Identification of a novel signal sequence that targets transmembrane proteins to the nuclear envelope inner membrane

The Knowns and Unknowns of Herpesvirus Nuclear Egress

Nuclear egress of herpesvirus capsids across the intact nuclear envelope is an exceptional vesicle-mediated nucleocytoplasmic translocation resulting in the delivery of herpesvirus capsids into the cytosol. Budding of the (nucleo)capsid at and scission from the inner nuclear membrane (INM) is mediated by the viral nuclear egress complex (NEC) resulting in a transiently enveloped virus particle in the perinuclear space followed by fusion of the primary envelope with the outer nuclear membrane (ONM). The dimeric NEC oligomerizes into a honeycomb-shaped coat underlining the INM to induce membrane curvature and scission. Mutational analyses complemented structural data defining functionally important regions. Questions remain, including where and when the NEC is formed and how membrane curvature is mediated, vesicle formation is regulated, and directionality is secured. The composition of the primary enveloped virion and the machinery mediating fusion of the primary envelope with the ONM is still debated. While NEC-mediated budding apparently follows a highly conserved mechanism, species and/or cell type-specific differences complicate understanding of later steps.

Endocytosis and Organelle Targeting of Nanomedicines in Cancer Therapy

Nanomedicines (NMs) have played an increasing role in cancer therapy as carriers to efficiently deliver therapeutics into tumor cells. For this application, the uptake of NMs by tumor cells is usually a prerequisite to deliver the cargo to intracellular locations, which mainly relies on endocytosis. NMs can enter cells through a variety of endocytosis pathways. Different endocytosis pathways exhibit different intracellular trafficking routes and diverse subcellular localizations. Therefore, a comprehensive understanding of endocytosis mechanisms is necessary for increasing cellular entry efficiency and to trace the fate of NMs after internalization. This review focuses on endocytosis pathways of NMs in tumor cells, mainly including clathrin- and caveolae-mediated endocytosis pathways, involving effector molecules, expression difference of those molecules between normal and tumor cells, as well as the intracellular trafficking route of corresponding endocytosis vesicles. Then, the latest strategies for NMs to actively employ endocytosis are described, including improving tumor cellular uptake of NMs by receptor-mediated endocytosis, transporter-mediated endocytosis and enabling drug activity by changing intracellular routes. Finally, active targeting strategies towards intracellular organelles are also mentioned. This review will be helpful not only in explicating endocytosis and the trafficking process of NMs and elucidating anti-tumor mechanisms inside the cell but also in rendering new ideas for the design of highly efficacious and cancer-targeted NMs.

Conservation of the glycoprotein B homologs of the Kaposi׳s sarcoma-associated herpesvirus (KSHV/HHV8) and old world primate rhadinoviruses of chimpanzees and macaques

The envelope-associated glycoprotein B (gB) is highly conserved within the Herpesviridae and plays a critical role in viral entry. We analyzed the evolutionary conservation of sequence and structural motifs within the Kaposi׳s sarcoma-associated herpesvirus (KSHV) gB and homologs of Old World primate rhadinoviruses belonging to the distinct RV1 and RV2 rhadinovirus lineages. In addition to gB homologs of rhadinoviruses infecting the pig-tailed and rhesus macaques, we cloned and sequenced gB homologs of RV1 and RV2 rhadinoviruses infecting chimpanzees. A structural model of the KSHV gB was determined, and functional motifs and sequence variants were mapped to the model structure. Conserved domains and motifs were identified, including an "RGD" motif that plays a critical role in KSHV binding and entry through the cellular integrin αVβ3. The RGD motif was only detected in RV1 rhadinoviruses suggesting an important difference in cell tropism between the two rhadinovirus lineages.

Nuclear Egress of Herpesviruses: The Prototypic Vesicular Nucleocytoplasmic Transport

Herpesvirus particles mature in two different cellular compartments. While capsid assembly and packaging of the genomic linear double-stranded DNA occur in the nucleus, virion formation takes place in the cytoplasm by the addition of numerous tegument proteins as well as acquisition of the viral envelope by budding into cellular vesicles derived from the trans-Golgi network containing virally encoded glycoproteins. To gain access to the final maturation compartment, herpesvirus nucleocapsids have to cross a formidable barrier, the nuclear envelope (NE). Since the ca. 120 nm diameter capsids are unable to traverse via nuclear pores, herpesviruses employ a vesicular transport through both leaflets of the NE. This process involves proteins which support local dissolution of the nuclear lamina to allow access of capsids to the inner nuclear membrane (INM), drive vesicle formation from the INM and mediate inclusion of the capsid as well as scission of the capsid-containing vesicle (also designated as "primary virion"). Fusion of the vesicle membrane (i.e., the "primary envelope") with the outer nuclear membrane subsequently results in release of the nucleocapsid into the cytoplasm for continuing virion morphogenesis. While this process has long been thought to be unique for herpesviruses, a similar pathway for nuclear egress of macromolecular complexes has recently been observed in Drosophila. Thus, herpesviruses may have coopted a hitherto unrecognized cellular mechanism of vesicle-mediated nucleocytoplasmic transport. This could have far reaching consequences for our understanding of cellular functions as again unraveled by the study of viruses.

