear NMR relaxation measurements, secondary chemical shifts, and sequential amide-amide connectivities. These data are consistent with a model for light chain binding coupled to intermediate chain dimerization through forming a coiled coil distant from the binding site.

Reconstitution of herpes simplex virus microtubule-dependent trafficking in vitro

Microtubule-mediated anterograde transport of herpes simplex virus (HSV) from the neuronal cell body to the axon terminal is crucial for the spread and transmission of the virus. It is therefore of central importance to identify the cellular and viral factors responsible for this trafficking event. In previous studies, we isolated HSV-containing cytoplasmic organelles from infected cells and showed that they represent the first and only destination for HSV capsids after they emerge from the nucleus. In the present study, we tested whether these cytoplasmic compartments were capable of microtubule-dependent traffic. Organelles containing green fluorescent protein-labeled HSV capsids were isolated and found to be able to bind rhodamine-labeled microtubules polymerized in vitro. Following the addition of ATP, the HSV-associated organelles trafficked along the microtubules, as visualized by time lapse microscopy in an imaging microchamber. The velocity and processivity of trafficking resembled those seen for neurotropic herpesvirus traffic in living axons. The use of motor-specific inhibitors indicated that traffic was predominantly kinesin mediated, consistent with the reconstitution of anterograde traffic. Immunocytochemical studies revealed that the majority of HSV-containing organelles attached to the microtubules contained the trans-Golgi network marker TGN46. This simple, minimal reconstitution of microtubule-mediated anterograde traffic should facilitate and complement molecular analysis of HSV egress in vivo.

Herpes simplex virus infection, with particular reference to the progression and complications of primary herpetic gingivostomatitis

Primary herpetic gingivostomatitis (PHGS) represents the clinically apparent pattern of primary herpes simplex virus (HSV) infection, since the vast majority of other primary infections are symptomless. PHGS is caused predominantly by HSV-1 and affects mainly children. Prodromal symptoms, such as fever, anorexia, irritability, malaise and headache, may occur in advance of disease. The disease presents as numerous pin-head vesicles, which rupture rapidly to form painful irregular ulcerations covered by yellow-grey membranes. Sub-mandibular lymphadenitis, halitosis and refusal to drink are usual concomitant findings. Following resolution of the lesions, the virus travels through the nerve endings to the nerve cells serving the affected area, whereupon it enters a latent state. When the host becomes stressed, the virus replicates and migrates in skin, mucosae and, in rare instances, the central nervous system. A range of morbidities, or even mortality, may then occur, i.e., recurrent HSV infections, which are directly or indirectly associated with PHGS. These pathological entities range from the innocuous herpes labialis to life-threatening meningoencephalitis.

The inner tegument promotes herpes simplex virus capsid motility along microtubules in vitro

After viral fusion, capsids of the neurotropic herpes simplex virus are transported along microtubules (MT) to the nuclear pores for viral genome uncoating, nuclear transcription and replication. After assembly and egress from the nucleus, cytosolic capsids are transported to host membranes for secondary envelopment or to the axon terminal for further viral spread. Using GFP-tagged capsids, Cy3-labelled MT and cytosol, we have reconstituted viral capsid transport in vitro. In the presence of ATP, capsids moved along MT up to 30 microm. Blocking the function of dynactin, a cofactor of dynein and kinesin-2, inhibited the transport. Removing outer tegument proteins from the capsids increased in vitro motility. In contrast, capsids isolated from infected nuclei that were devoid of inner as well as outer tegument proteins showed little interaction with dynein and its cofactor dynactin. Our data suggest that the inner tegument of alphaherpesviruses contains viral receptors for MT motors.

Roles for herpes simplex virus type 1 UL34 and US3 proteins in disrupting the nuclear lamina during herpes simplex virus type 1 egress

Cells infected with wild type HSV-1 showed significant lamin A/C and lamin B rearrangement, while UL34-null virus-infected cells exhibited few changes in lamin localization, indicating that UL34 is necessary for lamin disruption. During HSV infection, US3 limited the development of disruptions in the lamina, since cells infected with a US3-null virus developed large perforations in the lamin layer. US3 regulation of lamin disruption does not correlate with the induction of apoptosis. Expression of either UL34 or US3 proteins alone disrupted lamin A/C and lamin B localization. Expression of UL34 and US3 together had little effect on lamin A/C localization, suggesting a regulatory interaction between the two proteins. The data presented in this paper argue for crucial roles for both UL34 and US3 in regulating the state of the nuclear lamina during viral infection.

Molecular motors hijacking by intracellular pathogens

Cargoes are transported intracellularly along cytoskeletal tracks composed of actin or tubulin. Their movement involves the action of molecular motor proteins that generate directed movement along microtubules or actin filaments. The three classes of molecular motors--kinesins, dyneins and myosins--are involved in a multiplicity of biological movements such as mitosis, positioning of organelles, intracellular transports and also vesicular sorting through membrane tubulation and fission and delivery to their target compartment. Intracellular pathogens use this molecular machinery to reach their site of replication, to leave their host or to control the dynamics of membrane exchanges with their replication compartment.

The role of the cytoskeleton during viral infection

Upon infection, virions or subviral nucleoprotein complexes are transported from the cell surface to the site of viral transcription and replication. During viral egress, particles containing viral proteins and nucleic acids again move from the site of their synthesis to that of virus assembly and further to the plasma membrane. Because free diffusion of molecules larger than 500 kDa is restricted in the cytoplasm, viruses as well as cellular organelles employ active, energy-consuming enzymes for directed transport. This is particularly evident in the case of neurotropic viruses that travel long distances in the axon during retrograde or anterograde transport. Viruses use two strategies for intracellular transport: Viral components either hijack the cytoplasmic membrane traffic or they interact directly with the cytoskeletal transport machinery. In this review we describe how viruses--particularly members of the Herpesviridae, Adenoviridae, Parvoviridae, Poxviridae, and Baculoviridae--make use of the microtubule and the actin cytoskeleton. Analysing the underlying principles of viral cytosolic transport will be helpful in the design of viral vectors to be used in research as well as human gene therapy, and in the identification of new antiviral target molecules.

Viral stop-and-go along microtubules: taking a ride with dynein and kinesins

Incoming viral particles move from the cell surface to sites of viral transcription and replication. By contrast, during assembly and egress, subviral nucleoprotein complexes and virions travel back to the plasma membrane. Because diffusion of large molecules is severely restricted in the cytoplasm, viruses use ATP-hydrolyzing molecular motors of the host for propelling along the microtubules, which are the intracellular highways. Recent studies have revealed that, besides travelling inside endocytic or exocytic vesicles, viral proteins interact directly with dynein or kinesin motors. Understanding the molecular mechanisms of cytoplasmic viral transport will aid in the construction of viral vectors for human gene therapy and the search for new antiviral targets.

Human herpesvirus 6 open reading frame U14 protein and cellular p53 interact with each other and are contained in the virion

A mass spectroscopic analysis of proteins from human herpesvirus 6 (HHV-6)-infected cells showed that the HHV-6 U14 protein coimmunoprecipitated with the tumor suppressor p53. The binding of U14 to p53 was verified by coimmunoprecipitation experiments in both Molt-3 cells infected with HHV-6 and 293 cells cotransfected with U14 and p53 expression vectors. Indirect immunofluorescence assays (IFAs) showed that by 18 h postinfection (hpi) U14 localized to the dot-like structures observed in both the nucleus and cytoplasm where p53 was partly accumulated. Despite Northern blotting evidence that U14 follows late kinetics, the U14 protein was detected immediately after infection (at 3 hpi) by IFA. In addition, by Western blotting, U14 was detected at 0 hpi or in the presence of cycloheximide which completely abolished the expression of IE1 protein. In addition to U14, p53 was detected at 0 hpi although it was not detected in mock-infected cells. Furthermore, both U14 and p53 were clearly detected in the viral particles by Western blotting and immunoelectron microscopy, supporting the idea that U14 and p53 are incorporated into virions. Our study provides the first evidence of the incorporation of cellular p53 into viral particles and suggests that p53 may play an important role in viral infection.

Herpes simplex virus 1 ICP22 regulates the accumulation of a shorter mRNA and of a truncated US3 protein kinase that exhibits altered functions

The U(S)3 open reading frame of herpes simplex virus 1 (HSV-1) was reported to encode two mRNAs each directing the synthesis of the same protein. We report that the U(S)3 gene encodes two proteins. The predominant U(S)3 protein is made in wild-type HSV-1-infected cells. The truncated mRNA and a truncated protein designated U(S)3.5 and initiating from methionine 77 were preeminent in cells infected with a mutant lacking the gene encoding ICP22. Both the wild-type and truncated proteins also accumulated in cells transduced with a baculovirus carrying the entire U(S)3 open reading frame. The U(S)3.5 protein accumulating in cells infected with the mutant lacking the gene encoding ICP22 mediated the phosphorylation of histone deacetylase 1, a function of U(S)3 protein, but failed to block apoptosis of the infected cells. The U(S)3.5 and U(S)3 proteins differ with respect to the range of functions they exhibit.

Baculovirus as versatile vectors for protein expression in insect and mammalian cells

Today, many thousands of recombinant proteins, ranging from cytosolic enzymes to membrane-bound proteins, have been successfully produced in baculovirus-infected insect cells. Yet, in addition to its value in producing recombinant proteins in insect cells and larvae, this viral vector system continues to evolve in new and unexpected ways. This is exemplified by the development of engineered insect cell lines to mimic mammalian cell glycosylation of expressed proteins, baculovirus display strategies and the application of the virus as a mammalian-cell gene delivery vector. Novel vector design and cell engineering approaches will serve to further enhance the value of baculovirus technology.

Nuclear localization signal peptides induce molecular delivery along microtubules

Many essential processes in eukaryotic cells depend on regulated molecular exchange between its two major compartments, the cytoplasm and the nucleus. In general, nuclear import of macromolecular complexes is dependent on specific peptide signals and their recognition by receptors that mediate translocation through the nuclear pores. Here we address the question of how protein products bearing such nuclear localization signals arrive at the nuclear membrane before import, i.e., by simple diffusion or perhaps with assistance of cytoskeletal elements or cytoskeleton-associated motor proteins. Using direct single-particle tracking and detailed statistical analysis, we show that the presence of nuclear localization signals invokes active transport along microtubules in a cell-free Xenopus egg extract. Chemical and antibody inhibition of minus-end directed cytoplasmic dynein blocks this active movement. In the intact cell, where microtubules project radially from the centrosome, such an interaction would effectively deliver nuclear-targeted cargo to the nuclear envelope in preparation for import.

HSV trafficking and development of gene therapy vectors with applications in the nervous system

Herpes simplex virus type 1 (HSV-1) is a neurotropic double-stranded DNA virus that causes cold sores, keratitis, and rarely encephalitis in humans. Nonpathogenic HSV-1 gene transfer vectors have been generated by elimination of viral functions necessary for replication. The life cycle of the native virus includes replication in epithelial cells at the site of initial inoculation followed by retrograde axonal transport to the nuclei of sensory neurons innervating the area of cutaneous primary infection. In this review, we summarize the current understanding of the molecular basis for HSV cell entry, nuclear transport of the genome, virion egress following replication, and retrograde and anterograde axonal transport in neurons. We discuss how each of these properties has been exploited or modified to allow the generation of gene transfer vectors with particular utility for neurological applications. Recent advances in engineering virus entry have provided proof of principle that vector targeting is possible. Furthermore, significant and potentially therapeutic modifications to the pathological responses to various noxious insults have been demonstrated in models of peripheral nerve disease. These applications exploit the natural axonal transport mechanism of HSV, allowing transgene expression in the cell nucleus within the inaccessible trigeminal ganglion or dorsal root ganglion, following the noninvasive procedure of subcutaneous vector inoculation. These findings demonstrate the importance of understanding basic virology in the design of vector systems and the powerful approach of exploiting favorable properties of the parent virus in the generation of gene transfer vectors.

Host and viral proteins in the virion of Kaposi's sarcoma-associated herpesvirus

Infection of cultured cells with Kaposi's sarcoma associated herpesvirus (KSHV) typically establishes a latent infection, in which only a few viral genes are expressed. Recently, it has been reported that a subset of lytic genes are transiently expressed very early after viral entry but that this burst of abortive lytic gene expression is terminated with the supervention of latency (H. H. Krishnan, P. P. Naranatt, M. S. Smith, L. Zeng, C. Bloomer, and B. Chandran, J. Virol. 78:3601-3620, 2004). To identify molecules imported into cells by KSHV that might influence this gene expression program, we have examined the protein composition of the KSHV particle. Immunoblotting of virus particles demonstrated that RTA, the lytic switch protein, and RAP, a viral protein that is a transcriptional and cell cycle modulator, were both incorporated into virus particles. In a second approach, polypeptides isolated from purified virions were identified by mass-spectrometric analysis of their constituent tryptic peptides. With this approach we were able to identify 18 major virion proteins, including structural, regulatory, and signaling proteins of both viral and cellular origin.