Herpesvirus nuclear egress: Pseudorabies Virus can simultaneously induce nuclear envelope breakdown and exit the nucleus via the envelopment-deenvelopment-pathway

Herpesvirus replication takes place in the nucleus and in the cytosol. After entering the cell, nucleocapsids are transported to nuclear pores where viral DNA is released into the nucleus. After gene expression and DNA replication new nucleocapsids are assembled which have to exit the nucleus for virion formation in the cytosol. Since nuclear pores are not wide enough to allow passage of the nucleocapsid, nuclear egress occurs by vesicle-mediated transport through the nuclear envelope. To this end, nucleocapsids bud at the inner nuclear membrane (INM) recruiting a primary envelope which then fuses with the outer nuclear membrane (ONM). In the absence of this regulated nuclear egress, mutants of the alphaherpesvirus pseudorabies virus have been described that escape from the nucleus after virus-induced nuclear envelope breakdown. Here we review these exit pathways and demonstrate that both can occur simultaneously under appropriate conditions.

Identification of conserved amino acids in pUL34 which are critical for function of the pseudorabies virus nuclear egress complex

Nuclear egress of herpesvirus capsids is mediated by a conserved heterodimeric complex of two viral proteins, designated pUL34 and pUL31 in herpes simplex virus and pseudorabies virus (PrV). pUL34, a tail-anchored membrane protein, is targeted to the nuclear envelope and recruits pUL31 to the inner nuclear membrane (INM) to provide the docking and envelopment machinery for the nascent capsid. While the less conserved C-terminal part of pUL34 is required for correct positioning of the nuclear egress complex (NEC) at the INM, the conserved N-terminal part functions as a docking site for pUL31. Since no crystal structure of NEC is available yet, structure-function studies depend on mutational analyses, with several approaches already being performed for different herpesvirus NECs. Here, we extended our studies on PrV pUL34 and identified two asparagine residues (N75, N103) and a dileucine motif (LL166/167), adjacent to an endoplasmic reticulum retention signal, which are absolutely required for NEC function. While the pUL34-N75A substitution mutant is unable to interact with pUL31, the pUL34-N103A mutant is nonfunctional, despite continuing complex formation. Surprisingly, mutant pUL34-G77A, which does not efficiently recruit pUL31 to the nuclear rim after cotransfection, nonetheless complements a UL34 deletion mutant, indicating that the NEC may be stabilized by additional viral factors during infection.

IMPORTANCE

In the absence of a crystal structure of the nuclear egress complex (NEC) required for herpesvirus maturation, site-directed mutagenesis studies provide important information on critical amino acid residues. Here, we identify conserved amino acid residues in the membrane-bound component of the NEC which are relevant for its function.

The cytomegalovirus egress proteins pUL50 and pUL53 are translocated to the nuclear envelope through two distinct modes of nuclear import

The nucleocytoplasmic export of cytomegaloviral capsids is regulated by formation of a multi-component nuclear egress complex (NEC), essentially based on viral proteins pUL50 and pUL53. In this study, the generation of recombinant human cytomegaloviruses, expressing tagged versions of pUL50 and pUL53, enabled the investigation of NEC formation in infected primary fibroblasts. For these recombinant viruses, a wild-type-like mode of pUL50-pUL53 interaction and recruitment of both proteins to the nuclear envelope could be demonstrated. Importantly, pUL50 was translocated from an initial cytoplasmic distribution to the nuclear rim, whereas pUL53 accumulated in the nucleus before attaining overall rim colocalization with pUL50. Specified experimental settings illustrated that pUL50 and pUL53 were subject to different pathways of intracellular trafficking. Importantly, a novel nuclear localization signal (NLS) could be identified and functionally verified for pUL53 (amino acids 18-27), whereas no NLS was present in pUL50. Analysis of amino acid replacement mutants further illustrated the differential modes of nuclear import of the two essential viral egress proteins. Taken together, our findings suggest a combination of classical nuclear import (pUL53) and interaction-mediated recruitment (pUL50) as the driving forces for core NEC formation and viral nuclear egress.