Targeting of herpesvirus capsid transport in axons is coupled to association with specific sets of tegument proteins

The capsids of neurotropic herpesviruses have the remarkable ability to move in specific directions within axons. By modulating bidirectional capsid transport to favor either retrograde (minus-end) or anterograde (plus-end) motion, these viruses travel to sensory ganglia or peripheral tissue at specific stages of infection. By using correlative motion analysis to simultaneously monitor the trafficking of distinct viral proteins in living neurons, we demonstrate that viral "tegument" proteins are complexed to capsids moving in axons. The removal of a subset of tegument proteins from capsids invariably preceded retrograde transport to the cell body in sensory ganglia, whereas addition of these proteins was coupled to anterograde transport of progeny capsids to the distal axon. Although capsid transport never occurred without associated tegument proteins, anterograde-specific tegument proteins were competent to travel to the distal axon independent of capsids. These findings are compatible with a model of viral bidirectional transport in which tegument proteins direct capsid traffic to specific intracellular locations during the infectious cycle.

Identification of an essential domain in the herpes simplex virus 1 UL34 protein that is necessary and sufficient to interact with UL31 protein

Previous results have indicated that the herpes simplex virus 1 UL31 and UL34 proteins interact and form a complex at the inner nuclear membranes of infected cells, where both play important roles in the envelopment of nucleocapsids at the inner nuclear membrane. In the work described here, mapping studies using glutathione S-transferase pull-down assays indicated that amino acids 137 to 181 of the UL34 protein are sufficient to mediate an interaction with the UL31 protein. A recombinant virus (v3480) lacking UL34 codons 138 to 181 was constructed. Similar to a UL34 null virus, v3480 failed to replicate on Vero cells and grew to a limited extent on rabbit skin cells. A UL34-expressing cell line restored v3480 growth and plaque formation. Similar to the localization of UL31 protein in cells infected with a UL34 null virus, the UL31 protein was present in the nuclei of Hep2 cells infected with v3480. Hep2 cells infected with v3480 contained the UL34 protein in the cytoplasm, the nucleus, and the nuclear membrane, and this was noted to be similar to the appearance of cells infected with a UL31 null virus. In transient expression assays, the interaction between UL34 amino acids 137 to 181 and the UL31 protein was sufficiently robust to target green fluorescent protein and emerin to intranuclear sites that contained the UL31 protein. These data indicate that amino acids 137 to 181 of the UL34 protein are (i) sufficient to mediate interactions with the UL31 protein in vitro and in vivo, (ii) necessary for the colocalization of UL31 and UL34 in infected cells, and (iii) essential for normal viral replication.

Characterization and intracellular localization of the Epstein-Barr virus protein BFLF2: interactions with BFRF1 and with the nuclear lamina

We have reported in the accompanying paper that the BFRF1 protein of Epstein-Barr virus (EBV) is important for efficient primary viral envelopment and egress (A. Farina, R. Feederle, S. Raffa, R. Gonnella, R. Santarelli, L. Frati, A. Angeloni, M. R. Torrisi, A. Faggioni, and H.-J. Delecluse, J. Virol. 79:3703-3712). Here we describe the characterization of the product of the EBV BFLF2 gene, which belongs to a family of conserved herpesviral genes which include the UL31 genes of herpes simplex virus and of pseudorabies virus and whose products are known to interact with UL34, the positional homolog of BFRF1. BFLF2 is an early transcript and is expressed in a variety of cell lines upon EBV lytic cycle activation. Western blotting of purified virion preparations showed that BFLF2 is a component of intracellular virions but is absent from mature extracellular virions. Coimmunoprecipitation experiments indicated that BFLF2 interacts with BFRF1, which was confirmed by immunofluorescence confocal microscopy showing that the two proteins colocalize on the nuclear membrane not only upon cotransfection in epithelial cells but also during viral replication. In cells carrying an EBV mutant with the BFRF1 gene deleted (293-BFRF1-KO cells) BFLF2 expression was low, and it was restored to wild-type levels upon treatment of the cells with the proteasome inhibitor MG132. Furthermore, recomplementing the 293-BFRF1-KO cells by BFRF1 transfection restored BFLF2 expression to the wild-type level. In addition, when expressed alone BFLF2 was localized diffusely inside the nucleus, whereas in the presence of BFRF1 the two proteins colocalized at the nuclear rim. Finally, 293 epithelial cells transfected with either protein or cotransfected were analyzed by electron microscopy to investigate potential alterations in the morphology of the nuclear membrane. The ultrastructural analysis revealed that (i) BFRF1 caused duplications of the nuclear membrane, similar to those reported to occur during the course of herpesviral replication, and (ii) while BFLF2 alone did not cause any apparent alteration, coexpression of the two proteins dramatically induced profound convolutions of the duplicated nuclear membrane. Both biochemical and morphological analysis showed association of the BFRF1-BFLF2 complex with a component of the nuclear lamina, lamin B. Taken together, these results and those of the accompanying paper (Farina et al., J. Virol. 79:3703-3712) indicate an important role of BFRF1 and BFLF2 in the early steps of EBV maturation at the nuclear membrane.

BFRF1 of Epstein-Barr virus is essential for efficient primary viral envelopment and egress

The molecular mechanisms that underlie maturation and egress of Epstein-Barr virus (EBV) virions are only partially characterized. We have recently shown that the BFRF1 gene, the EBV positional homolog of herpes simplex virus type 1 and pseudorabies virus UL34, is expressed early during EBV lytic replication and that it is found predominantly on the nuclear membrane (A. Farina, R. Santarelli, R. Gonnella, R. Bei, R. Muraro, G. Cardinali, S. Uccini, G. Ragona, L. Frati, A. Faggioni, and A. Angeloni, J. Virol. 74:3235-3244, 2000). These data suggest that the BFRF1 protein might be involved in viral primary envelopment. To precisely determine the function of this protein, we have constructed an EBV mutant devoid of the BFRF1 gene (BFRF1-KO). 293 cells carrying BFRF1-KO showed no differences in comparison with wild-type EBV in terms of DNA lytic replication or expression of late viral proteins upon induction of the lytic cycle. However, binding assays and infection experiments using cell lines or human cord blood lymphocytes showed a clear reduction in the viral mutant titers. Complementation experiments with BFRF1-KO and a BFRF1 expression vector restored viral titers to levels similar to those for the wild-type control, showing that the modifications that we introduced were limited to the BFRF1 gene. Electron microscopic observations showed that the reduction in viral titers was due to sequestration of EBV nucleocapsids in the nuclei of lytically induced cells. This suggests that BFRF1 is involved in transport of the maturing virion across the nuclear membrane. This hypothesis was further supported by the observation that BFRF1 is present in maturing intracellular virions but not in their extracellular counterparts. This implies that BFRF1 is a key protein for EBV maturation.

Kaposi's sarcoma-associated herpesvirus modulates microtubule dynamics via RhoA-GTP-diaphanous 2 signaling and utilizes the dynein motors to deliver its DNA to the nucleus

Human herpesvirus 8 (HHV-8; also called Kaposi's sarcoma-associated herpesvirus), which is implicated in the pathogenesis of Kaposi's sarcoma (KS) and lymphoproliferative disorders, infects a variety of target cells both in vivo and in vitro. HHV-8 binds to several in vitro target cells via cell surface heparan sulfate and utilizes the alpha3beta1 integrin as one of its entry receptors. Interactions with cell surface molecules induce the activation of host cell signaling cascades and cytoskeletal changes (P. P. Naranatt, S. M. Akula, C. A. Zien, H. H. Krishnan, and B. Chandran, J. Virol. 77:1524-1539, 2003). However, the mechanism by which the HHV-8-induced signaling pathway facilitates the complex events associated with the internalization and nuclear trafficking of internalized viral DNA is as yet undefined. Here we examined the role of HHV-8-induced cytoskeletal dynamics in the infectious process and their interlinkage with signaling pathways. The depolymerization of microtubules did not affect HHV-8 binding and internalization, but it inhibited the nuclear delivery of viral DNA and infection. In contrast, the depolymerization of actin microfilaments did not have any effect on virus binding, entry, nuclear delivery, or infection. Early during infection, HHV-8 induced the acetylation of microtubules and the activation of the RhoA and Rac1 GTPases. The inactivation of Rho GTPases by Clostridium difficile toxin B significantly reduced microtubular acetylation and the delivery of viral DNA to the nucleus. In contrast, the activation of Rho GTPases by Escherichia coli cytotoxic necrotizing factor significantly augmented the nuclear delivery of viral DNA. Among the Rho GTPase-induced downstream effector molecules known to stabilize the microtubules, the activation of RhoA-GTP-dependent diaphanous 2 was observed, with no significant activation in the Rac- and Cdc42-dependent PAK1/2 and stathmin molecules. The nuclear delivery of viral DNA increased in cells expressing a constitutively active RhoA mutant and decreased in cells expressing a dominant-negative mutant of RhoA. HHV-8 capsids colocalized with the microtubules, as observed by confocal microscopic examination, and the colocalization was abolished by the destabilization of microtubules with nocodazole and by the phosphatidylinositol 3-kinase inhibitor affecting the Rho GTPases. These results suggest that HHV-8 induces Rho GTPases, and in doing so, modulates microtubules and promotes the trafficking of viral capsids and the establishment of infection. This is the first demonstration of virus-induced host cell signaling pathways in the modulation of microtubule dynamics and in the trafficking of viral DNA to the infected cell nucleus. These results further support our hypothesis that HHV-8 manipulates the host cell signaling pathway to create an appropriate intracellular environment that is conducive to the establishment of a successful infection.

Kaposi's sarcoma-associated herpesvirus modulates microtubule dynamics via RhoA-GTP-diaphanous 2 signaling and utilizes the dynein motors to deliver its DNA to the nucleus

Human herpesvirus 8 (HHV-8; also called Kaposi's sarcoma-associated herpesvirus), which is implicated in the pathogenesis of Kaposi's sarcoma (KS) and lymphoproliferative disorders, infects a variety of target cells both in vivo and in vitro. HHV-8 binds to several in vitro target cells via cell surface heparan sulfate and utilizes the alpha3beta1 integrin as one of its entry receptors. Interactions with cell surface molecules induce the activation of host cell signaling cascades and cytoskeletal changes (P. P. Naranatt, S. M. Akula, C. A. Zien, H. H. Krishnan, and B. Chandran, J. Virol. 77:1524-1539, 2003). However, the mechanism by which the HHV-8-induced signaling pathway facilitates the complex events associated with the internalization and nuclear trafficking of internalized viral DNA is as yet undefined. Here we examined the role of HHV-8-induced cytoskeletal dynamics in the infectious process and their interlinkage with signaling pathways. The depolymerization of microtubules did not affect HHV-8 binding and internalization, but it inhibited the nuclear delivery of viral DNA and infection. In contrast, the depolymerization of actin microfilaments did not have any effect on virus binding, entry, nuclear delivery, or infection. Early during infection, HHV-8 induced the acetylation of microtubules and the activation of the RhoA and Rac1 GTPases. The inactivation of Rho GTPases by Clostridium difficile toxin B significantly reduced microtubular acetylation and the delivery of viral DNA to the nucleus. In contrast, the activation of Rho GTPases by Escherichia coli cytotoxic necrotizing factor significantly augmented the nuclear delivery of viral DNA. Among the Rho GTPase-induced downstream effector molecules known to stabilize the microtubules, the activation of RhoA-GTP-dependent diaphanous 2 was observed, with no significant activation in the Rac- and Cdc42-dependent PAK1/2 and stathmin molecules. The nuclear delivery of viral DNA increased in cells expressing a constitutively active RhoA mutant and decreased in cells expressing a dominant-negative mutant of RhoA. HHV-8 capsids colocalized with the microtubules, as observed by confocal microscopic examination, and the colocalization was abolished by the destabilization of microtubules with nocodazole and by the phosphatidylinositol 3-kinase inhibitor affecting the Rho GTPases. These results suggest that HHV-8 induces Rho GTPases, and in doing so, modulates microtubules and promotes the trafficking of viral capsids and the establishment of infection. This is the first demonstration of virus-induced host cell signaling pathways in the modulation of microtubule dynamics and in the trafficking of viral DNA to the infected cell nucleus. These results further support our hypothesis that HHV-8 manipulates the host cell signaling pathway to create an appropriate intracellular environment that is conducive to the establishment of a successful infection.