Mapping of sequences in Pseudorabies virus pUL34 that are required for formation and function of the nuclear egress complex

The nuclear egress complex (NEC) is required for efficient translocation of newly synthesized herpesvirus nucleocapsids from the nucleus to the cytosol. It consists of the type II membrane protein pUL34 which interacts with pUL31 at the inner nuclear membrane (INM). To map regions within pUL34 required for nuclear membrane targeting and pUL31 interaction, we constructed deletion/substitution mutations. Previously, we showed that 50 C-terminal amino acids (aa) of pseudorabies virus (PrV) pUL34, including the transmembrane domain, could be functionally replaced by cellular lamina-associated polypeptide 2β (Lap2β) sequences. In contrast, replacement of the C-terminal 100 aa abrogated complementation but not pUL31 interaction. To further delineate essential sequences within this region, C-terminal pUL34 truncations of 60, 70, 80, 85, and 90 aa fused to Lap2β sequences were generated. While truncations up to 85 aa were functional, deletion of the C-terminal 90 aa abrogated function, which indicates that the important region is located between aa 171 and 176. Amino acids 173 to 175 represent RQR, a motif suggested to mediate INM targeting. Mutagenesis to RQG revealed that the mutant protein exhibited pronounced Golgi localization, but a fraction still reached the INM. Deletion mutations in the N-terminal domain of pUL34 demonstrated that absence of the first 4 aa was tolerated, while removal of 9 or more residues resulted in a nonfunctional protein. In addition, mutation of three conserved cysteines did not abrogate pUL34 function, whereas alteration of a conserved glutamine/tyrosine sequence yielded a nonfunctional protein.

Structural determinants for nuclear envelope localization and function of pseudorabies virus pUL34

Herpesvirus proteins pUL34 and pUL31 form a complex at the inner nuclear membrane (INM) which is necessary for efficient nuclear egress. Pseudorabies virus (PrV) pUL34 is a type II membrane protein of 262 amino acids (aa). The transmembrane region (TM) is predicted to be located between aa 245 and 261, leaving only one amino acid in the C terminus that probably extends into the perinuclear space. It is targeted to the nuclear envelope in the absence of other viral proteins, pointing to intrinsic localization motifs, and shows structural similarity to cellular INM proteins like lamina-associated polypeptide (Lap) 2ß and Emerin. To investigate which domains of pUL34 are relevant for localization and function, we constructed chimeric proteins by replacing parts of pUL34 with regions of cellular INM proteins. First the 18 C-terminal amino acids encompassing the TM were exchanged with TM regions and C-terminal domains of Lap2ß and Emerin or with the first TM region of the polytopic lamin B receptor (LBR), including the nine following amino acids. All resulting chimeric proteins complemented the replication defect of PrV-ΔUL34, demonstrating that the substitution of the TM and the extension of the C-terminal domain does not interfere with the function of pUL34. Complementation was reduced but not abolished when the C-terminal 50 aa were replaced by corresponding Lap2ß sequences (pUL34-LapCT50). However, replacing the C-terminal 100 aa (pUL34-LapCT100) resulted in a nonfunctional protein despite continuing pUL31 binding, pointing to an important functional role of this region. The replacement of the N-terminal 100 aa (pUL34-LapNT100) had no effect on nuclear envelope localization but abrogated pUL31 binding and function.

A classical NLS and the SUN domain contribute to the targeting of SUN2 to the inner nuclear membrane

Integral membrane proteins of the inner nuclear membrane (INM) are inserted into the endoplasmic reticulum membrane during their biogenesis and are then targeted to their final destination. We have used human SUN2 to delineate features that are required for INM targeting and have identified multiple elements that collectively contribute to the efficient localization of SUN2 to the nuclear envelope (NE). One such targeting element is a classical nuclear localization signal (cNLS) present in the N-terminal, nucleoplasmic domain of SUN2. A second motif proximal to the cNLS is a cluster of arginines that serves coatomer-mediated retrieval of SUN2 from the Golgi. Unexpectedly, also the C-terminal, lumenal SUN domain of SUN2 supports NE localization, showing that targeting elements are not limited to cytoplasmic or transmembrane domains of INM proteins. Together, SUN2 represents the first mammalian INM protein relying on a functional cNLS, a Golgi retrieval signal and a perinuclear domain to mediate targeting to the INM.