Recombinant baculoviruses used to study estrogen receptor function in human osteosarcoma cells

We report that modified baculoviruses, termed BacMam viruses, can efficiently deliver multiple genes into mammalian cells to generate a heterologous transcription factor/reporter gene system. Using human estrogen receptor (ER) as a model nuclear receptor, we demonstrate how this approach can be successfully applied to assay development in Saos-2 human osteosarcoma cells. BacMam viruses containing full-length cDNAs were constructed for both human ER subtypes, ERalpha and ERbeta, and a third BacMam virus containing an ER-responsive reporter gene cassette. Using these viruses, we found that BacMam-ER expression/reporter constructs could be used to profile the effects of the agonist 17beta-estradiol and the partial agonist raloxifene in human Saos-2 cells. A comparison of assay data obtained with the BacMam-based system with that using standard DNA transfections demonstrates that the two systems are functionally equivalent, giving comparable EC(50) and IC(50) values for estrogen and estrogen plus raloxifene treatments, respectively. Our results indicate that BacMam-mediated gene transfer offers a novel and efficient method for delivery of nuclear receptors and associated genes for mammalian cell-based assay development.

Local modulation of plus-end transport targets herpesvirus entry and egress in sensory axons

The core structures of many viruses move within cells by association with host cytoskeletal motor proteins; however, the mechanisms by which intracellular viral particles are transported toward sites of replication or the cell periphery at distinct stages of infection remain to be understood. The regulation of herpesvirus directional transport in sensory neurons was examined by tracking individual viral capsids within axons at multiple frames per s. After entry into axons, capsids underwent bidirectional and saltatory movement to the cell body independently of endosomes. A comparison of entry transport to a previous analysis of capsid axonal transport during egress revealed that capsid targeting in and out of cells occurs by modulation of plus-end, but not minus-end, motion. Entry transport was unperturbed by the presence of egressing virus from a prior infection, indicating that transport direction is not modulated globally by viral gene expression, but rather directly by a component of the viral particle.

Cytoplasmic dynein mediates adenovirus binding to microtubules

During infection, adenovirus (Ad) capsids undergo microtubule-dependent retrograde transport as part of a program of vectorial transport of the viral genome to the nucleus. The microtubule-associated molecular motor, cytoplasmic dynein, has been implicated in the retrograde movement of Ad. We hypothesized that cytoplasmic dynein constituted the primary mode of association of Ad with microtubules. To evaluate this hypothesis, an Ad-microtubule binding assay was established in which microtubules were polymerized with taxol, combined with Ad in the presence or absence of microtubule-associated proteins (MAPs), and centrifuged through a glycerol cushion. The addition of purified bovine brain MAPs increased the fraction of Ad in the microtubule pellet from 17.3% +/- 3.5% to 80.7% +/- 3.8% (P < 0.01). In the absence of tubulin polymerization or in the presence of high salt, no Ad was found in the pellet. Ad binding to microtubules was not enhanced by bovine brain MAPs enriched for tau protein or by the addition of bovine serum albumin. Enhanced Ad-microtubule binding was also observed by using a fraction of MAPs purified from lung A549 epithelial cell lysate which contained cytoplasmic dynein. Ad-microtubule interaction was sensitive to the addition of ATP, a hallmark of cytoplasmic dynein-dependent microtubule interactions. Immunodepletion of cytoplasmic dynein from the A549 cell lysate abolished the MAP-enhanced Ad-microtubule binding. The interaction of Ad with both dynein and dynactin complexes was demonstrated by coimmunoprecipitation. Partially uncoated capsids isolated from cells 40 min after infection also exhibited microtubule binding. In summary, the primary mode of Ad attachment to microtubules occurs though cytoplasmic dynein-mediated binding.

Conformational changes in the nuclear lamina induced by herpes simplex virus type 1 require genes U(L)31 and U(L)34

The herpes simplex virus type 1 (HSV-1) U(L)31 and U(L)34 proteins are dependent on each other for proper targeting to the nuclear membrane and are required for efficient envelopment of nucleocapsids at the inner nuclear membrane. In this work, we show that whereas the solubility of lamins A and C (lamin A/C) was not markedly increased, HSV induced conformational changes in the nuclear lamina of infected cells, as viewed after staining with three different lamin A/C-specific antibodies. In one case, reactivity with a monoclonal antibody that recognizes an epitope in the lamin tail domain was greatly reduced in HSV-infected cells. This apparent HSV-induced epitope masking required both U(L)31 and U(L)34, but these proteins were not sufficient to mask the epitope in uninfected cells, indicating that other HSV proteins are also required. In the second case, staining with a rabbit polyclonal antibody that primarily recognizes epitopes in the lamin A/C rod domain revealed that U(L)34 is required for HSV-induced decreased availability of epitopes for reaction with the antibody, whereas U(L)31 protein was dispensable for this effect. Still another polyclonal antibody indicated virtually no difference in lamin A/C staining in infected versus uninfected cells, indicating that the HSV-induced changes are more conformational than the result of lamin depletion at the nuclear rim. Further evidence supporting an interaction between the nuclear lamina and the U(L)31/U(L)34 protein complex includes the observations that (i) overexpression of the U(L)31 protein in uninfected cells was sufficient to relocalize lamin A/C from the nuclear rim into nucleoplasmic aggregates, (ii) overexpression of U(L)34 was sufficient to relocalize some lamin A/C into the cytoplasm, and (iii) both U(L)31 and U(L)34 could directly bind lamin A/C in vitro. These studies suggest that the U(L)31 and U(L)34 proteins modify the conformation of the nuclear lamina in infected cells, possibly by direct interaction with lamin A/C, and that other proteins are also likely involved. Given that the nuclear lamina potentially excludes nucleocapsids from envelopment sites at the inner nuclear membrane, the lamina alteration may reflect a role of the U(L)31/U(L)34 protein complex in perturbing the lamina to promote nucleocapsid egress from the nucleus. Alternatively, the data are compatible with a role of the lamina in targeting the U(L)31/U(L)34 protein complex to the nuclear membrane.

The Epstein-Barr virus BFRF1 and BFLF2 proteins interact and coexpression alters their cellular localization

The BFRF1 protein of Epstein-Barr virus (EBV) is a recently identified membrane protein that is the homolog of the alphaherpesvirus UL34 gene product. We report here that a yeast two-hybrid screen identified the BFLF2 gene product, a homolog of alphaherpesvirus UL31, as a protein that interacts with BFRF1. Expression of BFLF2 in mammalian cells revealed a protein of approximately 28 kDa that associated with BFRF1 in a noncovalently linked complex. When expressed alone, the BFRF1 protein was found in the cytoplasm and perinuclear region. BFLF2 was found diffusely in the nucleus in the absence of BFRF1, but coexpression of BFRF1 and BFLF2 resulted in colocalization of the two proteins at the nuclear rim. These data recapitulate the behavior of the alphaherpesvirus homologs of BFRF1 and BFLF2 and suggest that functional as well as structural and positional homology may be conserved.

Herpes simplex virus type 1 capsid protein VP26 interacts with dynein light chains RP3 and Tctex1 and plays a role in retrograde cellular transport

Cytoplasmic dynein is the major molecular motor involved in minus-end-directed cellular transport along microtubules. There is increasing evidence that the retrograde transport of herpes simplex virus type 1 along sensory axons is mediated by cytoplasmic dynein, but the viral and cellular proteins involved are not known. Here we report that the herpes simplex virus outer capsid protein VP26 interacts with dynein light chains RP3 and Tctex1 and is sufficient to mediate retrograde transport of viral capsids in a cellular model. A library of herpes simplex virus capsid and tegument structural genes was constructed and tested for interactions with dynein subunits in a yeast two-hybrid system. A strong interaction was detected between VP26 and the homologous 14-kDa dynein light chains RP3 and Tctex1. In vitro pull-down assays confirmed binding of VP26 to RP3, Tctex1, and intact cytoplasmic dynein complexes. Recombinant herpes simplex virus capsids were constructed either with or without VP26. In pull-down assays VP26+ capsids bound to RP3; VP26-capsids did not. To investigate intracellular transport, the recombinant viral capsids were microinjected into living cells and incubated at 37 degrees C. After 1 h VP26+ capsids were observed to co-localize with RP3, Tctex1, and microtubules. After 2 or 4 h VP26+ capsids had moved closer to the cell nucleus, whereas VP26-capsids remained in a random distribution. We propose that VP26 mediates binding of incoming herpes simplex virus capsids to cytoplasmic dynein during cellular infection, through interactions with dynein light chains.

Dynein: An ancient motor protein involved in multiple modes of transport

Cytoplasmic dynein has long been thought to be responsible for retrograde axonal transport. As the number of cellular roles for this multifunctional protein has expanded, the complexity of its contribution to axonal transport has increased. In this article the increasing evidence for a role for cytoplasmic dynein in anterograde as well as retrograde transport is discussed. The current status of the complex dynein cargo-binding mechanism is evaluated. Finally, recent genetic evidence supporting a role in axonal transport and revealing a role in neurodegenerative conditions is reviewed.

Intracellular localization of the Epstein-Barr virus BFRF1 gene product in lymphoid cell lines and oral hairy leukoplakia lesions

A novel protein encoded by the BFRF1 gene of the Epstein-Barr virus was identified recently [Farina et al. (2000) J Virol 74:3235-3244], which is antigenic "in vivo" and expressed early in the viral replicative cycle. In the present study, its subcellular localization was examined in greater detail comparing Epstein-Barr virus (EBV) induced producing and nonproducing cell lines by immunofluorescence: in 12-0-tetradecanoyl phorbol-13-acetate (TPA)-induced Raji and B95-8 cells, as well as in anti-IgG-stimulated Akata cells, the protein appeared to be localized over the cell nuclear membrane. A similar nuclear membrane localization was observed in epithelial cells of oral hairy leukoplakia, a pathological manifestation of permissive EBV infection. In contrast, upon transfection of BFRF1 in the EBV-negative Burkitt's lymphoma cell line DG75, the protein was localized predominantly over the plasma membrane. The membrane localization was abolished when DG75 cells were transfected with a C-terminal deletion mutant of BFRF1 lacking the transmembrane domain. Because induced Raji cells do not produce virus, the above observations indicate that the nuclear membrane localization is not associated with viral production, but requires the expression of EBV genes, and suggest that additional proteins, expressed early during viral lytic infection, might be necessary to target the protein to the nuclear membrane. Immunogold electron microscopy on ultrathin cryosections of induced B95-8 cells showed that BFRF1 on the nuclear membranes was concentrated over multilayered domains representing areas of active viral replication or at the sites of viral budding, suggesting that BFRF1 is involved in the process of viral assembly.

The pseudorabies virus Us2 protein, a virion tegument component, is prenylated in infected cells

The Us2 gene is conserved among alphaherpesviruses, but its function is not known. We demonstrate here that the pseudorabies virus (PRV) Us2 protein is synthesized early after infection and localizes to cytoplasmic vesicles and to the plasma membrane, despite the lack of a recognizable signal sequence or membrane-spanning domain. Us2 protein is also packaged as part of the tegument of mature virions. The Us2 carboxy-terminal four amino acids comprise a CAAX motif, a well-characterized signal for protein prenylation. Treatment of infected cells with lovastatin, a drug that disrupts protein prenylation, changed the relative electrophoretic mobility of Us2 in sodium dodecyl sulfate-polyacrylamide gels. In addition, lovastatin treatment caused a dramatic relocalization of Us2 to cytoplasmic punctate structures associated with microtubules, which appeared to concentrate over the microtubule organizing center. When the CAAX motif was changed to GAAX and the mutant protein was synthesized from an expression plasmid, it concentrated in punctate cytoplasmic structures reminiscent of Us2 localization in infected cells treated with lovastatin. We suggest that prenylation of PRV Us2 protein is required for proper membrane association. Curiously, the Us2 protein isolated from purified virions does not appear to be prenylated. This is the first report to describe the prenylation of an alphaherpesvirus protein.