p150/glued modifies nuclear estrogen receptor function

Estrogen modulates gene expression through interactions with estrogen receptors (ERs) that bind chromosomal target genes. Recent studies have suggested an interaction between the cytoskeletal system and estrogen signaling; these have implicated a role of cytoplasmic microtubules in scaffolding ERalpha and enhancing nongenomic function; in addition, other experiments demonstrate that dynein light chain 1 may chaperone ERalpha to the nucleus, indirectly increasing transcriptional potency. Actin/myosin and dynein light chain 1 are also required for estrogen-mediated chromosomal movement that is required for transcriptional up-regulation of ERalpha targets. We present evidence that the dynactin component, p150/glued, directly influences the potency of nuclear ER function. Increasing the stoichiometric ratio of p150/glued and ERalpha by overexpression enhances estrogen responses. ERalpha enhancement by p150/glued does not appear to be influenced by shifts in subcellular localization because microtubule disruption fails to increase nuclear ERalpha. Rather, we find that modest amounts of p150/glued reside in the nucleus of cells, suggesting that it plays a direct role in nuclear transcription. Notably, p150/glued is recruited to the pS2 promoter in the presence of hormone, and, in MCF-7 cells, knockdown of p150/glued levels reduces estrogen-dependent transcription. Our results suggest that p150/glued modulates estrogen sensitivity in cells through nuclear mechanisms.

Glycoproteins required for entry are not necessary for egress of pseudorabies virus

In the current perception of the herpesvirus replication cycle, two fusion processes are thought to occur during entry and nuclear egress. For penetration, glycoproteins gB and gH/gL have been shown to be essential, whereas a possible role of these glycoproteins in nuclear egress remains unclear. Viral envelope glycoproteins have been detected by immunolabeling in the nuclear membrane as well as in primary enveloped particles in several herpesviruses, indicating that they might be involved in the fusion process. Moreover, a herpes simplex virus type 1 mutant simultaneously lacking gB and gH was described to be deficient in nuclear egress (A. Farnsworth, T. W. Wisner, M. Webb, R. Roller, G. Cohen, R. Eisenberg, and D. C. Johnson, Proc. Natl. Acad. Sci. USA 104:10187-10192, 2007). To analyze the situation in the related alphaherpesvirus pseudorabies virus (PrV), mutants carrying single and double deletions of glycoproteins gB, gD, gH, and gL were constructed and characterized. We show here that the simultaneous deletion of gB and gD, gB and gH, gD and gH, or gH and gL has no detectable effect on PrV egress, implying that none of these glycoproteins either singly or in the tested combinations is required for nuclear egress. In addition, immunolabeling studies using different mono- or polyclonal sera raised against various PrV glycoproteins did not reveal the presence of viral glycoproteins in the inner nuclear membrane or in primary virions. Thus, our data strongly suggest that different fusion mechanisms are active during virus entry and egress.

Mutational analysis of ErbB2 intracellular localization

In the present study, the sub-cellular localization of ErbB2 and its mutants expressed as GFP-tagged proteins in MCF-7 cells or endogenous ErbB2 in SKBR3 cells was examined. The data presented here demonstrate that the full-length ErbB2 was localized at the cytoplasmic membrane and ErbB2 ICD localized in the nucleus predominantly. The sequence of ErbB2 ICD contains the information supporting its nuclear translocation and cytoplasmic retention. A region (residues 721-970) harboring an arginine triplet is essential for the cytoplasmic trafficking of ErbB2. The results indicate that differential sub-cellular localization of ErbB2 ICD and the full-length ErbB2 is dependent on their structural determinants. The present results give initial clues for further analysis of the mechanism of ErbB2 intracellular localization.

Importin-mediated Nuclear Translocation of Galectin-3

Galectin-3 (Gal-3), a member of a beta-galactoside-binding protein family, is involved in RNA processing and cell cycle regulation through activation of transcription factors when translocated to the nucleus. We have previously shown that Gal-3 can import into the nucleus through at least two pathways; via passive diffusion and/or active transport (Nakahara, S., Oka, N., Wang, Y., Hogan, V., Inohara, H, and Raz, A. (2006) Cancer Res. 66, 9995-10006). Here, we investigated the process mediated by the active nuclear transport of Gal-3 and have identified a nuclear localization signal (NLS)-like motif in its protein sequence, (223)HRVKKL(228), that resembles p53 and c-Myc NLSs ((378)SRHKKL(383), (322)AKRVKL(327)), respectively. Moreover, trimers of enhanced green fluorescence protein (3xGFP) fused with this NLS-like sequence, which is too large to passively diffuse through the nuclear pores, accumulated in the cell nuclei. To gain insights into this newly identified nuclear import mechanism, the interaction between Gal-3 and importins (importins alpha and beta) that carry the NLS harboring nuclear proteins into the nucleus, was investigated. Pull-down assays and bimolecular fluorescence complementation (BiFC) analysis revealed that wild-type Gal-3, but not mutant Gal-3 (R224A), binds to importin-alpha. Down-regulation of importin-beta by RNA interference (RNAi) efficiently abrogates its nuclear accumulation. Furthermore, we provide evidence that impaired nuclear translocation of mutant Gal-3 protein (R224A) results in accelerated degradation compared with the wild-type protein. Thus, these results suggest that Gal-3 is translocated to the nucleus, in part, via the importin-alpha/beta route and that Arg(224) amino acid residue of human Gal-3 is essential for its active nuclear translocation and its molecular stability.

Herpesvirus assembly: a tale of two membranes

Herpes virions are amongst the most complex virus particles: they comprise in excess of thirty virally encoded proteins, and also contain cellular components. Capsid formation and the cleavage and encapsidation of replicated viral DNA occur in the nucleus and resemble similar processes in tailed dsDNA (double-stranded DNA) bacteriophages, which indicates they might have common ancestry. In contrast, final virion maturation takes place in the cytoplasm. Nucleocapsids gain access to this compartment by envelopment at the inner nuclear membrane, which involves the interaction between viral and cellular proteins in order to locally alter nuclear architecture. Fusion of the primary viral envelope with the outer nuclear membrane results in translocation of the nucleocapsid to the cytoplasm. Here, the majority of the tegument - a structure, composed of a multitude of different proteins, that links the capsid and the envelope - is added to nucleocapsids, which obtain their final envelope by budding into glycoprotein-containing Golgi-derived vesicles. Thus, herpesvirus morphogenesis proceeds in two different cellular compartments, involving different viral and cellular proteins.

The human cytomegalovirus glycoprotein UL16 traffics through the plasma membrane and the nuclear envelope

The human cytomegalovirus (HCMV) UL16 gene encodes a glycoprotein that interferes with the immune response to the virus-infected cell. In vitro, UL16 interacts with MICB and ULBPs that are ligands for the stimulatory receptor NKG2D, expressed on NK cells and CD8(+)T cells. UL16 expression has been shown to promote intracellular accumulation of MICB, ULBP1 and 2 and thus, interfere with the immune response to HCMV-infected cells. The mechanism that has been suggested for UL16-mediated MICB downmodulation is retention in the ER. Here, we studied the intracellular localization and maturation of UL16 and MICB in HCMV-infected cells and transfectant systems. UL16 trafficked through the ER, TGN and progressed to the plasma membrane, after which the protein was internalized. Strikingly, UL16 was also observed in the inner nuclear membrane. MICB was also localized in the TGN in HCMV-infected cells. These data suggest that MICB trafficking might be affected after its transit through the ER.

Hide and run. Arginine-based endoplasmic-reticulum-sorting motifs in the assembly of heteromultimeric membrane proteins

Arginine-based endoplasmic reticulum (ER)-localization signals are sorting motifs that are involved in the biosynthetic transport of multimeric membrane proteins. After their discovery in the invariant chain of the major histocompatibility complex class II, several hallmarks of these signals have emerged. They occur in polytopic membrane proteins that are subunits of membrane protein complexes; the presence of the signal maintains improperly assembled subunits in the ER by retention or retrieval until it is masked as a result of heteromultimeric assembly. A distinct consensus sequence and their position independence with respect to the distal termini of the protein distinguish them from other ER-sorting motifs. Recognition by the coatomer (COPI) vesicle coat explains ER retrieval. Often, di-leucine endocytic signals occur close to arginine-based signals. Recruitment of 14-3-3 family or PDZ-domain proteins can counteract ER-localization activity, as can phosphorylation. This, and the occurrence of arginine-based signals in alternatively spliced regions, implicates them in the regulated surface expression of multimeric membrane proteins in addition to their function in quality control.

Maturation of Borna disease virus glycoprotein

The maturation of Borna disease virus (BDV) glycoprotein GP was studied in regard to intracellular compartmentalization, compartmentalization signal-domains, proteolytic processing, and packaging into virus particles. Our data show that BDV-GP is (i) predominantly located in the endoplasmic reticulum (ER), (ii) partially exists in the ER already as cleaved subunits GP-N and GP-C, (iii) is directed to the ER/cis-Golgi region by its transmembrane and/or cytoplasmic domains in CD8-BDV-GP hybrid constructs and (iv) is incorporated in the virus particles as authentic BDV glycoprotein exclusively in the cleaved form decorated with N-glycans of the complex type. Downregulation of BDV-glycoproteins on the cell surface, their limited proteolytic processing, and protection of antigenic epitopes on the viral glycoproteins by host-identical N-glycans are different strategies for persistent virus infections.