Herpes simplex virion entry into and intracellular transport within mammalian cells

Alphaherpesviruses, membrane-enveloped DNA viruses that are responsible for a host of human ailments, bind to, enter and are directly targeted to specific intracellular domains within their mammalian host cells. This review emphasizes recent work on the best studied of the alphaherpesviruses, Herpes simplex virus type 1 (HSV1). One area of focus is on recent work that has identified viral glycoproteins that are important in binding and internalization of the virus to the host cell. Complementary work on the receptors for those viral glycoproteins that reside on the host cell surface is also presented, with some discussion of how receptor variety might lead to the tissue tropism demonstrated by alphaherpes viruses. An additional area of focus in this review is how HSV uses the host cell transport systems to achieve intracellular targeting of the incoming virion toward the cell nucleus, and, after production of newly synthesized and assembled viral progeny, targeting them toward the plasma membrane for release.

Exploitation of microtubule cytoskeleton and dynein during parvoviral traffic toward the nucleus

Canine parvovirus (CPV), a model virus for the study of parvoviral entry, enters host cells by receptor-mediated endocytosis, escapes from endosomal vesicles to the cytosol, and then replicates in the nucleus. We examined the role of the microtubule (MT)-mediated cytoplasmic trafficking of viral particles toward the nucleus. Immunofluorescence and immunoelectron microscopy showed that capsids were transported through the cytoplasm into the nucleus after cytoplasmic microinjection but that in the presence of MT-depolymerizing agents, viral capsids were unable to reach the nucleus. The nuclear accumulation of capsids was also reduced by microinjection of an anti-dynein antibody. Moreover, electron microscopy and light microscopy experiments demonstrated that viral capsids associate with tubulin and dynein in vitro. Coprecipitation studies indicated that viral capsids interact with dynein. When the cytoplasmic transport process was studied in living cells by microinjecting fluorescently labeled capsids into the cytoplasm of cells containing fluorescent tubulin, capsids were found in close contact with MTs. These results suggest that intact MTs and the motor protein dynein are required for the cytoplasmic transport of CPV capsids and contribute to the accumulation of the capsid in the nucleus.

Chlamydia trachomatis uses host cell dynein to traffic to the microtubule-organizing center in a p50 dynamitin-independent process

Chlamydiae are pathogenic obligate intracellular bacteria with a biphasic developmental cycle that involves cell types adapted for extracellular survival (elementary bodies, EBs) and intracellular multiplication (reticulate bodies, RBs). The intracellular development of chlamydiae occurs entirely within a membrane-bound vacuole termed an inclusion. Within 2 hours after entry into host cells, Chlamydia trachomatis EBs are trafficked to the perinuclear region of the host cell and remain in close proximity to the Golgi apparatus, where they begin to fuse with a subset of host vesicles containing sphingomyelin. Here, we provide evidence that chlamydial migration from the cell periphery to the peri-Golgi region resembles host cell vesicular trafficking. Chlamydiae move towards the minus end of microtubules and aggregate at the microtubule-organizing center (MTOC). In mammalian cells the most important minus-end-directed microtubule motor is cytoplasmic dynein. Microinjection of antibodies to a subunit of cytoplasmic dynein inhibited movement of chlamydiae to the MTOC, whereas microinjection of antibodies to the plus-directed microtubule motor, kinesin, had no effect. Surprisingly, overexpression of the protein p50 dynamitin, a subunit of the dynactin complex that links vesicular cargo to the dynein motor in minus directed vesicle trafficking, did not abrogate chlamydial migration even though host vesicle transport was inhibited. Nascent chlamydial inclusions did, however, colocalize with the p150(Glued) dynactin subunit, which suggests that p150(Glued) may be required for dynein activation or processivity but that the cargo-binding activity of dynactin, supplied by p50 dynamitin subunits and possibly other subunits, is not. Because chlamydial transcription and translation were required for this intracellular trafficking, chlamydial proteins modifying the cytoplasmic face of the inclusion membrane are probable candidates for proteins fulfilling this function.

Effects of charged cluster mutations on the function of herpes simplex virus type 1 UL34 protein

Herpes simplex virus type 1 (HSV-1) is a DNA virus that acquires an envelope by budding into the inner nuclear membrane of an infected cell. Recombinant HSV-1 lacking the U(L)34 gene cannot undergo this event. U(L)34 and U(L)31, another viral protein, colocalize in an infected cell and are necessary and sufficient to target both proteins to the inner nuclear envelope. In order to define and characterize sequences of U(L)34 that are necessary for primary envelopment to occur, a library of 19 U(L)34 charged cluster mutants and a truncation mutant lacking the putative transmembrane domain (DeltaTM) were generated. Mutants in this library were analyzed in a complementation assay for their ability to function in the production of infectious virus. Seven of the mutants failed to complement a U(L)34-null virus. The remainder of the mutants complemented at or near wild-type U(L)34 levels. Failure of a mutant protein to function might be the result of incorrect subcellular localization. To address this possibility, confocal microscopy was used to determine the localization of the U(L)34 protein in charged cluster mutants and DeltaTM. In transfection-infection experiments, all of the functional U(L)34 mutants and four of the six noncomplementing mutants localized to the inner nuclear envelope in a manner indistinguishable from that of wild-type U(L)34. All of the noncomplementing U(L)34 mutants mediated proper localization of U(L)31. Charged clusters critical for U(L)34 function are dispersed throughout the protein sequence and do not correlate well with highly conserved regions of the protein. These data suggest that U(L)34 has at least one function in addition to mediating proper localization of U(L)31 in infected cells and provide further support for the role of U(L)34 in mediating proper localization of U(L)31 in infected cells.

Recognition of novel viral sequences that associate with the dynein light chain LC8 identified through a pepscan technique

Recent data from multiple laboratories indicate that upon infection, many different families of viruses hijack the dynein motor machinery and become transported in a retrograde manner towards the cell nucleus. In certain cases, one of the dynein light chains, LC8, is involved in this interaction. Using a library of overlapping dodecapeptides synthesized on a cellulose membrane (pepscan technique) we have analyzed the interaction of the dynein light chain LC8 with 17 polypeptides of viral origin. We demonstrate the strong binding of two herpesvirus polypeptides, the human adenovirus protease, vaccinia virus polymerase, human papillomavirus E4 protein, yam mosaic virus polyprotein, human respiratory syncytial virus attachment glycoprotein, human coxsackievirus capsid protein and the product of the AMV179 gene of an insect poxvirus to LC8. Our data corroborate the manipulation of the dynein macromolecular complex of the cell during viral infection and point towards the light chain LC8 as one of the most frequently used targets of virus manipulation.