Characterization of murine gammaherpesvirus 68 glycoprotein B

Murine gammaherpesvirus 68 (MHV-68) glycoprotein B (gB) was identified in purified virions by immunoblotting, immunoprecipitation, and immunoelectron microscopy. It was synthesized as a 120-kDa precursor in infected cells and cleaved into 65-kDa and 55-kDa disulfide-linked subunits close to the time of virion release. The N-linked glycans on the cleaved, virion gB remained partially endoglycosidase H sensitive. The processing of MHV-68 gB therefore appears similar to that of Kaposi's sarcoma-associated herpesvirus gB and human cytomegalovirus gB.

Herpes simplex virus type 1 glycoprotein B sorting in hippocampal neurons

Herpes simplex virus type 1 (HSV-1) is a neuroinvasive human pathogen that spreads in the nervous system in functionally connected neurons. Determining how HSV-1 components are sorted in neurons is critical to elucidate the mechanisms of virus neuroinvasion. By using recombinant viruses expressing glycoprotein B (gB) tagged with green fluorescent protein (GFP), the subcellular localization of this envelope protein was visualized in infected hippocampal neurons in culture. Results obtained using a fully infectious recombinant virus containing GFP inserted into the ectodomain of gB support the view that capsids and gB are transported separately in neuron processes. Moreover, they show that during infection gB is sorted to the dendritic tree and the axons of polarized hippocampal neurons. However, GFP insertion into the cytoplasmic tail of gB impaired the maturation of the resulting fusion protein and caused its retention in the endoplasmic reticulum. The defective protein did not gain access to axons of infected neurons. These results suggest that the cytoplasmic tail of gB plays a role in maturation and transport and subsequently in axonal sorting in differentiated hippocampal neurons.

Membrane restraint of estrogen receptor alpha enhances estrogen-dependent nuclear localization and genomic function

Estrogen receptor (ER) alpha localizes to both the nucleus and the plasma membrane, mediating estrogen-dependent genomic and nongenomic signaling, respectively. In some cells, ERalpha appears to be excluded from the nucleus, and it is unclear whether genomic signaling takes place. The purpose of this study was to determine whether membrane-associated ERalpha is capable of genomic signaling, or whether this pool of receptors strictly serves membrane-mediated signaling. ERalpha fused to the C-terminal cytoplasmic tail of bovine rhodopsin (Rh-ERalpha) activates ER response element-dependent transcription only in the presence of estrogen; the activity is antagonized by the estrogen antagonist ICI 182,780 and by the dominant-negative mutant of ERalpha and is unaffected by inhibitors of MAPKs and Akt signaling, indicating that this was due to direct genomic action. The activity of Rh-ERalpha containing the activating Y537S mutation was also estrogen dependent, suggesting that estrogen gated the entry of Rh-ERalpha into the nucleus. Indeed, cell fractionation studies demonstrated that Rh-ERalpha protein, in contrast to ERalpha that was nuclear at baseline, was excluded from the nucleus in the absence of hormone, and localized to the inner nuclear membrane on incubation with estrogen. These data demonstrate that membrane tethered ERalpha is capable of nuclear function and that its transcriptional activity is regulated by hormone-dependent entry into the inner nuclear membrane. Furthermore, these experiments provide evidence that under certain circumstances, membrane proteins are capable of nuclear function without detectable nucleoplasmic localization.

Identification of Lassa virus glycoprotein signal peptide as a trans-acting maturation factor

Lassa virus glycoprotein is translated as a precursor (pre-GP-C) into the lumen of the endoplasmic reticulum and is cotranslationally cleaved into the signal peptide and GP-C, before GP-C is proteolytically processed into its subunits GP1 and GP2. The signal peptide of pre-GP-C comprises 58 amino acids. The substitution of Lassa virus pre-GP-C signal peptide with another signal peptide still mediates translocation and the release of signal peptide but abolishes the proteolytic cleavage of GP-C into GP1 and GP2. Remarkably, cleavage of GP-C from these hybrid pre-GP-C substrates was restored on coexpression of the wild-type pre-GP-C signal peptide, indicating that the signal peptide functions as a trans-acting factor to promote Lassa virus GP-C processing. To our knowledge, this is the first report on a signal peptide that is essential for proteolytic processing of a secretory pathway protein.

Identification of Lassa virus glycoprotein signal peptide as a trans-acting maturation factor

Lassa virus glycoprotein is translated as a precursor (pre-GP-C) into the lumen of the endoplasmic reticulum and is cotranslationally cleaved into the signal peptide and GP-C, before GP-C is proteolytically processed into its subunits GP1 and GP2. The signal peptide of pre-GP-C comprises 58 amino acids. The substitution of Lassa virus pre-GP-C signal peptide with another signal peptide still mediates translocation and the release of signal peptide but abolishes the proteolytic cleavage of GP-C into GP1 and GP2. Remarkably, cleavage of GP-C from these hybrid pre-GP-C substrates was restored on coexpression of the wild-type pre-GP-C signal peptide, indicating that the signal peptide functions as a trans-acting factor to promote Lassa virus GP-C processing. To our knowledge, this is the first report on a signal peptide that is essential for proteolytic processing of a secretory pathway protein.

A three-residue signal confers localization of a reporter protein in the inner nuclear membrane

By alignment to the carboxy-terminal-deduced aa sequence of human cytomegalovirus glycoprotein B (gB), conserved hexameric aa motifs with putative function for localization in the inner nuclear membrane (INM) were identified in the nucleoplasmic tails of herpes simplex virus type 1 gB and of the cellular lamin B receptor. Fusion of the respective hexamers to CD8 as a reporter redirected transport of the chimeras into the INM, suggesting their functioning as modular signal motifs. Consecutive experiments showed that the three-residue motif RxR represents a consensus sequence which is sufficient for localization of the CD8 reporter in the INM.

A role for human cytomegalovirus glycoprotein O (gO) in cell fusion and a new hypervariable locus

A cell fusion assay using fusion-from-without (FFWO) recombinant adenoviruses (RAds) and specific antibody showed a role in fusion modulation for glycoprotein gO, the recently identified third component of the gH/gL gCIII complex of human cytomegalovirus (HCMV). As in HCMV, RAd gO expressed multiple glycosylated species with a mature product of 125 kDa. Coexpression with gH/gL RAds showed gCIII reconstitution in the absence of other HCMV products and stabilisation by intermolecular disulfide bonds. Properties of HCMV clinical isolate, Pt, also implicated gO in cell spread. Compared to laboratory strain AD169, Pt was resistant to gH antibody plaque inhibition, but mature gH was identical. However, the gO sequences were highly divergent (20%), with further variation in laboratory strain Towne gO (34%). Thus, gO forms gCIII with gH/gL, performs in cell fusion, and is a newly identified HCMV hypervariable locus which may influence gCIII's function in mediating infection.

Proteolytic processing of human cytomegalovirus glycoprotein B is dispensable for viral growth in culture

Glycoprotein B (gB) of human cytomegalovirus (HCMV), which is considered essential for the viral life cycle, is proteolytically processed during maturation. Since gB homologues of several other herpesviruses remain uncleaved, the relevance of this property of HCMV gB for viral infectivity is unclear. Here we report on the construction of a viral mutant in which the recognition site of gB for the cellular endoprotease furin was destroyed. Because mutagenesis of essential proteins may result in a lethal phenotype, a replication-deficient HCMV gB-null genome encoding enhanced green fluorescent protein was constructed, and complementation by mutant gBs was initially evaluated in transient-cotransfection assays. Cotransfection of plasmids expressing authentic gB or gB with a mutated cleavage site (gB-DeltaFur) led to the formation of green fluorescent miniplaques which were considered to result from one cycle of phenotypic complementation of the gB-null genome. To verify these results, two recombinant HCMV genomes were constructed: HCMV-BAC-DeltaMhdI, with a deletion of hydrophobic domain 1 of gB that appeared to be essential for viral growth in the cotransfection experiments, and HCMV-BACDeltaFur, in which the gB cleavage site was mutated by amino acid substitution. Consistent with the results of the cotransfection assays, only the DeltaFur mutant replicated in human fibroblasts, showing growth kinetics comparable to that of wild-type virus. gB in mutant-infected cells was uncleaved, whereas glycosylation and transport to the cell surface were not impaired. Extracellular mutant virus contained exclusively uncleaved gB, indicating that proteolytic processing of gB is dispensable for viral replication in cell culture.

Multimerization potential of the cytoplasmic domain of the human cytomegalovirus glycoprotein B

In the present study the coding sequence of the cytoplasmic tail of the human cytomegalovirus glycoprotein B (gB) was expressed. The secondary structure of the purified recombinant protein was analyzed by circular dichroism, and the quaternary structure was investigated by gel permeation chromatography, and electron microscopy. Our data indicate that the cytoplasmic gB domain contains alpha-helix structures and assembles into tetramers, suggesting that the authentic gB may represent a homotetramer.