Cytoskeletal motors and cargo in membrane trafficking: opportunities for high specificity in drug intervention

Membrane trafficking comprises the directed transport of vesicle and/or organelle cargos to specific locations throughout the cell, and is primarily driven by molecular motors tracking along microtubules and microfilaments. The mechanisms by which specific motor complexes attach to their respective vesicular cargo is of great interest, and is only now starting to be unraveled. The proteins identified as links between the molecular motors and the vesicular cargo are viable drug targets and represent opportunities to regulate small groups of related proteins or even single proteins, such as receptors and transporters, at the cytosolic trafficking level. Ultimately, continued development in this area will lead to greater success in directing endocytosed drugs to the desired intracellular targets, such as the cell nucleus or the basolateral membrane.

Break ins and break outs: viral interactions with the cytoskeleton of Mammalian cells

The host cytoskeleton plays important roles in the entry, replication, and egress of viruses. An assortment of viruses hijack cellular motor proteins to move on microtubules toward the cell interior during the entry process; others reverse this transport during egress to move assembling virus particles toward the plasma membrane. Polymerization of actin filaments is sometimes used to propel viruses from cell to cell, while many viruses induce the destruction of select cytoskeletal filaments apparently to effect efficient egress. Indeed, the tactics used by any given virus to achieve its infectious life cycle are certain to involve multiple cytoskeletal interactions. Understanding these interactions, and their orchestration during viral infections, is providing unexpected insights into basic virology, viral pathogenesis, and the biology of the cytoskeleton.

The effects of cyclic stretch on gene transfer in alveolar epithelial cells

Cyclic stretch has been shown to alter cell physiology, cytoskeletal structure, signal transduction, and gene expression in a variety of cell types. To determine the effects of stretch on the gene transfer process, we compared the transfection efficiencies of human A549 cells grown either statically or exposed to 10% cyclic stretch (Delta surface area) at 60 cycles/min (1 Hz) for 24 hours prior to, and/or after transfection with pEGFP-N1 and pCMV-lux-DTS using lipoplex or electroporation. Stretching the cells prior to transfection had no effect on gene transfer. By contrast, cyclic, but not continuous, stretch applied immediately after transfection for as little as 30 minutes resulted in a 10-fold increase in gene transfer and expression by either transfection technique. These stretch conditions did not result in rupture of the plasma membrane based on the fact that DNA was unable to enter stretched cells unless either an electric field was applied or the DNA was complexed with liposomes. Taken together with the timing of the stretch response and the known effects of stretch on transcription, these findings suggest that cyclic stretch may be altering the intracellular transport of plasmids to increase gene expression.

Cargo-carrying motor vehicles on the neuronal highway: Transport pathways and neurodegenerative disease

Within axons vital cargoes must be transported over great distances along microtubule tracks to maintain neuronal viability. Essential to this system are the molecular motors, kinesin and dynein, which transport a variety of neuronal cargoes. Elucidating the transport pathways, the identity of the cargoes transported, and the regulation of motor-cargo complexes are areas of intense investigation. Evidence suggests that essential components, including signaling proteins, neuroprotective and repair molecules, and vesicular and cytoskeletal components are all transported. In addition newly emerging data indicate that defects in axonal transport pathways may contribute to the initiation or progression of chronic neuronal dysfunction. In this review we concentrate on microtubule-based motor proteins, their linkers, and cargoes and discuss how factors in the axonal transport pathway contribute to disease states. As additional cargo complexes and transport pathways are identified, an understanding of the role these pathways play in the development of human disease will hopefully lead to new diagnostic and treatment strategies.

Interaction of HCF-1 with a cellular nuclear export factor

HCF-1 is a cellular protein required by VP16 to activate the herpes simplex virus (HSV) immediate-early genes. VP16 is a component of the viral tegument and, after release into the cell, binds to HCF-1 and translocates to the nucleus to form a complex with the POU domain protein Oct-1 and a VP16-responsive DNA sequence. This VP16-induced complex boosts transcription of the viral immediate-early genes and initiates lytic replication. In uninfected cells, HCF-1 functions as a coactivator for the cellular transcription factors LZIP and GABP and also plays an essential role in cell proliferation. VP16 and LZIP share a tetrapeptide HCF-binding motif recognized by the beta-propeller domain of HCF-1. Here we describe a new cellular HCF-1 beta-propeller domain binding protein, termed HPIP, which contains a functional HCF-binding motif and a leucine-rich nuclear export sequence. We show that HPIP shuttles between the nucleus and cytoplasm in a CRM1-dependent manner and that overexpression of HPIP leads to accumulation of HCF-1 in the cytoplasm. These data suggest that HPIP regulates HCF-1 activity by modulating its subcellular localization. Furthermore, HPIP-mediated export may provide the pool of cytoplasmic HCF-1 required for import of virion-derived VP16 into the nucleus.

The roadblock light chain binds a novel region of the cytoplasmic Dynein intermediate chain

Cytoplasmic dynein is the major minus-end directed microtubule-based motor in eukaryotic cells. It is composed of a number of different subunits including three light chain families: Tctex1, LC8, and roadblock. The incorporation of the roadblock light chains into the cytoplasmic dynein complex had not been determined. There are two roadblock genes in mammals, ROBL-1 and ROBL-2. We find that both members of the roadblock family bind directly to all of the intermediate chain isoforms of mammalian cytoplasmic dynein. This was determined with three complementary approaches. A yeast two-hybrid assay demonstrated that both roadblock light chains interact with intermediate chain isoforms from the IC74-1 and IC74-2 genes in vivo. This was confirmed in vitro with both a solid phase blot overlay assay and a solution-binding assay. The roadblock-binding domain on the intermediate chain was mapped to an approximately 72 residue region. The binding domain is downstream of each of the two alternative splice sites in the intermediate chains. This location is consistent with the finding that both roadblock-1 and roadblock-2 show no binding specificity for a single IC74-1 or IC74-2 intermediate chain isoform. In addition, this roadblock-binding domain is significantly downstream from both the Tctex1- and LC8-binding sites, supporting the hypothesis that multiple light chain family members can bind to the same intermediate chain.