Intracellular redistribution of nucleolin upon interaction with the CD3epsilon chain of the T cell receptor complex

T cell activation through the antigen receptor (TCR) involves the cytoplasmic tails of the CD3 subunits CD3gamma, CD3delta, CD3epsilon, and CD3zeta. Whereas the biological significance of the cytoplasmic tails of these molecules is suggested, in part, by their evolutionarily conserved sequences, their interactions with signal transduction molecules are not completely understood. We used affinity chromatography columns of glutathione S-transferase fused to the CD3epsilon cytoplasmic tail to isolate proteins that specifically interact with this subunit. In this way, we identified the shuttling protein nucleolin as a specific CD3epsilon-interacting molecule. Using competition studies and affinity chromatography on peptide columns, we were able to identify a central proline-rich sequence as the nucleolin-interacting sequence in CD3epsilon. Transfection in COS cells of wild type CD3epsilon, but not of nonbinding mutants of CD3epsilon, resulted in redistribution of nucleolin from the nucleus and nucleoli to the cytoplasm. This property was transferred to a CD8 protein chimera by appending the cytoplasmic tail of CD3epsilon. We also found that nucleolin associated with the TCR complex. This association was increased upon TCR engagement, suggesting that the CD3epsilon/nucleolin interaction may have a role in T cell activation.

Role of the cytoplasmic tails of pseudorabies virus glycoproteins B, E and M in intracellular localization and virion incorporation

The cytoplasmic domains of several herpesviral glycoproteins encompass potential intracellular sorting signals. To analyse the function of the cytoplasmic domains of different pseudorabies virus (PrV) glycoproteins, hybrid proteins were constructed consisting of the extracellular and transmembrane domains of envelope glycoprotein D (gD) fused to the cytoplasmic tails of gB, gE or gM (designated gDB, gDE and gDM), all of which contain putative endocytosis motifs. gD is a type I membrane protein required for binding to and entry into target cells. Localization of hybrid proteins compared to full-length gB, gE and gM as well as carboxy-terminally truncated variants of gD was studied by confocal laser scanning microscopy. The function of gD hybrids was assayed by trans-complementation of a gD-negative PrV mutant. The carboxy-terminal domains of gB and gM directed a predominantly intracellular localization of gDB and gDM, while full-length gD and a tail-less gD mutant (gDc) were preferentially expressed on the cell surface. In contrast gDE, and a gDB lacking the putative gB endocytosis signal (gDB Delta 29), were predominantly located in the plasma membrane. Despite the different intracellular localization, all tested proteins were able to complement infectivity of a PrV gD(-) mutant. Cells which stably express full-length gD and plasma-membrane-associated gD hybrids exhibit a significant resistance to PrV infection, while cells expressing predominantly intracellularly located forms do not. This suggests that the assumed sequestration of receptors by gD, which is supposed to be responsible for the interference phenomenon, occurs at the cell surface.

UNCL, the mammalian homologue of UNC-50, is an inner nuclear membrane RNA-binding protein

We isolated a mammalian homologue of the C. elegans gene unc-50 that we have named UNCL. The 777 kb rat UNCL cDNA encodes a 259 amino acid protein that is expressed in a wide variety of tissues with highest mRNA levels in brain, kidney and testis. Hydropathy plot analysis and in vitro translation experiments with microsomal membranes indicate that UNCL is a transmembrane protein. Hemagglutinin tagged UNCL was stably transfected into SaOS-2 osteosarcoma cells and exhibited a nuclear rim staining pattern which was retained following extraction with 1% Triton X-100, suggesting that UNCL localizes to the inner nuclear membrane. UNCL-HA was extractable in 350 mM NaCl, suggesting that UNCL is not associated with the nuclear matrix. Homopolymer RNA-binding assays performed on in vitro translated UNCL protein and 'structural modeling by homology' suggest that UNCL binds RNA via an amino-terminal RNA Recognition-like Motif. Since unc-50 is required for expression of assembled muscle-type nicotinic receptors in the nematode we investigated whether UNCL had a similar function for mammalian nicotinic receptors. When UNCL was co-expressed with neural nicotinic receptors in Xenopus oocytes or COS cells it increased expression of functional cell surface receptors up to 1. 6-fold. We conclude that UNCL is a novel inner nuclear membrane protein that associates with RNA and is involved in the cell-surface expression of neuronal nicotinic receptors. UNCL plays a broader role because UNCL homologues are present in two yeast and a plant species, none of which express nicotinic receptors and it is also found in tissues that lack nicotinic receptors.